Unit Four: Matter

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Liquid state

Although the kinetic theory explains the behaviors of gas particles, some of the assumptions of the theory apply to liquids and solids as well. The particles of a substance in the liquid state, shown in Figure 2, are also constantly moving, although they are not moving as quickly as they would be if the substance were in the gas state. Therefore, the particles that make up a substance have less kinetic energy when in its liquid state than when in its gas state. Because they have less energy, the particles are less able to overcome their attractions to each other. They can slide past each other, allowing a liquid to flow and take the shape of its container. However, because the particles that make up a liquid have not completely overcome the attractive forces between them, the particles cling together, giving the liquid a definite volume.

Properties of Metalloids

Can an element be a metal and a nonmetal? In a sense, the metalloid elements are. Metalloids are elements that have some properties of metals and some properties of nonmetals. They can form ionic bonds and covalent bonds. Some metalloids can conduct electricity better than most nonmetals but not as well as many metals. The metalloids are located along the stair-step line on the periodic table. In this book, the metalloid element blocks are green. Groups 13 through 17 are mixed groups and contain metals, nonmetals, and metalloids. The Boron Group Boron, a metalloid, is the first element in group 13. You might find the boron compounds borax and boric acid in your home. Borax is a water softener used in some laundry products. Boric acid is a mild antiseptic. Boron also is used to make heatresistant glass, such as the lab equipment in Figure 15. Aluminum, a metal in group 13, is the most abundant metal in Earth's crust. Because it is strong and light, aluminum is used in soft-drink cans, foil, pans, bicycle frames, and airplanes. Gallium is a metal used in electronic components. The last two group 13 elements are the rare metals indium and thallium.

The Oxygen Group

Group 16 is the oxygen group. These elements have six electrons in their outer energy levels. They will accept two electrons from a metal or bond covalently with other nonmetals. Oxygen Oxygen, a nonmetal, exists in the air as diatomic molecules ( O 2 ). Nearly all living things on Earth, including people, need O 2 for respiration. During electrical storms, some oxygen molecules change into ozone molecules ( O 3 ). A layer of ozone around Earth protects living things from some of the Sun's radiation. There are also many useful compounds that contain oxygen. Water ( H 2 O) is another essential substance for living organisms. Hydrogen peroxide ( H 2 O 2 ), which is used as a disinfectant, is another oxygen compound. Plants absorb carbon dioxide (C O 2 ) and give off the oxygen we breathe. Other group 16 elements The second element in the oxygen group is sulfur. Sulfur is a nonmetal that has several allotropes. It exists as different-shaped crystals and as a noncrystalline solid. Sulfur combines with metals to form sulfides of distinctive colors that are used as pigments in paints. The nonmetal selenium and two metalloids tellurium and polonium are the other group 16 elements. Selenium is the most common of these three. You need trace amounts of selenium in your diet. Selenium is found in many multivitamins. However, selenium is toxic if too much of it gets into your system.

Thermal expansion

Average atomic mass

How do scientists account for and represent the different atomic masses of isotopes? As an example, models of two naturally occurring isotopes of boron are shown in Figure 7. Because most elements, including boron, naturally occur as more than one isotope, each element can be described by an average atomic mass of the isotopes. The average atomic mass of an element is the weighted average mass of all naturally occurring isotopes of an element, measured in atomic mass units (amu), according to their natural abundances. For example, eighty percent or four out of five atoms of boron are boron-11 and twenty percent or one out of five is boron-10. The following calculation gives the weighted average of these two masses. _4 5 (11 amu) + _1 5 (10 amu) = 10.8 amu The average atomic mass of the element boron is 10.8 amu. Note that the average atomic mass of boron is closest to the mass of its most abundant isotope, boron-11.

The Atom and the Periodic Table

The modern periodic table consists of boxes, each containing information such as element name, symbol, atomic number, and atomic mass. A typical box is shown in Figure 10. As you have learned, elements on the periodic table are organized based on similarities in their physical and chemical properties. The horizontal rows of elements in the periodic table are called periods and are numbered 1 through 7. The vertical columns in the periodic table are called groups (also called families), and they are numbered 1 through 18. Elements in each group share similar properties. For example, the elements in group 11—including copper, silver, and gold—are all similar. Each element is a shiny metal and a good conductor of heat and electricity. Why are these elements so similar? Electron cloud structure You have learned about the nucleus of an atom and the fact that protons and neutrons are located there. But where are the electrons? How many are there? Because an atom does not have an overall charge, the number of electrons is equal to the number of protons. Therefore, a carbon atom has six protons and six electrons. An oxygen atom has eight protons and eight electrons. Electrons are located in an area surrounding the nucleus called the electron cloud. Energy Levels Scientists have discovered that electrons within the electron cloud have different amounts of energy. Scientists model the energy differences between electrons by placing electrons in energy levels, as shown in Figure 11. Electrons located in energy levels close to the nucleus have less energy than electrons in energy levels farther away. Electrons occupy energy levels in a predictable pattern from the inner to the outer levels. Elements in the same group have the same number of electrons in their outermost energy levels. These electrons are called valence electrons. It is the number of valence electrons that determines the chemical properties of each individual element. It is important to understand the relationship between the location of an element in the periodic table, the element's chemical properties, and the element's atomic structure.

The Atom and the Periodic Table

Atomic models

400 B.C. Democritus Model Democritus fi rst proposed that elements consisted of tiny, solid particles that could not be subdivided. He called these particles atomos, meaning "uncuttable." Democritus's ideas were criticized by Aristotle, who believed that empty space could not exist. Because Aristotle was one of the most infl uential philosophers of his time, Democritus's atomic theory was rejected. 1904 Thomson Model English physicist Joseph John Thomson proposed a model that consisted of a spherical atom containing small, negatively charged particles. He thought these "electrons" (in red) were evenly embedded throughout a positively charged sphere, much like chocolate chips in a ball of cookie dough. 1911 Rutherford Model English physicist Ernest Rutherford proposed that all the positive charge of an atom was concentrated in a central atomic nucleus that was surrounded by electrons. 1913 Bohr Model Danish physicist Niels Bohr hypothesized that electrons traveled in fi xed orbits. The electrons could jump between orbits as they absorbed or released specifi c amounts of energy. The Bohr model worked very well for hydrogen, but did not work as well for atoms with many electrons.

Heating curves

A graph of temperature v. time for heating of 1.0 kg of water is shown in Figure 7. This type of graph is called a heating curve. It shows how temperature changes over time as thermal energy is continuously added. Notice the two areas on the graph where the temperature does not change. At 0°C, ice is melting. All of the energy put into the ice at this temperature is used to overcome the attractive forces between the particles. The flat line on the graph indicates that temperature remains constant during melting. After the attractive forces are overcome, the particles move more freely and their temperature increases. At 100°C, water is boiling, the temperature remains constant again, and the graph is flat. All of the energy that is put into the water goes to overcoming the remaining attractive forces between the particles. When all of the attractive forces between the particles are overcome, the energy goes into increasing the temperature again.

Bonding in nonmetals

Nonmetals become negative ions when they gain electrons from metals. An example of an ionic compound is calcium fluoride (CaF2), which is shown in Figure 11. Calcium fluoride forms from the nonmetal fluoride and the metal calcium. When bonded with other nonmetals, atoms of nonmetals usually share electrons to form covalent bonds. Compounds made of atoms that are covalently bonded are called covalent compounds. For example, the covalent compound carbon dioxide (CO2) is shown in Figure 11. Carbon dioxide is a gas that you exhale and that plants need to survive.

Subatomic Particles

An element is matter that is composed of only one type of atom. An atom is the smallest particle of an element that retains the element's properties. For example, the element iron is composed of only iron atoms and the element hydrogen is composed of only hydrogen atoms. Atoms are composed of even smaller particles—subatomic particles—called protons, neutrons, and electrons, as shown in Figure 1. The small, positively charged center of the atom is called the nucleus. The nucleus contains protons and neutrons. Protons are particles in the nucleus with an electric charge of 1+. The number of protons in the nucleus is unique for each element. Neutrons are electrically neutral particles in the nucleus; they do not have a charge. Electrons are particles with an electric charge of 1−. They occupy the space surrounding the nucleus of an atom. Quarks—even smaller particles Are the protons, electrons, and neutrons that make up atoms the smallest particles that exist? Scientists have inferred that protons and neutrons are composed of smaller particles called quarks. Electrons, however, are not made of smaller particles. So far, scientists have confirmed the existence of six uniquely different quarks. A particular arrangement of three of these quarks produces a proton. Another arrangement of three quarks produces a neutron. The search for the composition of protons and neutrons is a continuing effort.

Viscosity

Another property exhibited by a fluid is its tendency to flow. While all fluids flow, they vary in the rates at which they flow. Viscosity is the resistance of a fluid to flowing. For example, when you take syrup out of the refrigerator and pour it, as shown in Figure 18, the flow of syrup is slow. But if this syrup were heated, it would flow much faster. Water flows easily because it has low viscosity. Cold syrup flows slowly because it has high viscosity. What causes viscosity? When a container of liquid is tilted to allow flow to begin, the flowing portion of the liquid transfers energy to the portion of the liquid that is stationary. In effect, the flowing portion of the liquid is pulling the stationary portion of the liquid, causing it to flow, too. If the flowing portion of the liquid does not effectively pull the other portions of the liquid into motion, then the liquid has a high viscosity, which is a high resistance to flow. If the flowing portion of the liquid pulls the other portions of the liquid into motion easily, then the liquid has low viscosity, or a low resistance to flow.

