CHEM 101 - Test 1: CHEM101 Ch. 1 and 2, CHEM 101 Ch. 20, CHEM101 Ch. 3
An atom is characterized by its atomic number Z, and its mass number, A. The atomic number, written as a subscript to the left of the element symbol, gives the number of protons in the nucleus. The mass number, written as a superscript to the left of the elemental symbol, gives the total number of particles in the nucleus, or nucleons, a general term for both protons (p) and neutrons (n). The most abundant isotope of carbon, for example, has 6 protons and 6 neutrons for a total of 12 nucleons.
Atoms with identical atomic numbers but different mass numbers are called isotopes (Section 2.8), and the nucleus of a given isotope is called a nuclide.
Scientists have known since 1896 that many nuclei are radioactive—they undergo a spontaneous decay and emit some form of radiation.
Early studies of radioactive isotopes, or radioisotopes, by Ernest Rutherford in 1897 showed that there are three common types of radiation with markedly different properties: alpha (α) beta (β) and gamma (γ) radiation, named after the first three letters of the Greek alphabet.
Group 8A—Noble gases
Helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) are colorless gases with very low chemical reactivity. Helium and neon don't combine with any other element; argon, krypton, and xenon combine with very few.
Naming Compounds with Polyatomic Ions
Ionic compounds that contain polyatomic ions are named in the same way as binary ionic compounds: First the cation is identified and then the anion.
Dalton
John Dalton (1766-1844) was exploring along similar lines. His work led him to propose what has come to be called the law of multiple proportions. The key to Dalton's proposition was his realization that the same elements sometimes combine in different ratios to give different chemical compounds. For example, oxygen and nitrogen can combine either in a 7:8 mass ratio to make the compound we know today as nitric oxide (NO) or in a 7:16 mass ratio to make the compound we know as nitrogen dioxide (NO2). The second compound contains exactly twice as much oxygen as the first.This result makes sense only if we assume that matter is composed of discrete atoms that have characteristic masses and combine with one another in specific and well-defined ways.
Priestley
Joseph Priestley (1733-1804) that the next great leap was made. Priestley prepared and isolated the gas oxygen in 1774 by heating the compound mercury oxide (HgO) according to the chemical equation we would now write as 2 HgO→2 Hg+O2.
Proust
Joseph Proust (1754-1826) to formulate a second fundamental chemical principle that we now call the law of definite proportions
Group 1A—Alkali metals
Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) are soft, silvery metals. All react rapidly, often violently, with water to form products that are highly alkaline, or basic—hence the name alkali metals. Because of their high reactivity, the alkali metals are never found in nature in the pure state but only in combination with other elements. Francium (Fr) is also an alkali metal, but, as noted previously, it is so rare that little is known about it.Note that group 1A also contains hydrogen (H) even though, as a colorless gas, it is completely different in appearance and behavior from the alkali metals. We'll see the reason for this classification in Section 5.13.
SI Unit - Mole
One mole of any element is the amount whose mass in grams, called its molar mass, is numerically equal to its atomic weight. One mole of carbon atoms has a mass of 12.011 g, and one mole of silver atoms has a mass of 107.868 g. Molar mass thus acts as a conversion factor that lets you convert between mass in grams and number of atoms. Whenever you have the same number of moles of different elements, you also have the same number of atoms. Experiments show that one mole of any element contains 6.022 141×1023 atoms, a value called Avogadro's number, abbreviated N>A.
Binary molecular compounds—those made of only two covalently bonded elements—are named in much the same way as binary ionic compounds.
One of the elements in the compound is more electron-poor, or cationlike, and the other element is more electron-rich, or anionlike. As with ionic compounds, the cationlike element takes the name of the element itself, and the anionlike element takes an -ide ending. The compound HF, for example, is called hydrogen fluoride.Because nonmetals often combine with one another in different proportions to form different compounds, numerical prefixes are usually included in the names of binary molecular compounds to specify the numbers of each kind of atom present. The compound CO, for example, is called carbon monoxide, and CO2 is called carbon dioxide. Mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca.