Isotopes

Atoms of the same element can have different mass numbers. For example, carbon atoms can have a mass number of 12 and also a mass number of 14. The number of protons for each element never changes. So, for an atom's mass number to differ, the number of neutrons must change. Atoms of the same element that have different numbers of neutrons are called isotopes. To identify isotopes, scientists write the name of the element followed by the element's mass number. Carbon with a mass number of 12 is written as carbon-12. Carbon-12 has six protons and six neutrons. Carbon-14 has six protons and eight neutrons. Carbon-12 and carbon-14 are isotopes of the element carbon. Some properties of carbon-12 and carbon-14 are unique due to differences in the number of neutrons that each element contains. For example, carbon-14 is radioactive, but carbon-12 is not. Suppose you have a sample of the element boron. Naturally occurring isotopes of boron have mass numbers of 10 or 11. How many neutrons does each isotope contain? Locate boron in Table 4 on the previous page and determine the number of protons in an atom of boron. You can then calculate that boron-10 has 5 neutrons and boron-11 has 6 neutrons.

Pascal's Principle and Pressure

Blaise Pascal (1623-1662), a French Scientist, discovered the pressure applied to a fluid is transmitted throughout the fluid. This is why when you squeeze one end of a toothpaste tube, toothpaste emerges from the other end. Pressure has been transmitted. The pressure has been transmitted through the toothpaste. In order to understand Pascal's principle, you must first understand pressure. Pressure Right now, pressure from the air is pushing on you from all sides like the pressure you feel underwater in a swimming pool. Pressure is force exerted per unit area. Pressure: Force exerted per unit of area. Pressure(Pa)= Force (N)/area(m squared) P= F/A The SI unit of pressure is the Pascal (Pa). Because pressure is the amount of force divided by area, one pascal is one newton per square meters (N/m squared). Most pressures are given in kilo-pascals (kPa) because one Pa is a very small amount of pressure. Pascal's principle The idea that pressure is transferred through a fluid can be written as an equation: pressure in = pressure out. Since pressure is force over area, Pascal's principle can be written another way. Pressure In= Pressure Out Pascals Principle: Input Force (N)/Input Area (m squared)= output force(N)/output area (m squared) F (in)/A(in)= F(out)/A(out) Hydraulic lifts Auto repair shops often make use of hydraulic lifts, which move heavy loads in accordance with Pascal's principle. A pipe that is filled with fluid connects small and large cylinders, as shown in Figure 15. Pressure applied to the small cylinder is transferred through the fluid to the large cylinder. With a hydraulic lift, you could use your weight to lift something much heavier than you.

The electron cloud model

By 1926, scientists developed the electron cloud model of the atom, which is the model that is accepted today. An electron cloud is the area around the nucleus of an atom where electrons are most likely to be found. The electron cloud is 100,000 times larger in diameter than the diameter of the nucleus of an atom. In contrast, each electron in the cloud is significantly smaller in mass than a single proton or single neutron. Reading Check Explain the difference between the Bohr model and the electron cloud model. Because an electron's mass is negligible compared to the nucleus and the electron is moving so quickly around the nucleus, it is impossible to describe its exact location in an atom at any moment. Picture the spokes on a moving bicycle wheel. The spokes are moving so quickly that you cannot pinpoint any single spoke in the wheel. All that you see is a blur that contains all the spokes somewhere with-in it. In a similar way, an electron cloud is a blur of activity containing all of an atom's electrons somewhere within it. Figure 5 illustrates the location of the nucleus and the electron cloud in the electron cloud model of the atom.

Compound

Two or more elements combine to form these. A compound is a substance in which the atoms of two or more elements are chemically combined in a fixed proportion. For example, water is a compound in which two atoms of the element hydrogen combine with one atoms of the element oxygen. Cannot be separated by physical means.

Solid state

Unlike a gas or a liquid, a solid has a definite shape and volume. The particles that make up a solid are closely packed together, as shown in Figure 2. They are still in motion, but they have so little kinetic energy that the particles are unable to overcome their attractions to each other. Many solids are crystalline, which means their particles have specific geometric arrangements. Figure 3 shows the geometric arrangement of ice. Notice that the hydrogen and oxygen atoms alternate in the arrangement.

Properties of Metals

Coins, paper clips, and some baseball bats are made of metals. Metals are elements that are shiny, malleable, ductile, and good conductors of heat and electricity. Except for mercury, metals are solids at room temperature. The shiny property of metals is called metallic luster. Metals are malleable (MA lee uh bul), which means they can be hammered or rolled into sheets. Metals are also ductile, which means they can be drawn into wires. Figure 1 shows the malleability and ductility of metals. These properties make metals suitable for use in objects ranging from eyeglass frames to computers to buildings. The specific properties of a metal depend on its unique configuration of electrons, protons, and neutrons. However, elements in the same group of the periodic table have similar properties . Metals and the periodic table In the periodic table, metals are found to the left of the stair-step line. In the periodic tables in this book, the metal element blocks are colored blue. Notice that most elements are metals. Except for hydrogen, all the elements in groups 1 through 12 are metals, as are the elements under the stair-step line in groups 13 through 15

Bernoulli's Principle

Daniel Bernoulli (1700-1782) was a Swiss scientist who studied the properties of moving fluids, such as water and air. Bernoulli found that fluid velocity increases when the flow of the fluid is restricted. Placing your thumb over a running garden hose, as shown in Figure 16, demonstrates this effect. When the opening of the hose is decreased in size, the water flows out more quickly. Bernoulli examined the relationship between fluid flow and pressure. You might think that increasing the velocity of a fluid's flow would increase its pressure, but Bernoulli found the opposite to be true. According to Bernoulli's principle, as the velocity of a fluid increases, the pressure exerted by that fluid decreases. He published this discovery in 1738. One application of Bernoulli's principle is the hose-end sprayer. This sprayer is used to apply fertilizers and pesticides to yards and gardens. To use this sprayer, a concentrated solution of the chemical that is to be applied is placed in the sprayer. The sprayer is attached to a garden hose, as shown in Figure 17. A straw-like tube is attached to the lid of the unit. The end of the tube is submerged into the concentrated chemical. The water to the garden hose is turned to a high flow rate. When you are ready to apply the chemicals to the lawn or plant area, you must push a trigger on the sprayer attachment. This allows the water in the hose to flow at a high rate of speed, creating a low pressure area above the straw-like tube. The concentrated chemical solution is sucked up through the straw and into the stream of water. The concentrated solution mixes with water, reducing the concentration to the appropriate level and creating a spray that is easy to apply

Scientific Shorthand

Do you use abbreviations for long words, street addresses, or the names of states? Scientists also use abbreviations. In fact, scientists have developed their own shorthand for naming the elements. Do the letters C, Al, Ne, and Au mean anything to you? Each letter or pair of letters is a chemical symbol, which is a short or abbreviated name of an element. Chemical symbols, such as those in Table 1, consist of one capital letter or a capital letter plus one or two lowercase letters. For some elements, the symbol is the first letter of the element's name. For other elements, the symbol is the first letter of the name plus another letter from its name. Some symbols are derived from Latin. For instance, argentum is Latin for silver. The chemical symbol for silver is Ag. Elements are named in a variety of ways. Some elements are named to honor scientists, for places, or for their properties. For example, the element curium was named to honor Pierre and Marie Curie, scientists who researched radioactivity. Other elements, like germanium, were named after a country. Regardless of the origin of the name, scientists derived the international system of symbols for convenience. It is much easier to write H for hydrogen, O for oxygen, and H2O for dihydrogen monoxide (water). Because scientists worldwide use this system, everyone recognizes what these symbols represent.

The Carbon Group

Each element in group 14, the carbon family, has four electrons in its outer energy level, but this is where much of the similarity ends. Carbon is a nonmetal, silicon and germanium are metalloids, and tin and lead are metals. Carbon Carbon, a nonmetal, occurs as an element in coal and as a compound in oil, natural gas, and foods. Carbon in these materials can combine with oxygen to produce carbon dioxide (C O 2 ). In the presence of sunlight, plants utilize C O 2 to make food. Carbon compounds, many of which are essential to life, can be found in you and all around you. All organic compounds contain carbon, but not all carbon compounds are organic. Allotropes of carbon What do the diamond in a diamond ring and the graphite in your pencil have in common? Both are carbon. Diamond and graphite, as shown in Figure 16, are allotropes of carbon. Allotropes are different molecular structures of the same element. Reading Check Define the term allotrope. Graphite and diamond Graphite is a black powder that consists of layers of hexagonal structures of carbon atoms. In the hexagons, each carbon atom is bonded to three other carbon atoms. The fourth electron of each atom is bonded weakly to the layer next to it. This structure allows the layers to slide easily past one another, making graphite an excellent lubricant. A diamond is transparent and extremely hard. In a diamond, each carbon atom is bonded to four other carbon atoms at the vertices, or corner points, of a tetrahedron. In turn, many tetrahedrons join together to form a giant molecule in which the atoms are held tightly in a strong crystalline structure. This structure accounts for the hardness of diamond.

Metals in Earth's Crust

Earth's hardened outer layer, called the crust, contains many compounds and a few uncombined metals such as gold and copper. Metals that are found in Earth's crust are minerals. Minerals are often found in ores. Ores are mixtures of minerals, clay, and rock that occur naturally in Earth's crust. Most metals must be mined and separated from their ores. After an ore is mined, the mineral is separated from the rock and clay. Then the mineral often is converted to another physical form. This step usually involves heat and is called roasting. Finally, the metal is refined into a pure form. Later, it can be alloyed with other metals. Removing the waste rock can be expensive. If the cost of removing the waste rock becomes greater than the value of the desired material, the mineral mixture is no longer classified as an ore. Mines can be found throughout the world. The western United States has several copper mines, like the one in Figure 9. Most of the world's platinum is found in South Africa. Chromium is important because it is used to harden steel, to manufacture stainless steel, and to form other alloys. The United States imports most of its chromium from South Africa, the Philippines, and Turkey.

Electron dot diagrams

Elements in the same group have the same number of electrons in their outermost energy levels. These electrons determine the chemical properties of an element. They are so significant that American chemist G.N. Lewis created a diagram to represent an element's outermost electrons while teaching a college chemistry class. An electron dot diagram uses the chemical symbol of an element surrounded by dots to represent the number of electrons in the outermost energy level. Figure 13 shows the electron dot diagrams for the group 1 elements. Same group—similar properties The electron dot diagrams for the elements in group 1 show that all members of a group have the same number of outermost electrons. Remember that the number of outermost electrons determines the chemical properties for each element. A common chemical property of group 1 metals is the tendency to react with nonmetals in group 17. The nonmetals in group 17 have electron dot diagrams similar to chlorine, as shown in Figure 14. For example, the group 1 element sodium reacts easily with the group 17 element chlorine. The result is the formation of the compound sodium chloride (NaCl)—ordinary table salt. Group 18 Not all elements will combine easily with other elements. The elements in group 18 have complete outermost energy levels, meaning that they cannot hold any more electrons. This special configuration makes many of the group 18 elements unreactive. Figure 14 shows the electron dot diagram for neon, a member of group 18.

Rows on the periodic table

Energy levels coincide with the number of rows on the periodic table. These energy levels are named using numbers one to seven. The maximum number of electrons that can be placed in each of the first four levels is shown in Figure 11. For example, energy level one can hold a maximum of two electrons. Energy level two can hold a maximum of eight electrons. For energy levels two and higher, the outer energy level is stable when it holds eight electrons. Notice, however, that energy levels three and four can contain more than eight electrons. The way in which energy levels split into sublevels allows for energy levels three and higher to contain more than eight electrons. These additional electrons are added to inner sublevels; the outer energy level is still stable when it contains eight electrons. Filling the first row Remember that the atomic number found on the periodic table is equal to the number of electrons in a neutral atom. Look at the elements in Figure 12. The first row has hydrogen with one electron and helium with two electrons, both in energy level one. Because energy level one is the outermost energy level containing an electron, hydrogen has one outer electron. Helium has two outer electrons. Recall from Figure 11 that energy level one can hold a maximum of two electrons. Therefore, helium has a full outer energy level and is chemically stable. ? Inquiry MiniLab SC.912.P.8.5: Relate properties of atoms and their position in the periodic table to the arrangement of their electrons. The elements in group 1 have one electron in their outermost energy levels. ■ Figure 14 Electron dot diagrams show the electrons in an element's outermost energy level. Ne Neon, a member of group 18, has a full outer energy level. Neon has eight electrons in its outer energy level, making it unreactive. Cl The electron dot diagram for group 17 consists of three sets of paired dots and one single dot. Cl - Na + Sodium combines with chlorine to give each element a complete outer energy level in the resulting compound. Filling higher rows The second row starts with lithium, which has three electrons—two in the first energy level and one in the second energy level. Lithium is followed by beryllium with two outer electrons, boron with three, and so on. Neon has a complete outermost energy level with eight outer electrons. Electrons begin filling energy level three for elements in the third row. The row ends with argon, which has eight outer electrons.

The Kinetic Theory

Explanation of how particles in gases behave. You encounter solids, liquids, and gases every day. Look at Figure 1. Can you identify the states of matter present? The tea is in the liquid state. The ice cubes dropped into the tea to cool it are in the solid state. Surrounding the glass, as part of the air, is water in the gas state. How do these states compare? Gas state To understand the states of matter, we must think about the particles that make up matter. Consider the air around you: it is composed of nitrogen, oxygen, and water, along with other gases. These atoms and molecules—the particles that make up the air—are constantly moving. The kinetic theory is an explanation of how the particles in gases behave. To explain the behavior of particles, it is necessary to make some basic assumptions. The assumptions of the kinetic theory are as follows: Figure 2 demonstrates the kinetic theory, showing the particles that make up a substance in the gas state. Because their particles are in constant motion, colliding with each other and with the walls of their container, gases do not have a fixed volume or shape. Instead, the particles that make up a gas spread out so that they fill whatever container they are in. The Assumptions of the Theory: 1) All matter is composed of tiny particles (atoms, molecules, ions) 2) These particles are in constant, random motion 3) The particles collide with each other and with the walls of any container in which they are hld 4) The amount of energy that the particles lose from these collisions is negligible

The Halogens

Fluorine, chlorine, bromine, iodine, and astatine are called halogens and make up group 17. They are very reactive in their elemental forms, and their compounds have many uses. For example, halogen lightbulbs contain small amounts of bromine or iodine vapor. Because an atom of a halogen has seven electrons in its outer energy level, only one electron is needed to complete this energy level. If a halogen gains an electron from a metal, an ionic compound called a salt is formed. An example of this is sodium chloride (NaCl). You know this compound as table salt. In the gaseous state, the halogens form reactive diatomic molecules and can be identified by their distinctive colors. Chlorine is greenish-yellow, bromine is reddish- orange, and iodine is violet. Fluorine Fluorine is the most chemically active of the nonmetal elements. As you can see in Figure 12, fluorine compounds have many uses. Fluorine compounds, called fluorides, are added to toothpastes and to city water systems to help prevent tooth decay. Hydrofluoric acid, a mixture of hydrogen fluoride and water, is used to etch glass and to frost the inner surfaces of lightbulbs. It is also used in the fabrication of semiconductors

Boyle's Law- Volume and Pressure

Have you ever seen a weather balloon, like the one shown in Figure 19? They carry sensing instruments to very high altitudes to detect weather information. A weather balloon is inflated near Earth's surface with a low-density gas. Recall that a gas completely fills its container. The balloon remains inflated because of collisions that the gas particles inside the balloon have with the balloon itself. In other words, these collisions between gas particles and the container wall cause the gas to exert pressure on the container. As the balloon rises, the atmospheric pressure outside the balloon decreases. This decrease in pressure allows the balloon to expand, eventually reaching a volume between 30 and 200 times its original size. Boyle's law describes the relationship between gas pressure and volume that explains the behavior of weather balloons. Volume and pressure Because a balloon is flexible, its volume can change. In the case of the weather balloon, the volume increases as the external pressure decreases. The volume of the gas inside the weather balloon continues to increase until the balloon can no longer contain it. At this point, the balloon ruptures and the sensing instruments it was carrying fall to the ground. From the weather balloon, we know what happens to volume when you decrease pressure. What happens to the pressure from a gas if you decrease its volume—for example, by decreasing the size of the container in which the gas is held? Think about the kinetic theory of matter. The pressure from a gas depends on how often its particles strike the walls of the container. If you squeeze gas into a smaller space, its particles will strike the walls more often, causing increased pressure. The opposite is also true, too. If you give the particles that make up the gas more space, increasing the volume, they will hit the walls less often and the pressure from the gas will be reduced. Robert Boyle (1627-1691), a British scientist, described this property of gases. According to Boyle's law, if you decrease the volume of a container of gas and hold the temperature constant, the pressure from the gas will increase. An increase in the volume of the container causes the pressure to drop, if the temperature remains constant. Figure 20 shows this relationship as the volume of a gas is decreased from 10 L to 5 L to 2.5 L. Note the points on the graph that correspond to each of these volumes. An equation for Boyle's law Boyle's law can be expressed with a mathematical equation. When the temperature of a gas is constant, then the product of the pressure and volume of that gas does not change. Boyle's Law Equation initial pressure × initial volume = final pressure × final volume P iV i = P fV f The product of the initial pressure and volume—designated with the subscript i—is equal to the product of the final pressure and volume—designated with the subscript f. You can use this equation to find one unknown value when you have the other three. The equation will work with any units for either volume or pressure, as long as you use the same pressure units for Pi and Pf and the same volume units for Vi and Vf.

Thermal Expansion

Have you ever wondered why a concrete sidewalk has seams? When thermal energy is transferred to a concrete sidewalk, the concrete expands. Without the seams, a concrete sidewalk would crack in hot weather. The kinetic theory can help to explain this behavior. Recall that particles move faster and farther apart as the temperature rises. This separation of particles results in an expansion of the entire object, known as thermal expansion. Thermal expansion is an increase in the size of a substance when the temperature is increased. Substances also contract when they cool. Thermometers A common example of liquids undergoing thermal expansion occurs in thermometers, like the one shown in Figure 8. The addition of energy causes the particles that make up the liquid in the thermometer to move faster. As their motion increases, the particles that make up the liquid in the narrow thermometer tube start to move farther apart. This causes the liquid in a thermometer to expand and rise as the temperature increases. Hot-air balloons An application of gases undergoing thermal expansion is shown in Figure 9. Hot-air balloons are able to rise due to the thermal expansion of air. The air in the balloon is heated, causing the distance between the particles that make up the air to increase. As the hot-air balloon expands, the number of particles per cubic centimeter decreases. This expansion results in a decreased density of hot air. Because the density of the air in the hot-air balloon is lower than the density of the cooler air outside, the balloon will rise.