A Summary of Radioactive Decay Processes
Process, symbol, change in atomic number, change in mass number, change in neutron number Alpha emission, 4/2He or alpha, -2, -4, -2 Beta emission 0/-1e or beta-, 1, 0, -1 Gamma emission 0/0gamma or gamma, 0, 0, 0 Positron emission 0/1e or beta+, -1, 0, 1 Electron capture E.C., -1, 0, 1
Length
The meter (m) is the standard unit of length in the SI system. One meter is 39.37 inches, about 10% longer than an English yard and much too large for most measurements in chemistry. Other more commonly used measures of length are the centimeter (cm; 1 cm=0.01 m, a bit less than half an inch), the millimeter (mm; 1 mm=0.001 m, about the thickness of a U.S. dime), the micrometer (μm; 1 μm=10−6 m), the nanometer (nm; 1 nm=10−9 m), and the picometer (pm; 1 pm=10−12 m). Thus, a chemist might refer to the diameter of a sodium atom as 372 pm (3.72×10−10 m).
Rutherford's work involved the use of alpha (α) particles, a type of emission previously found to be given off by a number of naturally occurring radioactive elements, including radium, polonium, and radon. Rutherford knew that alpha particles are about 7000 times more massive than electrons and that they have a positive charge that is twice the magnitude of the charge on an electron but opposite in sign.
When Rutherford directed a beam of alpha particles at a thin gold foil, he found that almost all the particles passed through the foil undeflected. A very small number, however (about 1 of every 20,000), were deflected at an angle, and a few actually bounced back toward the particle source.Rutherford explained his results by proposing that a metal atom must be almost entirely empty space and have its mass concentrated in a tiny central core that he called the nucleus. If the nucleus contains the atom's positive charges and most of its mass, and if the electrons are a relatively large distance away, then it is clear why the observed scattering results are obtained: most alpha particles encounter empty space as they fly through the foil. Only when a positive alpha particle chances to come near a small but massive positive nucleus is it repelled strongly enough to make it bounce backward.
Many reactions are carried out using an excess amount of one reactant—more than is actually needed according to stoichiometry.
Whenever the ratios of reactant molecules used in an experiment are different from those given by the coefficients of the balanced equation, a surplus of one reactant is left over after the reaction is finished. Thus, the extent to which a chemical reaction takes place depends on the reactant that is present in limiting amount—the limiting reactant. The other reactant is said to be the excess reactant.
Four steps in balancing a chemical equation are:
Write an unbalanced equation using the correct chemical formula unit for each reactant and product. Find suitable coefficients—the numbers placed before formulas to indicate how many formula units of each substance are required to balance the equation. Report coefficients to their smallest whole-number values. Check your answer by making sure that the numbers and kinds of atoms are the same on both sides of the equation.
Further work by Rutherford in the late 1800s showed that beta (β) radiation
consists of a stream of particles that are attracted to a positive electrode (Figure 20.1), repelled by a negative electrode, and have a mass-to-charge ratio identifying them as electrons, 0/-1e or β−. Beta emission occurs when a neutron in the nucleus spontaneously decays into a proton plus an electron, which is then ejected. The product nucleus has the same mass number as the starting nucleus because a neutron has turned into a proton, but it has a higher atomic number because it has the newly created proton.
Volume
A cubic meter equals 264.2 U.S. gallons, much too large a quantity for normal use in chemistry. As a result, smaller, more convenient measures are commonly employed. Both the cubic decimeter (dm3) (1 dm3=0.001 m3), equal in size to the more familiar metric liter (L), and the cubic centimeter (cm3) (1 cm3=0.001 dm3=10−6 m3), equal in size to the metric milliliter (mL), are particularly convenient. Slightly larger than 1 U.S. quart, a liter has the volume of a cube 1 dm on edge. Similarly, a milliliter has the volume of a cube 1 cm on edge.
Balancing a chemical equation involves finding out how many formula units of each different substance take part in the reaction.
A formula unit, as its name implies, is one unit—whether atom, ion, or molecule—corresponding to a given formula.
Mixture v Compound
A mixture is simply a blend of two or more substances added together in some arbitrary proportion without chemically changing the individual substances themselves. A chemical compound, in contrast to a mixture, is a pure substance that is formed when atoms of different elements combine in a specific way to create a new material with properties completely unlike those of its constituent elements.
Nuclear reactions are distinguished from chemical reactions in several ways:
A nuclear reaction involves a change in an atom's nucleus, usually producing a different element. A chemical reaction, by contrast, involves only a change in the way that different atoms are combined. A chemical reaction never changes the nuclei themselves or produces a different element. Different isotopes of an element have essentially the same behavior in chemical reactions but often have completely different behavior in nuclear reactions. The energy change accompanying a nuclear reaction is far greater than that accompanying a chemical reaction. The nuclear transformation of 1.0 g of uranium-235 (U92235) releases more than one million times as much energy as the chemical combustion of 1.0 g of methane.
Group 7A—Halogens
Fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) are colorful, corrosive nonmetals. They are found in nature only in combination with other elements, such as with sodium in table salt (sodium chloride, NaCl). In fact, the group name halogen is taken from the Greek word hals, meaning "salt." Astatine (At) is also a halogen, but it exists in such tiny amounts that little is known about it.
The process involves calculating the amount of product that can be made if each reactant is completely used up. The limiting reactant can be determined by comparing the amount of product formed from each reactant. The limiting reactant will form the lowest amount of product, which is the theoretical yield. Just as in previous stoichiometry examples, amounts of reactants and products are related from the balanced equation and number of moles.
For the balanced equation aA + bB --> cC Grams of A Use molar mass of A as a conversion factor Moles of A Use coefficients in the balanced equation to find the A:C mole ratio Moles of C product Use molar mass of C as a conversion factor Grams of C product Grams of B Use molar mass of B as a conversion factor Moles of B Use coefficients in the balanced equation to find the B:C mole ratio Moles of C product Use molar mass of C as a conversion factor Grams of C product Compae - the reactant that produces the lowest amount of the product C is limiting. The mass of C found from the limiting reactant is the theoretical yield.
Modern measurements show that an atom has a diameter of roughly 10−10 m and that a nucleus has a diameter of about 10−15 m.
Further experiments by Rutherford and others between 1910 and 1930 showed that a nucleus is composed of two kinds of particles, called protons and neutrons. Protons have a mass of 1.672 622×10−24 g (about 1836 times that of an electron) and are positively charged. Because the charge on a proton is opposite in sign but equal in size to that on an electron, the numbers of protons and electrons in a neutral atom are equal. Neutrons (1.674 927×10−24 g) are almost identical in mass to protons but carry no charge, and the number of neutrons in a nucleus is not directly related to the numbers of protons and electrons. Table 2.4 compares the three fundamental subatomic particles, and Figure 2.9 gives an overall view of the atom.
Prefixes and Conversions
10^12 tera T 10^9 giga G 10^6 mega M 10^3 kilo k 10^2 hecto h 10 deka da 10^-1 deci d 10^-2 centi c 10^-3 milli m 10^-6 micro mu 10^-9 nano n 10^-12 pico p 10^-15 femto f
Properties of nanoscale particles are different depending on the size, which can be tuned to different applications. Ex. Fuel Cells
A catalyst is a substance that speeds up the rate of a chemical reaction. A fuel cell is a device that uses a fuel like H to produce electricity. The reactions in the fuel cell involve a transfer of electrons and are called redox reactions. They are zero emission, but they have a slow rate of water conversion. Platinum and palladium are some alternate solutions as a catalyst. Alloys are a mixture of metals sometimes used for this.
Precision v Accuracy
Accuracy refers to how close to the true value a given measurement is, whereas precision refers to how well a number of independent measurements agree with one another. To indicate the uncertainty in a measurement, the value you record should use all the digits you are sure of plus one additional digit that you estimate. The total number of digits recorded for a measurement is called the measurement's number of significant figures. All digits but the last are certain; the final digit is an estimate, which we generally assume to have an error of plus or minus one (±1).
Some Polyatomic Ions
Ammonium NH4+ Acetate CH3CO2- Cyanide CN- Hypochlorite ClO- Chlorite ClO2- Chlorate ClO3- Perchlorate ClO4- Dihydrogen Phosphate H2PO4- Hydrogen Carbonate/Bicarbonate HCO3- Hydrogen Sulfate/Bisulfate HSO4- Hydroxide OH- Permanganate MnO4- Nitrite NO2- Nitrate NO3- Carbonate CO3-2 Chromate CrO4-2 Dichromate Cr2O7-2 Peroxide O2-2 Hydrogen Phosphate HPO4-2 Sulfite SO3-2 Sulfate SO4-2 Phosphate PO4-3 note that several of the ions form a series of oxoanions
118 presently known elements (90 naturally)
An element is a fundamental substance that can't be chemically changed or broken down into anything simpler while still retaining the properties of that element. Furthermore, only 83 of the 90 or so naturally occurring elements are found in any appreciable abundance. Hydrogen is thought to account for approximately 75% of the observed mass in the universe; oxygen and silicon together account for 75% of the mass of the Earth's crust; and oxygen, carbon, hydrogen, and nitrogen make up more than 95% of the mass of the human body (Figure 2.1). By contrast, there is probably less than 20 grams of the element francium (Fr) dispersed over the entire Earth at any one time. Francium is an unstable radioactive element, atoms of which are continually being formed and destroyed.