Vaporization and condensation

How does a liquid become a gas? Remember that the particles that make up a liquid are constantly moving. When particles move fast enough to escape the attractive forces of other particles, they enter the gas state. This process is called vaporization. Vaporization can occur in two ways: evaporation and boiling. The process in which a gas becomes a liquid is called condensation. Condensation is the reverse of vaporization. Evaporation Evaporation occurs at the surface of a liquid and can happen at nearly any temperature. To evaporate, particles must be at the liquid's surface and have enough kinetic energy to escape the attractive forces of the liquid. Boiling Shown in Figure 5, boiling is the second way that a liquid can vaporize. Unlike evaporation, boiling occurs throughout a liquid at a specific temperature, depending on the pressure on the surface of the liquid. The boiling point of a liquid is the temperature at which the pressure of the vapor in the liquid is equal to the external pressure acting on the surface of the liquid. This external pressure pushes down on the liquid, keeping particles from escaping. Particles require energy to overcome this pressure. The heat of vaporization is the amount of energy required for the liquid at its boiling point to become a gas. Sublimation At certain pressures, some substances can change directly from solids into gases without going through the liquid phase. Sublimation is the process of a solid changing directly to a gas without forming a liquid. Figure 6 shows frozen carbon dioxide, also known as dry ice, which is a common substance that undergoes sublimation

Hydrogen

If you could count all the atoms in the universe, you would find that about 90 percent of them are hydrogen atoms. Most hydrogen on Earth is found in the compound water. In fact, the word hydrogen comes from the Greek word hydro, which means "water." When water is broken down into its elements, hydrogen becomes a gas composed of diatomic molecules. A diatomic molecule consists of two atoms of the same element in a covalent bond. Reading Check Describe what a diatomic molecule is. Hydrogen is highly reactive. A hydrogen atom has a single electron, which the atom shares when it combines with other nonmetals. For example, hydrogen burns in oxygen to form water (H2O), in which hydrogen shares electrons with oxygen. Hydrogen can gain an electron when it combines with alkali and alkaline earth metals. The compounds formed are hydrides, such as sodium hydride (NaH).

Detecting Chemical Change

If you leave a pan of chili cooking unattended on the stove for too long, your nose soon tells you that something is wrong. Instead of a spicy aroma, you detect an unpleasant smell that alerts you that something is burning. This burnt odor is a clue that a new substance has formed. The identity changes The smell of rotten eggs and the formation of rust on bikes and car fenders are also signs that a chemical change has taken place. A change of one substance to another is a chemical change. Bubble formation produced by the foaming of an antacid tablet in a glass of water is a sign of new substances being produced. In some chemical changes, a rapid release of energy—detected as heat, light, and sound—is a clue that changes are occurring. A display of fireworks in the night sky is an example. Figure 17 illustrates another visual clue—the formation of a solid precipitate. What is another example of a chemical change forming a solid? Reading Check Define What is a chemical change? Clues such as heat, cooling, or the formation of bubbles or solids in a liquid are indicators that a reaction is taking place. However, the only sure proof is that a new substance is produced. Consider the following examples. The heat, light, and sound produced when hydrogen gas combines with oxygen in a rocket engine are clear evidence that a chemical reaction has taken place. But no clues announce the onset of the reaction that combines iron with oxygen to form rust. The only clue that iron has changed into a new substance is the visible presence of rust. Burning and rusting are chemical changes because new substances form Using chemical changes One case where you might separate substances using a chemical change is in cleaning tarnished silver, such as jewelry. Tarnishing, a chemical reaction between silver metal and sulfur compounds in the air, results in silver sulfide. A chemical reaction in a warm water bath with baking soda and aluminum can change silver sulfide back into silver. Separating substances using chemical changes is rarely done in the home, but it is commonly done in industrial and laboratory settings. For example, many metals are separated from their ores and then purified using chemical changes.

Charles Law- Temperature and Volume

If you've watched a hot-air balloon inflated, you know that gases expand when they are heated. Jacques Charles (1746-1823), a French Scientist, also noticed this. According to his law, the volume of a gas increases with increasing temperature as long as the pressure on the gas does not change. Ad with Boyle's Law the reverse is also true. The volume of a gas shrinks with decreasing temperature. Can be explained with the Kinetic Theory of Matter. As a gas is heated, the particles that make up the gas move faster and faster. Because the particles that make up the gas move faster they strike the walls of their container more often and with more force. In the hot-air balloon the walls have room to expand. So instead of increased pressure, there is increased volume. Like Boyle's law, Charles's law can be expressed mathematically. When the pressure on a gas is constant, then the ratio of the volume to the absolute temperature does not change. The absolute temperature is the temperature measured in kelvins. Equation: Initial Volume/Initial Temperature (K) = Final Volume/Final Temperature (K) Vi/Ti = Vf/Tf This shows that the ratio of the initial volume to the initial temperature is equal to the ratio of the final volume to the final temperature. Remember that temperature must be in kelvins.

Buckyballs

In the mid-1980s, a new allotrope of carbon called buckminsterfullerene was discovered. This soccer-ball-shaped molecule is shown in Figure 17 and is informally called a buckyball. It was named after R. Buckminster Fuller, an architect-engineer who designed buildings with similar shapes. In 1991, scientists were able to use buckyballs to synthesize extremely thin, graphite-like tubes, like those in Figure 17. These tubes, called nanotubes, are about one-billionth of a meter (1 nanometer) in diameter. You could stack tens of thousands of nanotubes to get the thickness of one piece of paper. Nanotubes might be used someday to make stronger building materials and to make computers that are smaller and faster. Silicon and germanium Silicon, a metalloid, is second only to oxygen in abundance in Earth's crust. Silicon is found in sand (Si O 2 ) and in almost all rocks and soil. The crystal structure of silicon dioxide is similar to the structure of diamond. Silicon occurs as two allotropes. One allotrope of silicon is a hard, gray substance, and the other is a brown powder. Silicon is the main component in semiconductors. Semiconductors are elements that conduct an electric current under certain conditions. Many of the electronics that you use every day, such as computers, need semiconductors to run. The diode in Figure 18 contains silicon. Germanium, the other metalloid in group 14, is also used to make semiconductors. Tin and lead Tin, a metal, is used to coat other metals, such as steel cans used to store food, to prevent corrosion. Tin also is combined with other metals to make bronze and pewter. The metal lead was once widely used in paint; however, it is no longer is used, due to its toxicity. Lead is also used in car batteries

Liquid crystals

Liquid crystals form another group of materials that do not change states in the usual manner. Normally, the ordered geometric arrangement of a solid is lost when the substance goes from the solid state to the liquid state. Liquid crystals start to flow during the melting phase, similar to a liquid. But they do not lose their ordered arrangement completely, as most substances do. Liquid crystals will retain their geometric order in specific directions. Liquid crystals are placed in classes, depending upon the type of order they maintain when they liquify. They are highly responsive to temperature changes and electric fields. Scientists use the unique properties of liquid crystals to make liquid crystal displays (LCD) for cell phones, calculators, and netbooks, shown in Figure 12. LCD screens are composed of individual crystal picture elements, or "pixels" for short. Varying the amount of electricity that passes through the pixel determines how the crystals are aligned and whether light is able to pass through them

The Transition Elements

Many transition elements, such as iron and gold, are familiar because they are less reactive than the metals in groups 1 and 2 and often occur in nature as uncombined elements. Transition elements are the elements in groups 3 through 12 in the periodic table. They are called transition elements because they are considered to be in transition between the main group elements. The main group elements are groups 1 and 2 and groups 13 through 18. Main group elements are sometimes called the representative elements. A glowing lightbulb filament is made from the transition element tungsten. Titanium, another transition element, is used in bike frames, ships, and golf clubs because of its lightness and strength. Chromium is used in making steel and in chrome plating. Platinum is used to make jewelry because it is rare, resistant to corrosion, and more valuable than gold. Transition elements often form colored compounds, as shown in Figure 6. The gems' colors come from chromium compounds. The blue glass gets its color from cobalt. Cadmium yellow and cobalt blue paints are made from compounds of transition elements. However, their uses are limited because they are so toxic. Iron, cobalt, and nickel Iron, cobalt, and nickel form a unique cluster of transition elements sometimes called the iron triad. They are the most common magnetic elements and are used in steel and other metal mixtures. Iron is the second most abundant metal in Earth's crust and the most widely used of all metals. It is the main component of steel. Other metals, such as nickel and cobalt, are added to steel to give it various characteristics. Nickel is also used to give a shiny, protective coating to other metals.

Mendeleev's predictions

Mendeleev had to leave blank spaces in his periodic table. He studied the physical and chemical properties and the atomic masses of the elements surrounding all the blank spaces. From this information, he was able to predict the probable properties and the atomic masses for the missing elements that had not yet been discovered. Table 5 shows a few of Mendeleev's predicted physical and chemical properties for germanium, which he called ekasilicon. His predictions proved to be accurate when compared to the actual properties of germanium. Scientists later confirmed the identities of missing elements and found that their properties were similar to what Mendeleev had suggested. Changes in the periodic table: Although Mendeleev's arrangement of the elements was a success, it required some changes. The atomic mass gradually increased from left to right on Mendeleev's table. If you look at the modern periodic table, in Figure 9, on the next page, you can locate instances where atomic mass decreases from left to right, such as nickel and cobalt. You might also observe that the atomic number always increases from left to right. In 1913, a young English scientist named Henry G.J. Moseley arranged all the known elements based on increasing atomic number instead of atomic mass. This new arrangement seemed to solve the problem of fluctuating mass. The modern periodic table uses Moseley's arrangement of the elements.

Comparing mixtures and substances

Mixtures, unlike compounds, do not always contain the same proportions of the substances of which they are made. Additionally, mixtures can be physically separated unlike pure substances. Recall that a substance is matter that has a fixed composition. In contrast, mixtures can have widely different compositions. For example, you could dissolve a small amount of salt in a tank of water, or a large amount of salt in the same tank. Mixtures are not substances, but they are composed of two or more substances. The differences between mixtures and substances are summarized in Figure 11.

Properties of Nonmetals

Most of your body's mass is made of oxygen, carbon, hydrogen, and nitrogen, as shown in Figure 10. Calcium, phosphorus, sulfur, and chlorine are among the other elements found in your body. Except for the metal calcium, these elements are nonmetals. Nonmetals are elements that are usually gases or solids at room temperature. Solid nonmetals are not malleable or ductile but are brittle or powdery. Nonmetals are poor conductors of heat and electricity because the electrons in nonmetals are not free to move as they do in metals. Nonmetals and the periodic table In the periodic table, all nonmetals except hydrogen are found to the right of the stair-step line. On the table in back of your book, the nonmetal element blocks are colored yellow. The noble gases, group 18, make up the only group of elements that are all nonmetals. Group 17 elements are called the halogens. The halogens, except astatine, are also nonmetals. The other nonmetals are found in groups 13 through 16.

Organizing the Elements

On a clear night, you can see one of the various phases of the Moon. Each month, the Moon appears to grow larger and then smaller in a predictable pattern. This type of change is periodic. Periodic means "repeated in a pattern." For example, a calendar is a periodic table of the days and months of the year. The days of the week are also periodic because they repeat themselves every seven days. In the late 1800s, a Russian chemist named Dmitri Mendeleev presented a way to organize all the known elements. While studying the physical and chemical properties of the elements, Mendeleev found that these properties repeated in predictablepatterns based on an element's atomic mass. Because the pattern repeated, it was considered to be periodic. Figure 8 shows one of Mendeleev's early periodic charts. Mendeleev arranged elements in rows based on increasing atomic mass and in columns based on elements that shared similar physical and chemical properties. Today, this arrangement is called the periodic table of elements. In the modern periodic table, the elements are arranged by increasing atomic number—not atomic mass—and by periodic changes in physical and chemical properties.

Water's strange behavior

Ordinarily, substances contract as their temperatures decrease. However, an exception to this rule is water. Over a small range of temperatures, water expands as the temperature decreases. At first, it behaves like other substances. As the temperature begins to drop, the particles that make up the water move closer together. This continues until the water reaches 4°C. Water molecules are unusual in that they have highly positive and highly negative areas. These charged regions affect the behavior of water. As the temperature of water continues to drop under 4°C, the molecules line up so that only positive and negative areas are near each other, as shown in Figure 10. As a result, empty spaces occur in the structure. Water expands as it cools from about 4°C to 0°C and becomes less dense than liquid water. That is why ice floats in liquid water.

Solid or Liquid?

Other substances also show unusual behaviors when changing states. Amorphous solids and liquid crystals are two classes of materials that do not react as you would expect when they are changing states. Amorphous solids Ice melts at 0°C, and lead melts at 327°C. But not all solids have a specific temperature at which they melt. Consider a stick of butter. Instead of having a specific melting point, butter softens and melts over a range of temperatures. Some solids are like butter. Instead of having a specific melting point, they soften and gradually turn into a liquid over a temperature range. These solids lack a crystalline structure and are called amorphous solids. One common amorphous solid is glass, shown in Figure 11.

Physical Change

Physical properties can change while composition remains fixed. If you tear a piece of chewing gum, you change some of its physical properties—its size and shape. However, you have not changed the identity of the materials that make up the gum. The identity remains the same When a substance, such as water, freezes, boils, evaporates, or condenses, it undergoes a physical change. A change in size, shape, or state of matter in which the identity of the substance remains the same is called a physical change. These changes might involve energy changes, but the kind of substance—the identity of the element or compound—does not change. Because all substances have distinct properties such as density, specific heat, and melting and boiling points, these properties can often be used to help identify a substance when a particular mixture contains more than one unknown material. A substance can change states if it absorbs or releases enough energy. Iron, for example, will melt at high temperatures. Yet, whether in solid or liquid state, iron has physical properties that identify it as iron. Color changes are physical changes, too. For example, when iron is first heated, it glows red. Then, if it is heated to a higher temperature, it turns white, as shown in Figure 14. Reading Check Infer Does a change in state mean that a new substance has formed? Explain. Using physical changes A cool drink of water is something most people take for granted; but in some parts of the world, drinkable water is scarce. Not enough drinkable water can be obtained from wells. Many such areas that lie close to the sea obtain drinking water by using physical properties of water to separate it from the salt. One method, which uses the property of boiling point, is a type of distillation.

Atomic number

Recall that an element is made of one type of atom. What determines the type of atom? In fact, the number of protons identifies the type of atom. For example, every carbon atom has six protons. Also, any atom with six protons is a carbon atom. Atoms of different elements have different numbers of protons. For example, atoms with eight protons are oxygen atoms. The number of protons in an atom's nucleus is equal to its atomic number. The atomic number of carbon is six. Oxygen has an atomic number equal to eight, as shown in Table 4. Therefore, if you are given any one of the following—the name of the element, the number of protons for the element, or the atomic number of the element—you can identify the other two. For example, if your teacher asked you to identify an atom with an atomic number of 11, you would know that the atom has eleven protons and it is sodium, as indicated in Table 4. Reading Check Interpret Table 4 to identify the element and the atomic number of the element with 29 protons. Mass Number The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus of an atom. Mass number = number of protons + number of neutrons For example, you can calculate the mass number of the copper atom listed in Table 4: 29 protons plus 34 neutrons equals a mass number of 63. Also, if you know the mass number and the atomic number of an atom, you can calculate the number of neutrons in the nucleus. The number of neutrons is equal to the mass number minus the atomic number. In fact, if you know two of the three numbers—mass number, atomic number, number of neutrons—you can always calculate the third.

Mixture

Salad dressings, such as the examples shown in Figure 5, are mixtures. A mixture is matter composed of two or more substances that can be separated by physical means. Heterogeneous mixtures In the salad dressing, all of the items in the dressing are in contact, but they do not react with one another. If the dressing is allowed to sit undisturbed long enough, the oil and vinegar will separate. Because the different components remain distinct, this salad dressing is considered an example of a heterogeneous mixture. A mixture in which different materials remain distinct is called a heterogeneous (he tuh ruh JEE nee us) mixture. Like the dressing, salad is a heterogeneous mixture. The vegetables in a salad remain distinct. You can remove the vegetables if you do not care to eat them. Some components of heterogeneous mixtures are easy to see, like the components of the salad, but others are not. For example, the shirt shown in Figure 6 is also a heterogeneous mixture, but you cannot see its individual components. However, with the help of a microscope, you can see the distinct cotton and polyester threads.

Transuranium elements

Since 1940, scientists have been working to find elements with more and more protons. Except for technetium (43) and promethium (61), each synthetic element has more than 92 protons. Elements that have more than 92 protons, the atomic number of uranium, are called transuranium elements. Figure 23 shows a small sample of the transuranium element americium. These elements do not belong exclusively to the metal, nonmetal, or metalloid group. These elements are found toward the bottom of the periodic table. Some are in the actinide series, and some are on the bottom row of the main periodic table. All the transuranium elements are synthetic and radioactive, and many of them disintegrate quickly. Seeking stability If you made something that fell apart less than a second after you made it, you might think you were not successful. However, nuclear scientists do just that when they synthesize atoms. Many synthetic elements last for only fractions of seconds after they are constructed and can be made in only small amounts. Making new elements is beneficial, however. By studying how the synthesized elements form and disintegrate, scientists can gain an understanding of the forces holding the nucleus together. In the 1960s, scientists theorized that more stable synthetic elements could exist. Finding a large, stable synthetic element might help scientists understand how the forces inside the atom work.

Plasma State

So far, you have learned about the three familiar states of matter—solids, liquids, and gases. However, there is a state of matter beyond the gas state. Plasma is matter that has enough energy to overcome not just the attractive forces between its particles but also the attractive forces within its atoms. The atoms that make up a plasma collide with such force that the electrons are completely stripped off the atoms. You may be surprised to learn that most of the ordinary matter in the universe is in the plasma state. Every star that you can see in the sky, including the Sun, is composed of matter in the plasma state. Most of the matter between the stars and galaxies is also in the plasma state. The familiar states of matter—solid, liquid, and gas—are extremely rare in the universe.