Lavoisier
Antoine Lavoisier (1743-1794) showed that oxygen is the key substance involved in combustion. Furthermore, Lavoisier demonstrated with careful measurements that when combustion is carried out in a closed container, the mass of the combustion products exactly equals the mass of the starting reactants. When hydrogen gas burns and combines with oxygen to yield water (H2O), for instance, the mass of the water formed is equal to the mass of the hydrogen and oxygen consumed. Called the law of mass conservation, this principle is a cornerstone of chemical science.
Derived Quantities and Units
Area m^2 Volume m^3 Density kg/m^3 Speed m/s Acceleration m/s^2 Force (kg*m)/s^2 --N Pressure kg/(m*s^2) --Pa Energy (kg*m^2)/s^2 --J
Group 2A—Alkaline earth metals
Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) are also lustrous, silvery metals but are less reactive than their neighbors in group 1A. Like the alkali metals, the alkaline earths are never found in nature in the pure state.
Temperature
Celsius degree (°C) is slowly replacing the degree Fahrenheit (°F) as the common unit for temperature measurement. In scientific work, however, the kelvin (K) has replaced both. (Note that we say only "kelvin," not "degree kelvin.") For all practical purposes, the kelvin and the degree Celsius are the same—both are one-hundredth of the interval between the freezing point of water and the boiling point of water at standard atmospheric pressure. The only real difference between the two units is that the numbers assigned to various points on the scales differ. Whereas the Celsius scale assigns a value of 0 °C to the freezing point of water and 100 °C to the boiling point of water, the Kelvin scale assigns a value of 0 K to the coldest possible temperature, −273.15 °C, sometimes called absolute zero. Thus, 0 K=−273.15 °C and −273.15 K=0 °C. For example, a warm spring day with a Celsius temperature of 25 °C has a Kelvin temperature of 25+273.15=298 K. In contrast to the Kelvin and Celsius scales, the common Fahrenheit scale specifies an interval of 180° between the freezing point (32 °F) and the boiling point (212 °F) of water. Thus, it takes 180 degrees Fahrenheit to cover the same range as 100 degrees Celsius (or kelvins), and a degree Fahrenheit is therefore only 100/180=5/9 as large as a degree Celsius. Figure 1.6 compares the Fahrenheit, Celsius, and Kelvin scales. Temperature in K = Temperature in °C+273.15 Temperature in °C = Temperature in K−273.15 1 °C=(9/5)°F and 1 °F=(5/9)°C.
Density
Density is calculated as the mass of an object divided by its volume and is expressed in the SI derived unit g/mL for a liquid or g/cm3 for a solid. Ice .917 Water 1 Gold 19.31 Helium .000164 Air .001185 Human fat .94 Human muscle 1.06 Cork .22-.26 Balsa wood .12 Earth 5.54 g/cm^3 When reporting density, temperature must also be specified. Although most substances expand when heated and contract when cooled, water behaves differently. Water contracts when cooled from 100 °C to 3.98 °C, but below this temperature it begins to expand again. Thus, the density of liquid water is at its maximum of 1.0000 g/mL at 3.98 °C but decreases to 0.999 87 g/mL at 0 °C (Figure 1.9). When freezing occurs, the density drops still further to a value of 0.917 g/cm3 for ice at 0 °C. Ice and any other substance with a density less than that of water will float, but any substance with a density greater than that of water will sink.
Law of definite proportions
Different samples of a pure chemical compound always contain the same proportion of elements by mass. Every sample of water (H2O) contains 1 part hydrogen and 8 parts oxygen by mass, every sample of carbon dioxide (CO2) contains 3 parts carbon and 8 parts oxygen by mass, and so on. Elements combine in specific proportions, not in random proportions.