Homogeneous Mixture

Soft drinks contain water, sugar, flavoring, coloring, and carbon dioxide gas. Figure 10 will help you visualize some of these particles in a liquid soft drink. A soft drink in a sealed bottle is an example of a homogeneous mixture. A homogeneous (hoh muh JEE nee us) mixture is a mixture that remains constantly and uniformly mixed and has particles that are so small that they cannot be seen with a microscope. Due to the interactions between particles, particles in a homogeneous mixture will never settle to the bottom of their container. A solution is the same thing as a homogeneous mixture. The most familiar solutions might be solids dissolved in liquids, but solutions can also be mixtures of a solid and a gas, a solid and a solid, a gas and a liquid, and so on. Tea, vinegar, steel alloys, and the compressed gas used by divers are all examples of solutions

Archimedes' Principle and Buoyancy

Some ships are like floating cities. For example, aircraft carriers are large enough to allow airplanes to take off and land on their decks. Despite their weights, these ships float. There is a force pushing up on the ship that opposes the gravitational force, pulling the ship down. What is the force pushing up on the ship? It is called the buoyant force. If the buoyant force is equal to the object's weight, the object will float. If the buoyant force is less than the object's weight, the object will sink. Buoyancy is the ability of a fluid—a liquid or a gas—to exert an upward force on an object immersed in it. In third century B.C. a Greek Mathematician named Archimedes made a discovery about buoyancy. Found that the buoyant force on an object is equal to the weight of the fluid displaced by the object. For example if you place a block of wood on water, it will push the water out of the way as it begins to sink, but only until the weight of the water displaced equals the blocks' weight When the weight of water displaced—the buoyant force—becomes equal to the weight of the block, it floats. If the weight of the water displaced is less than that of the block, the object sinks. Figure 13 shows the forces that affect objects in fluids Comparing buoyancy and weight Look again at the wood and steel blocks in Figure 13. They both displace the same volume and weight of water when submerged. Therefore, the buoyant forces on the blocks are equal. Yet, the steel block sinks and the wood block floats. What is different? The steel block weighs much more than the wood block. The gravitational force on the steel block is enough to make the steel block sink. The gravitational force on the wood block is not enough to make the wood block sink. Density and buoyancy One way to know whether an object will float or sink is to compare its density to the density of the fluid in which it is placed. An object floats if its density is less than that of the fluid. Remember that density is mass per unit volume. The density of the steel block is greater than the density of water. The wood block's density is less than that of water. Suppose you formed the steel block into the shape of the hull of a ship filled with air, as in Figure 14. Now the same mass takes up a larger volume. The overall density of the steel boat and air is less than the density of water. The boat will now float.

Suspension and Colloids

Suspensions A suspension is a heterogeneous mixture made of a liquid and solid particles that settle. Recall the oil and vinegar salad dressing in Figure 5. The dressing also has seasoning particles. The seasoning particles in the liquid will settle to the bottom of the container if allowed to sit undisturbed. River deltas are large scale examples of how particles in a suspension settle. Rivers flow swiftly through narrow channels, picking up soil and sediment along the way. As the river widens, it flows more slowly. Suspended particles settle and form deltas at the mouth of the river, as shown in Figure 7. Colloids Milk is an example of another kind of heterogeneous mixture called a colloid. It contains water, fats, and proteins in varying proportions. Unlike a suspension, however, its components will not settle if left standing. A colloid (KAH loyd) is a heterogeneous mixture with particles that never settle. Paint is a liquid colloid with suspended particles. Gases and solids can contain colloidal particles, too. Fog, like that shown in Figure 8, consists of particles of liquid water suspended in air. Smoke contains solids suspended in air. Identifying colloids One way to identify a colloid is by its appearance. Fog appears white because its particles are large enough to scatter light, as shown in Figure 8. Sometimes, it is not so obvious that a liquid is a colloid. These colloids can look very much like solutions, which are also mixtures where the particles cannot be seen. You can identify whether a liquid is a colloid by passing a beam of light through it. A light beam is invisible as it passes through a solution but can be seen as it passes through a colloid, as shown in Figure 9. This occurs because the particles in the colloid are large enough to scatter light, but those in the solution are not. The scattering of a light beam as it passes through a colloid is called the Tyndall effect.

The atomic model

You now know that all matter is composed of atoms, but this was not always accepted as truth. Around 400 b.c., the Greek philosopher Democritus proposed the idea that atoms are tiny particles that make up all matter. Another philosopher, Aristotle, disputed Democritus's idea and proposed that matter was uniform throughout and was not composed of such small particles. Aristotle's incorrect idea was accepted for about 2,000 years. In the 1800s, the English scientist John Dalton was able to present evidence to suggest that atoms exist. Dalton's atomic theory, highlighted in Table 2, led to his model of the atom. This model has changed somewhat over time with further investigations by other scientists, as shown on the next page in Figure 4. Dalton's modernization of Democritus' idea of the atom provided a physical explanation for chemical reactions. Due to this discovery, scientists could finally express these reactions in quantitative terms using chemical symbols and equations

The Alkali Metals

The Alkali Metals The elements in group 1 of the periodic table are the alkali (AL kuh li) metals. Like other metals, alkali metals are shiny, malleable, ductile, and good conductors of heat and electricity. However, they are softer than most other metals and are the most reactive metals. They react rapidly and sometimes violently with oxygen and water, as shown in Figure 4. Because they are so reactive, alkali metals do not occur naturally in their elemental forms, and pure samples must be stored in oil to prevent reaction with oxygen and water in the air. Atomic structure explains the reactive nature of alkali metals. Each atom of an alkali metal has one electron in its outer energy level. This electron is easily given up when an alkali metal combines with a nonmetal. As a result, the alkali metal atom becomes a positively charged ion in a compound such as sodium chloride (NaCl) or potassium bromide (KBr). Lithium, sodium, and potassium Look carefully at the nutritional information on a cereal box. You will notice that sodium and potassium are often listed. You and other living things need potassium and sodium compounds to stay healthy. Lithium can also benefit health. Lithium compounds are sometimes used to treat bipolar disorder. The lithium helps regulate chemical levels that are important to mental health. Rubidium, cesium, and francium The operation of some light-detecting sensors depends upon rubidium or cesium compounds. Cesium is used in atomic clocks because some of its isotopes are radioactive. A radioactive element is one in which the nucleus breaks down and gives off particles and energy. Francium is also radioactive and is extremely rare. Scientists estimate that Earth's crust contains less than 30 g of francium at one time.

The Alkaline Earth Metals

The alkaline earth metals make up group 2 of the periodic table. Like most metals, these metals are shiny, malleable, and ductile. Like the alkali metals, they combine readily with other elements and are not found as free elements in nature. Each atom of an alkaline earth metal has two electrons in its outer energy level. These electrons are given up when an alkaline earth metal combines with a nonmetal. The alkaline earth metal atom becomes the positively charged ion in a compound, such as calcium fluoride (CaF2). Some compounds of the alkaline earth metals are used to color fireworks, like those in Figure 5. Reading Check Compare and contrast the alkali metals and the alkaline earth metals. Magnesium Magnesium's lightness and strength make it a good material for cars, planes, spacecraft, household ladders, and baseball and softball bats. Most life on Earth depends upon chlorophyll, a magnesium-containing compound that enables plants to absorb light and make food. Calcium Calcium is seldom used as a free metal, but its compounds are useful and essential for life. Marble statues and some countertops are made of calcium carbonate (CaCO3). Calcium carbonate is the major component of the mineral limestone, which is found in many caves. You might take a vitamin with calcium. Calcium phosphate (CaPO4) in your bones helps make them strong. Other alkaline earth metals The compound barium sulfate (BaSO4) is used to diagnose some digestive disorders because it absorbs X-ray radiation well. First, the patient swallows a barium compound. Next, an X-ray is taken while the barium compound is going through the digestive tract. A doctor can then see where the barium is in the body and can use this information to diagnose internal abnormalities. Radium, the last element in group 2, is radioactive and is found associated with uranium. It was once used to treat cancers but is being replaced with more readily available radioactive elements.

Bonding in metals

The atoms of metals generally have one to three electrons in their outer energy levels. In chemical reactions, metals tend to give up electrons easily because they are not strongly held by the protons in the nucleus. Bonding with nonmetals When metals combine with nonmetals, the atoms of the metals tend to lose electrons to the atoms of nonmetals. The metal atoms become positive ions, and the nonmetal atoms become negative ions. Ions are charged particles with more or fewer electrons than the neutral atom. Both metals and nonmetals become more chemically stable when they form ions. A positively charged metal ion and a negatively charged nonmetal ion are attracted because of the electric force between them. They form a bond called an ionic bond. For example, a sodium atom can lose an electron to a chlorine atom. As shown in Figure 2, the sodium ion and chloride ion bond to form the compound sodium chloride (NaCl), also known as table salt. Metallic bonding A different type of bonding occurs between the atoms of metals. In metallic bonding, positively charged metallic ions are surrounded by a sea of electrons. Outer-level electrons are not held tightly to the nucleus of an atom but move freely among many positively charged ions, as shown in Figure 3. Metallic bonding explains many of the properties of metals. For example, when a metal is hammered into a sheet or drawn into a wire, it does not break because the ions are in layers that slide past one another without losing their attraction to the electron sea. Metals are also good conductors of heat and electricity because the outer-level electrons are weakly held and travel relatively freely

Discovering and Making Elements

The first elements known were those that occur naturally in their elemental forms, such as gold, lead, tin, and carbon. Gold, for example, has been known for at least 6,000 years. Most elements, however, were discovered after the birth of modern chemistry in the 1700s. Figure 21, on the previous page, shows the history of the discovery of many common elements. Synthetic elements By 1935, only four elements with fewer than 92 protons were missing from the periodic table. In 1939, Francium (87) was discovered. Elements with 43, 61, and 85 protons could not be found. Scientists believed that such elements could exist and began trying to create them in the lab. Elements created in the lab are called synthetic elements. By smashing existing elements with particles accelerated in a heavy ion accelerator, like the one in Figure 22, scientists have created elements that are not typically found on Earth. The first element created in the lab was technetium (43) in 1939. Astatine (85) was created in 1940. Later, it was found that small amounts of astatine occur in nature. Promethium (61), the last missing element, was synthesized in 1947. Why make elements? Bombarding uranium with neutrons can make neptunium, element 93. Half of the synthesized atoms of neptunium disintegrate in about two days. This might not sound useful, but when neptunium atoms disintegrate, they form plutonium. Plutonium has been produced in control rods of nuclear reactors and has been used in bombs. Plutonium also can be changed to americium, element 95, which is used in home smoke detectors. Synthetic elements are also useful because they are radioactive. For example, technetium's radioactivity makes it ideal for many medical applications.