Dalton to propose a new theory of matter. He reasoned as follows:
Elements are made up of tiny particles called atoms. Although Dalton didn't know what atoms were like, he nevertheless felt they were necessary to explain why there were so many different elements. Each element is characterized by the mass of its atoms. Atoms of the same element have the same mass, but atoms of different elements have different masses. Dalton realized that there must be some feature that distinguishes the atoms of one element from those of another. Because Proust's law of definite proportions showed that elements always combine in specific mass ratios, Dalton reasoned that the distinguishing feature among atoms of different elements must be mass. The chemical combination of elements to make different chemical compounds occurs when whole numbers of atoms join in fixed proportions. Only if whole numbers of atoms combine will different samples of a pure chemical compound always contain the same proportion of elements by mass (the law of definite proportions and the law of multiple proportions). Fractional parts of atoms are never involved in chemical reactions. Chemical reactions only rearrange how atoms are combined in chemical compounds; the atoms themselves don't change. Dalton realized that atoms must be chemically indestructible for the law of mass conservation to be valid. If the same numbers and kinds of atoms are present in both reactants and products, then the masses of reactants and products must also be the same.
Law of multiple proportions
Elements can combine in different ways to form different chemical compounds whose mass ratios are simple whole-number multiples of each other.
Atomic Number and Mass Number
Elements differ from one another according to the number of protons in the nucleus, a value called the element's atomic number (Z). That is, all atoms of a given element contain the same number of protons in their nuclei. Elements differ from one another according to the number of protons in the nucleus, a value called the element's atomic number (Z).Atoms with identical atomic numbers but different mass numbers are called isotopes.The number of neutrons in an isotope is not given explicitly but can be calculated by subtracting the atomic number (subscript) from the mass number (superscript). The number of neutrons in an atom has relatively little effect on the atom's chemical properties. The chemical behavior of an element is determined almost entirely by the number of electrons it has, which in turn is determined by the number of protons in its nucleus. All three isotopes of hydrogen therefore behave similarly (although not identically) in their chemical reactions.
Energy
Energy is the capacity to supply heat or do work. Kinetic energy (EK) is the energy of motion. The amount of kinetic energy in a moving object with mass m and velocity υ is given by the equation EK=(1/2)mυ^2 Potential energy (Ep), by contrast, is stored energy. The units for energy, (kg⋅m^2)/s^2, follow from the expression for kinetic energy. - Joule is 4.184 for 1 cal (temperature of 1 g of water by 1 °C)
Scientific Notation
Exponential format called scientific notation utilizes powers of ten to illustrate large and small numbers.
Thomson was able to measure only the ratio of charge to mass, not charge or mass itself, and it was left to the American R. A. Millikan (1868-1953) to devise a method for measuring the mass of an electron.
In Millikan's experiment, a fine mist of oil was sprayed into a chamber, and the tiny droplets were allowed to fall between two horizontal plates. Observing the droplets through a telescopic eyepiece made it possible to determine how rapidly they fell through the air, which in turn allowed their masses to be calculated. The droplets were then given a negative charge by irradiating them with X rays. X rays knocked electrons from gas molecules in the surrounding air, and the electrons stuck to the oil droplet. By applying a voltage to the plates, with the upper plate positive, it was possible to counteract the downward fall of the charged droplets and keep them suspended. With the voltage on the plates and the mass of the droplets known, Millikan was able to show that the charge on a given droplet was always a small whole-number multiple of e, whose modern value is 1.602 177×10−19 C. Substituting the value of e into Thomson's charge-to-mass ratio then gives the mass m of the electron as 9.109 382×10^−28 g:
Positron Emission and Electron Capture
In addition to α,β, and γ radiation, two other types of radioactive decay processes also occur commonly: positron emission and electron capture. Positron emission occurs when a proton in the nucleus changes into a neutron plus an ejected positron (e+10 or β+), a particle that can be thought of as a positive electron. A positron has the same mass as an electron but an opposite charge. The result of positron emission is a decrease in the atomic number of the product nucleus but no change in the mass number. Potassium-40, for example, undergoes positron emission to yield argon-40, a nuclear reaction important in geology for dating rocks. Note once again that the sum of the two subscripts on the right of the nuclear equation (18+1=19) is equal to the subscript in the K1940 nucleus on the left. You might already know that the acronym PET used in medical imaging stands for positron emission tomography. PET can be used to detect tumors by injecting a chemical compound containing a positron-emitting isotope such as F18 into the body. The compound binds specifically to a protein on the surface of a cancer cell and accumulates in a tumor. When decay occurs, the emitted positron reacts with a nearby electron and is instantly annihilated, releasing gamma rays whose position in the body can be detected. Electron capture is a process in which the nucleus captures one of the surrounding electrons in an atom, thereby converting a proton into a neutron. The mass number of the product nucleus is unchanged, but the atomic number decreases by 1, just as in positron emission. The conversion of mercury-197 into gold-197.
Sig Figs in Calculations
In carrying out a multiplication or division, the answer can't have more significant figures than either of the original numbers. In carrying out an addition or subtraction, the answer can't have more digits to the right of the decimal point than either of the original numbers. If the first digit you remove is less than 5, round down by dropping it and all following digits. If the first digit you remove is 5 or greater, round up by adding 1 to the digit on the left.
Modern Table
In the modern periodic table, shown in Figure 2.3, elements are placed on a grid with seven horizontal rows, called periods, and 18 vertical columns, called groups. When organized in this way, the elements in a given group have similar chemical properties.There are actually 32 groups in the periodic table rather than 18, but to make the table fit manageably on a page, the 14 elements beginning with lanthanum (the lanthanides) and the 14 beginning with actinium (the actinides) are pulled out and shown below the others. These groups are not numbered. Groups 1, 2, and 13-18 are called the main groups. Most of the elements on which life is based—carbon, hydrogen, nitrogen, oxygen, and phosphorus, for instance—are main-group elements. Group 3-12 in the middle of the table are called the transition metal groups. Most of the metals you're probably familiar with—iron, copper, zinc, and gold, for instance—are transition metals. And the 14 groups shown separately at the bottom of the table are called the inner transition metal groups.
one mole (mol) of any element is the amount whose mass in grams, or molar mass, is numerically equal to the element's atomic weight
In the same way, one mole of any chemical compound is the amount whose mass in grams is numerically equal to the compound's molecular weight (or formula weight) and contains Avogadro's number (NA) of formula units (6.022×10^23).The mole is the fundamental SI unit for measuring the amount of matter. One mole of any substance—atom, ion, or molecule—is the amount whose mass in grams is numerically equal to the substance's atomic or formula weight. One mole contains Avogadro's number of formula units.
Standard Chemical Formats
In this standard format for writing chemical transformations, each compound is described by its chemical formula, which lists the symbols of its constituent elements and uses subscripts to indicate the number of atoms of each. If no subscript is given, the number 1 is understood. Thus, sodium chloride (table salt) is written as NaCl, water as H2O, and sucrose (table sugar) as C12H22O11. A chemical reaction is written in a standard format called a chemical equation, in which the reactant substances undergoing change are written on the left, the product substances being formed are written on the right, and an arrow is drawn between them to indicate the direction of the chemical transformation.
Mass
Mass is defined as the amount of matter in an object. Matter, in turn, is a catchall term used to describe anything with a physical presence. Mass is measured in SI units by the kilogram (kg; 1 kg=2.205 U.S. lb). Because the kilogram is too large for many purposes in chemistry, the metric gram (g; 1 g=0.001 kg), the milligram (mg; 1 mg=0.001 g=10−6 kg), and the microgram (μg; 1 μg=0.001 mg=10−6 g=10−9 kg) are more commonly used. (The symbol μ is the lowercase Greek letter mu.) One gram is a bit less than half the mass of a new U.S. dime. Mass is a physical property that measures the amount of matter in an object, whereas weight measures the force with which gravity pulls on an object.
Law of mass conservation
Mass is neither created nor destroyed in chemical reactions. Thus, the combined mass of the reactants is exactly equal to the combined mass of the products.
SI Units Standard
Mass kg Length m Temperature K Amount of substance mol Time s Electric current A Luminous intensity cd
You might note that in writing this and other equations, the designations (g) for gas, (l) for liquid, (s) for solid, and (aq) for aqueous solutions are often appended to the symbols of reactants and products to show their physical state. In the laboratory, you can't count the reactant molecules; you have to weigh them. That is, you must convert a number ratio of reactant molecules, as given by coefficients in the balanced equation, into a mass ratio to be sure that you are using the right amounts.
Mass ratios are determined by using the molecular weights of the substances involved in a reaction. Just as the atomic weight of an element is the average mass of the element's atoms (Section 2.9), the molecular weight of a substance is the average mass of the substance's molecules. Numerically, molecular weight (or, more generally, formula weight to include both ionic and molecular substances) equals the sum of the atomic weights of all atoms in the molecule. Molecular weight Sum of atomic weights of all atoms in a molecule. Formula weight Sum of atomic weights of all atoms in a formula unit of any compound, molecular or ionic. Remember . . . The atomic weight of an element is the weighted average mass of the element's naturally occurring isotopes. Although atomic weight is usually written as dimensionless, the unit of atomic weight is the unified atomic mass unit (u).