Models

—Tools for Scientists Scientists and engineers use models to represent objects or ideas that are difficult to visualize or to picture in your mind. You might have seen models or blueprints of buildings, planetary models of the solar system, or even a model airplane. These are scaled-down models. Scaled-down models allow you to see something that is too large to visualize all at once or something that has not yet been built. Scaled-up models are often used to visualize things that are too small to see. Models of atoms are examples of scaled-up models. To give you an idea of how small the atom is, it would take about 50,000 aluminum atoms stacked one on top of the other to equal the thickness of a sheet of aluminum foil. To study the atom, scientists have developed scaled-up models that they can use to visualize an atom. For a model of the atom to be useful, it must support the accepted ideas about atomic structure and behavior. As new discoveries about atoms are made, scientists must include these new details in the model.

Weathering

The forces of nature continuously shape Earth's surface. Rocks split, deep canyons are carved, sand dunes shift, and limestone formations decorate caves. Do you think these changes, referred to as weathering, are physical or chemical? The answer is both. Geologists, who use the same criteria that you have learned in this chapter, say that some weathering changes are physical and some are chemical. Reading Check Determine Is weathering a physical change or a chemical change? Physical weathering Large rocks can split when water seeps into small cracks, freezes, and expands. Streams can smooth and sculpt hard rock, as shown at left in Figure 18. These are physical changes because the rock does not change into another substance . Chemical weathering In other cases, the change is chemical. For example, solid calcium carbonate, a compound found in limestone, reacts with water if it is slightly acidic, such as when it contains some dissolved carbon dioxide. The calcium carbonate reacts to form calcium bicarbonate. This change in limestone is a chemical change because the identity of the substances changes. This chemical change contributes to the weathering of the White Cliffs of Dover, shown in Figure 18, and also produces the icicle-shaped rock formations that are found in caves.

The Nitrogen Group

The nitrogen family makes up group 15. Each element has five electrons in its outer energy level. These elements tend to share electrons and form covalent compounds with other nonmetallic elements. Nitrogen and phosphorous Nitrogen is a nonmetal that is used to make ammonia (N H 3 ) and nitrates, which are compounds that contain the nitrate ion (N O 3 − ). Nitrates and ammonia are used in fertilizers. Nitrogen is the fourth most abundant element in your body. Each breath that you take is about 80 percent gaseous nitrogen in the form of diatomic molecules ( N 2 ). But animals and plants cannot use nitrogen in its diatomic form. The nitrogen must be combined into compounds, such as amino acids. Phosphorus is a nonmetal that has three allotropes. Phosphorous compounds can be used for many things from water softeners to fertilizers. Phosphorus compounds are also used in match heads and fine china, as shown in Figure 19. Arsenic, antimony, and bismuth Arsenic and antimony are metalloids, and bismuth is a metal. Antimony and bismuth are used with other metals to lower their melting points. Because of this property, bismuth is used in automatic fire-sprinkler heads, such as the one in Figure 20. The thermal energy from the fire melts the bismuth, which releases the plug and allows water to escape and put out the fire. Arsenic is used in some semiconductors. Arsenic was once used as a pigment in paints, but it is no longer used because many arsenic compounds are toxic.

The Noble Gases

The noble gases exist as isolated atoms. They are stable because their outermost energy levels are full. No naturally occurring noble gas compounds are known, but several compounds of argon, krypton, and xenon, primarily with fluorine, have been created in a laboratory. The stability of noble gases makes them useful. For example, helium is less dense than air but does not burn in oxygen. This makes it safer than hydrogen to use in blimps and balloons. An electric current will cause the noble gases to glow, as shown in Figure 14. For this reason, some noble gases, such as neon and argon, are used for brightly colored signs. The noble gases are also used in many lasers. One common type of laser is a helium-neon laser. Helium neon lasers produce beams of intense, red light

Atomic Mass

The nucleus contains almost all of the atom's mass because protons and neutrons are far more massive than electrons. The mass of a proton is roughly the same as that of a neutron— about 1.67 × 10-24 g, as shown in Table 3. The mass of each is more than 1,800 times greater than the mass of the electron. The electron's mass is so small that it can be ignored when evaluating the mass of an atom. If you were asked to estimate the height of your school building, you would probably not give an answer in kilometers. Considering the scale of the building, you would more likely give the height in meters. When thinking about the mass of an atom, scientists discovered that even grams were not small enough to use for measurement. Scientists needed a more manageable unit. The unit of measurement used to quantify an atom's mass is the atomic mass unit (amu). The mass of a proton or a neutron is almost equal to 1 amu. This is not coincidence—the unit was defined that way. The atomic mass unit is defined as one-twelfth of the mass of a carbon atom containing six protons and six neutrons, as shown in Figure 6. Remember that the mass of an atom is contained almost entirely in the mass of the protons and neutrons in the nucleus. Therefore, each of the 12 particles in the carbon nucleus must have a mass nearly equal to 1 amu.

Chlorine and bromine

The odor that you sometimes smell near a swimming pool is chlorine. Chlorine compounds, like the one shown in Figure 13, disinfect water. Chlorine, the most abundant halogen, is obtained from seawater at ocean-salt recovery sites. Household and industrial bleaches that are used to whiten flour, clothing, and paper also contain chlorine compounds. Bromine, the only nonmetal that is a liquid at room temperature, also is extracted from compounds in seawater. Some hot tubs use bromine compounds instead of chlorine compounds to disinfect water. Bromine compounds were once used in cosmetics and as flame retardants. But, due to health concerns, these compounds are being used less frequently. Another bromine compound is used to study genetic material, such as DNA. The bromine compound binds to the DNA in certain areas and acts as a sort of tag. Under fluorescent light, the bromine compound absorbs the fluorescent light and emits a reddish visible light, as shown in Figure 13. Reading Check Name some uses of chlorine and bromine compounds. Iodine and astatine Iodine, a shiny purple-gray solid at room temperature, is obtained from seawater. When heated, iodine sublimes to a purple vapor, as seen in Figure 13. Iodine is essential in your diet for the production of the hormone thyroxin and to prevent goiter, an enlarging of the thyroid gland in the neck. Iodine compounds are also used as disinfectants. Astatine is the last member of group 17. It is radioactive and rare but has many properties similar to those of the other halogens. Because it is so rare in nature, scientists usually make astatine for research purposes. Medical researchers are investigating the possibility of using astatine's radioactive properties to treat cancer

Regions of the Periodic Table

The periodic table has areas with specific names. Recall that the horizontal rows of elements are called periods. The elements increase by one proton and one electron as you move from left to right across a period. All the elements in the blue squares in Figure 15 are metals. Iron, zinc, and copper are examples of a few common metals. Most metals occur as solids at room temperature. They are shiny, can be drawn into wires, can be pounded into sheets, and are good conductors of heat and electricity. The elements on the right side of the periodic table that appear in the yellow squares are classified as nonmetals. Oxygen, bromine, and carbon are examples of nonmetals. Most nonmetals are gases at room temperature or brittle solids. They are poor conductors of heat and electricity. The elements in the green squares are metalloids. They exhibit properties of metals and nonmetals. Boron and silicon are examples of metalloids. New elements Scientists around the world continue their research into the synthesis of elements. In 1994, scientists at the Heavy Ion Research Laboratory in Darmstadt, Germany, discovered element 111. The International Union of Pure and Applied Chemistry (IUPAC) confirmed the discovery in 2003. The name Roentgenium (Rg) was officially approved in 2004. Element number 112 was discovered at the same laboratory. Synthesis of the element was reported in 1996. IUPAC confirmed the discovery in 2009, and the element was officially named Copernicium (Cn) in 2010. These elements are produced in the laboratory by joining smaller atoms into a single, larger atom. The search for elements with higher atomic numbers continues. Scientists think that they have synthesized elements 113, 114, 115, 116, and 118.

Distillation

The process of separating substances, such as salt and water, in a mixture by evaporating a liquid and recondensing its vapor is distillation. Distillation is done in the laboratory using an apparatus similar to the one shown in Figure 15. Two liquids with different boiling points can be separated in this way. The mixture is heated slowly until it begins to boil. The liquid with the lowest boiling point vaporizes first and is condensed and collected. Then, as temperature increases the second liquid boils, vaporizes, condenses, and is collected. Distillation is often used in industry. For instance, crude oil obtained from drilling is distilled to separate many different compounds in order to make products, such as the gasoline used to fuel automobiles.