As indicated in Figure 2.3, the elements of the periodic table are often divided into three major categories: metals, nonmetals, and semimetals.
Metals Metals, the largest category of elements, are found on the left side of the periodic table, bounded on the right by a zigzag line running from boron (B) at the top to astatine (At) at the bottom. The metals are easy to characterize by their appearance. All except mercury are solid at room temperature, and most have the silvery shine we normally associate with metals. In addition, metals are generally malleable rather than brittle, can be twisted and drawn into wires without breaking, and are good conductors of heat and electricity. Nonmetals Except for hydrogen, nonmetals are found on the right side of the periodic table and, like metals, are easy to characterize by their appearance. Eleven of the seventeen nonmetals are gases, one is a liquid (bromine), and only five are solids at room temperature (carbon, phosphorus, sulfur, selenium, and iodine). None are silvery in appearance, and several are brightly colored. The solid nonmetals are brittle rather than malleable and are poor conductors of heat and electricity. Semimetals Seven of the nine elements adjacent to the zigzag boundary between metals and nonmetals—boron, silicon, germanium, arsenic, antimony, tellurium, and astatine—are called semimetals because their properties are intermediate between those of their metallic and nonmetallic neighbors. Although most are silvery in appearance and all are solid at room temperature, semimetals are brittle rather than malleable and tend to be poor conductors of heat and electricity. Silicon, for example, is a widely used semiconductor, a substance whose electrical conductivity is intermediate between that of a metal and an insulator.
Look carefully at the sequence of steps in the calculation just completed. Moles (numbers of molecules) are given by the coefficients in the balanced equation, but grams are used to weigh reactants in the laboratory.
Moles tell us how many molecules of each reactant are needed, whereas grams tell us how much mass of each reactant is needed. Given Grams of A Use molar mass as a conversion factor Moles of A Use coefficients in the balanced euqation to find mole ratios Moles of B Use molar mass as a conversion factor Find Grams of B
Empirical v. Molecular Formulas with Formulas
Multiplying the subscripts by small integers in a trial-and-error procedure until whole numbers are found then gives the empirical formula, which gives the smallest whole-number ratios of atoms in the compound. The subscripts may not always be exact integers because of small errors in the data, but the discrepancies should be small. An empirical formula determined from percent composition tells only the ratios of atoms in a compound. The molecular formula, which tells the actual numbers of atoms in a molecule, can be either the same as the empirical formula or a multiple of it. To determine the molecular formula, it's necessary to know the molecular weight of the substance. To find the multiple, divide the molecular weight by the empirical formula weight. Then multiply the subscripts in the empirical formula by this multiple to obtain the molecular formula. Just as we can find the empirical formula of a substance from its percent composition, we can also find the percent composition of a substance from its empirical (or molecular) formula. The strategies for the two kinds of calculations are exactly opposite. Assume we start with mole. Dividing the mass of each element by the total mass and multiplying by 100 percent gives percent composition. The answer can be checked by confirming that the sum of the mass percentages is within a rounding error of 100 percent.
Organizing the Elements
Russian chemist Dmitri Mendeleev created the forerunner of the modern periodic table. Mendeleev's theory about how known chemical information could be organized passed all tests. Not only did the periodic table arrange data in a useful and consistent way to explain known facts about chemical reactivity, it also led to several remarkable predictions that were later found to be accurate.Mendeleev arranged the known elements in order of the relative masses of their atoms (called their atomic weights, Section 2.9), with hydrogen=1, and then grouped them according to their chemical reactivity. On so doing, he realized that there were several "holes" in the table, some of which are shown in Figure 2.2. The chemical behavior of aluminum (relative mass≈27.3) is similar to that of boron (relative mass≈11), but there was no element known at the time that fit into the slot below aluminum. In the same way, silicon (relative mass≈28) is similar in many respects to carbon (relative mass≈12), but there was no element known that fit below silicon. Furthermore, he predicted with remarkable accuracy what the properties of these unknown elements would be.
Boyle
The Englishman Robert Boyle (1627-1691) is generally credited with being the first to study chemistry as a separate intellectual discipline and the first to carry out rigorous chemical experiments. Through a careful series of experiments into the nature and behavior of gases, Boyle provided clear evidence for the atomic makeup of matter. In addition, Boyle was the first to clearly define an element as a substance that cannot be chemically broken down further and to suggest that a substantial number of different elements might exist. Atoms of these different elements, in turn, can join together in different ways to yield a vast number of different substances we call chemical compounds.