Temperature

The term used to explain how hot or cold an object is. Represents the average kinetic energy of the particles that make up a substance. On average, molecules of water at Zero degrees Celsius have less kinetic energy than molecules of water at 100 degree Celsius

The Inner Transition Elements

The two rows of elements that seem to be disconnected from the rest of the periodic table are called the inner transition elements. They are called this because they fit within the transition metals on the periodic table. The inner transition elements are located between groups 3 and 4 in periods 6 and 7. To save room, they are usually listed below the table. Figure 8 shows what the periodic table would look like if the inner transition elements were not written below the table. The lanthanides The first row of the inner transition elements includes elements with atomic numbers of 58 to 71. These elements are called the lanthanide series because they follow the element lanthanum. Lanthanum, cerium, praseodymium, and samarium are used with carbon to make a compound that is used extensively for movie lighting. Compounds of europium, gadolinium, and terbium are used as colored phosphors. Recall that phosphors change ultraviolet light into visible light. Televisions once used these compounds to produce the colors that you see. Reading Check Explain why the first row of inner transition elements is called the lanthanide series. The actinides The second row of inner transition elements includes elements with atomic numbers 90 to 103. These elements are called the actinide series because they follow the element actinium. All of the actinides are radioactive and unstable. Because they are unstable, the actinides are rare or nonexistent in nature. Their instability also makes them difficult to research. Thorium and uranium are the only actinides found in the Earth's crust in usable quantities. Thorium is used in making the glass for high-quality camera lenses because it bends light without much distortion. Uranium is best known for its use in nuclear reactors and in weapons, but one of its compounds has been used as photographic toner.

Thermal Energy

Think about the ice in Figure 3. How can frozen, solid ice have motion? The particles that make up solids are held tightly in place by the attractions between the particles. Those attractions give solids a definite shape and volume. However, the particles that make up a solid are still in constant motion. The particles' thermal energy causes them to vibrate. Thermal energy is the total energy of a material's particles. This includes both the kinetic energy of the particles as well as their potential energy. Energy from the motions of individual particles and energy from forces that act within or between particles are both forms of thermal energy. Energy from the motion of an object as a whole and energy from its interactions with its surroundings are not thermal energy

The search for quarks

To study quarks, scientists accelerate charged particles to tremendous speeds and then force them to collide with—or smash into—protons. These collisions cause the protons to break apart. The Fermi National Accelerator Laboratory in Batavia, Illinois, houses a machine that can generate the forces that are required to create this type of collision. This particle accelerator, called the Tevatron, is shown in Figure 2. Electric and magnetic fields inside the Tevatron are used to accelerate, focus, and collide fast-moving particles. The Large Hadron Collider (LHC), also shown in Figure 2, is a particle accelerator in Geneva, Switzerland. The ultimate goal of the LHC is to discover new particles and to further explain the formation of the universe. Scientists use a variety of collection devices to obtain detailed information about the particles created in a collision. Just as police investigators can reconstruct traffic accidents from tire marks and other evidence at the scene, scientists are able to examine data collectors for evidence of the tiniest of particles. Scientists use inference to identify subatomic particles and to reveal information about each particle's structure. For example, the wire chambers in Figure 3 help scientists examine the varying tracks made by different types of particles formed in high-speed collisions. The sixth quark Finding evidence for the existence of quarks was not an easy task. Scientists discovered five quarks and hypothesized that a sixth quark existed. However, it took several years for a team of nearly 450 scientists from around the world to find the sixth quark. The tracks of the sixth quark were hard to detect because only about one-billionth of a percent of the proton collisions performed showed the presence of a sixth quark, typically referred to as the top quark.

Changes of State

What happens to a solid when thermal energy is added to it? Think about the iced water in Figure 4. The particles that make up the water are moving fast and colliding with the particles that make up the ice cube. Those collisions transfer energy from the water to the ice. The particles at the surface of the ice cube vibrate faster, transferring energy to other particles in the ice cube. Melting and freezing Soon, the particles that make up the ice have enough kinetic energy to overcome the attractive forces holding them in their crystalline structure. The ice melts. The melting point is the temperature at which a solid becomes a liquid. Energy is required for the particles to slip out of the ordered arrangement of a solid. The heat of fusion is the energy required to change a substance from solid to liquid at its melting point. The transfer of energy between particles of liquid and particles of solid causes the ice to melt, but what happens to the particles of liquid after they collide with the solid? They slow down because they have less kinetic energy. As more of these collisions occur, the average kinetic energy of the particles of the liquid decreases, and the liquid cools. Freezing is the reverse of melting. When a liquid's temperature is lowered, the average kinetic energy of the molecules decreases. When enough energy has been removed, the molecules become fixed into position. The freezing point is the temperature at which a liquid turns into a solid.

Elements in the Universe

With the development of new technologies, scientists have been able to study the chemistry of the universe. Because the universe is so vast, they have been able to study only a small section of the universe. However, scientists have learned that many of the same elements are found throughout the universe. These include lightweight elements, such as hydrogen and helium, and heavier elements, such as silicon, oxygen, and iron. Many scientists think that hydrogen and helium are the building blocks of all other elements. Atoms fuse together within stars to produce heavier elements with atomic numbers greater than the atomic numbers of hydrogen and helium. Exploding stars, called supernovas, like the one shown in Figure 16, provide evidence to support this theory. When stars explode, a mixture of elements, including heavy elements like iron, are expelled into the galaxy. Many scientists think that supernovas have scattered heavy, naturally occurring elements throughout the universe. Promethium, technetium, and elements with atomic numbers greater than 92 are rare or are not found on Earth. Some of these elements, such as neptunium and plutonium, are found only in trace amounts in Earth's crust as a result of uranium decay. Others have been found only in stars.

The Conservation of Mass

Wood burns, which means it undergoes combustion. Combustion is a chemical change. Suppose you burn a large log in a fireplace until nothing is left but a small pile of ashes. Smoke, heat, and light are given off, and the changes in the composition of the log confirm that a chemical change took place. At first, you might think that matter was lost as the log burned because the pile of ashes looks much smaller than the log looked. In fact, the mass of the ashes is less than that of the log. However, suppose that you could collect all of the oxygen in the air that was combined with the log during the burning and all of the smoke and gases that escaped from the burning log and measure their masses too. You would find that no mass was lost after all. Mass is not gained or lost during any chemical change. In fact, matter is neither created nor destroyed during a chemical change. According to the law of conservation of mass, the mass of all substances that are present before a chemical change, known as the reactants, equals the mass of all of the substances that remain after the change, which are called the products. The Law of Conservation of Mass total mass of the reactants = total mass of the products Figure 19 illustrates the law of conservation of mass. Solid sodium bicarbonate in the balloon reacts with liquid hydrochloric acid in the flask. The gas, carbon dioxide, is released, which expands the balloon. Without the balloon in place, the gas would escape and you might think that mass was not conserved. With the balloon to collect the gas, the mass on the scale remains the same. The mass of the reactants is the same as the mass of the products.

Physical Properties

You can stretch a rubber band, but you cannot stretch a piece of string much, if at all. You can bend a piece of wire, but you cannot easily bend a matchstick. The abilities to stretch and to bend substances are physical properties. The identity of the substances—rubber, string, wire, wood—does not change. Any characteristic of a material that you can observe without changing the identity of the substance is a physical property. Some examples of physical properties are color, shape, size, density, melting point, and boiling point. Appearance The appearance of substances, such as the ones shown in Figure 12, is a physical property. How would you describe a tennis ball? You could begin by describing its shape, color, and state of matter. You might describe the tennis ball as a brightly colored, hollow sphere. You can measure some physical properties—for example, the diameter of the ball. What physical property of the ball is measured with a balance? To describe a soft drink in a cup, you could start by calling it a liquid with a brown color. You could measure its volume and temperature. Each of these characteristics is a physical property of that soft drink. Behavior Some physical properties describe the behavior of a material or a substance. As you might know, objects that contain iron, such as a safety pin, are attracted by a magnet. Attraction to a magnet is a physical property of iron. Every substance has a specific combination of physical properties that make it useful for certain tasks. Some metals, such as copper, can be drawn out into wires. Others, such as gold, can be pounded into sheets as thin as 0.1 micrometers (μm), about four-millionths of an inch. This property of gold makes it useful for decorating picture frames and other objects. Gold that has been beaten or flattened in this way is called gold leaf. Think again about a soft drink. If you knock over the cup, the drink will spread over the table or floor. If you knock over a jar of molasses, however, it does not flow as easily. Viscosity, the resistance to flow, is a physical property of liquids. Using physical properties to separate mixtures Removing the seeds from a watermelon can be done easily based on the physical properties of the seeds compared to the rest of the fruit. Figure 13 shows a mixture of sesame seeds and sunflower seeds. You can identify the two kinds of seeds by differences in color, shape, and size. By sifting the mixture, you can quickly separate the sesame seeds from the sunflower seeds because their sizes differ. Now look at the mixture of iron filings and sand shown in Figure 13. You probably will not be able to sift out the iron filings because they are similar in size to the sand particles. What you can do is pass a magnet through the mixture. The magnet attracts only the iron filings and pulls them from the sand. This is an example of how a physical property, such as magnetic attraction, can be used to separate substances in a mixture. A similar method is used to separate iron from aluminum and other refuse for recycling. Strong magnets are used in scrap yards and landfills to remove iron for recycling and reuse in an effort to conserve natural resources.

Chemical Change and Property

You have probably seen warnings on cans of paint thinner and lighter fluid for charcoal grills that state these liquids are flammable (FLA muh buhl). The tendency of a substance to burn, called its flammability, is an example of a chemical property. Any characteristic of a material that you can observe that produces one or more new substances is a chemical property. Flammability is a chemical property because burning produces new substances. As a result, a chemical change, also called a chemical reaction, has occurred. Many other substances used around the home are flammable. Knowing which ones are flammable helps you to use them safely. A less dramatic chemical change can affect some medicines. Look at Figure 16. You have probably seen bottles like this in a pharmacy. Many medicines are stored in dark bottles because the medicines contain compounds that can chemically change if they are exposed to light.

Unit Four: Matter

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