Determining the formula of a new compound begins with analyzing the substance to discover what elements it contains and how much of each element is present—that is, to find its composition.
The percent composition of a compound is expressed by identifying the elements present and giving the mass percent of each. Knowing a compound's percent composition makes it possible to calculate the compound's chemical formula. As shown in Figure 3.3, the strategy is to find the relative number of moles of each element in the compound and then use those numbers to establish the mole ratios of the elements. The mole ratios, in turn, correspond to the subscripts in the chemical formula. Gives Mass Percents Use molar masses as conversion factors Moles Mole ratios Find Subscripts
Binary ionic compounds—those made of only two elements—are named by identifying first the positive ion and then the negative ion.
The positive ion takes the same name as the element, while the negative ion takes the first part of its name from the element and then adds the ending -ide. For example, KBr is named potassium bromide: potassium for the K+ ion and bromide for the negative Br− ion derived from the element bromine.Notice, for instance, that metals tend to form cations and nonmetals tend to form anions. Also note that elements within a given group of the periodic table form ions with the same charge and that the charge is related to the group number. Main-group metals usually form cations whose charge is equal to the group number. some metals form more than one kind of cation. Iron, for instance, forms both the doubly charged Fe2+ ion and the triply charged Fe3+ ion. In naming these ions, we distinguish between them by using a Roman numeral in parentheses to indicate the number of charges (ous v ic).
Nanoscience
The production and study of structures that have at least one dimension between one and one hundred nm, where one nanometer is one billionth of a meter.
Scientific Method
The scientific method is an iterative process involving the formulation of questions arising from observations, careful design of experiments, and thoughtful analysis of results. The scientific method involves identifying ways to test the validity of new ideas, and seldom is there only one way to go about it. Observation. Observations are a systematic recording of natural phenomena and may be qualitative, descriptive in nature, or quantitative, involving measurements. Hypothesis. A hypothesis is a possible explanation for the observation developed based upon facts collected from previous experiments as well as scientific knowledge and intuition. The hypothesis may not be correct, but it must be testable with an experiment. Experiment. An experiment is a procedure for testing the hypothesis. Experiments are most useful when they are performed in a controlled manner, meaning that only one variable is changed at a time while all others remain constant. Theory. A theory is developed from a hypothesis consistent with experimental data and is a unifying principle that explains experimental results. It also makes predictions about related systems, and new experiments are carried out to verify the theory. All theory is...All a theory can do is provide the best explanation that we can come up with at the present time. It can be modified.
Chemistry
The study of the composition, properties, and transformations of matter.
J. J. Thomson (1856-1940)
Thomson's experiments involved the use of cathode-ray tubes (CRTs), early predecessors of the tubes found in older televisions and computer displays. Experiments by a number of physicists in the 1890s had shown that cathode rays can be deflected by bringing either a magnet or an electrically charged plate near the tube (Figure 2.6b). Because the beam is produced at a negative electrode and is deflected toward a positive plate, Thomson proposed that cathode rays must consist of tiny, negatively charged particles, which we now call electrons. Furthermore, because electrons are emitted from electrodes made of many different metals, all these different metals must contain electrons. Thomson reasoned that the amount of deflection of the electron beam in a cathode-ray tube due to a nearby magnetic or electric field should depend on three factors: The strength of the deflecting magnetic or electric field. The stronger the magnet or the higher the voltage on the charged plate, the greater the deflection. The magnitude of the negative charge on the electron. The larger the charge on the particle, the greater its interaction with the magnetic or electric field and the greater the deflection. The mass of the electron. The lighter the particle, the greater its deflection (just as a Ping-Pong ball is more easily deflected than a bowling ball). By carefully measuring the amount of deflection caused by electric and magnetic fields of known strength, Thomson was able to calculate the ratio of the electron's electric charge to its mass: its charge-to-mass ratio, e/m. The modern value is e/m=1.758 820×10^8C/g where e is the magnitude of the charge on the electron in coulombs (C) and m is the mass of the electron in grams.
Bonds - Covalent and Ionic
Thus, it's the electrons that form the connections, or chemical bonds, that join atoms together in compounds. Chemical bonds between atoms are usually classified as either covalent or ionic-As a general rule, covalent bonds occur primarily between nonmetal atoms, while ionic bonds occur primarily between metal and nonmetal atoms. A covalent bond, the most common kind of chemical bond, results when two atoms share several (usually two) electrons. The unit of matter that results when two or more atoms are joined by covalent bonds is called a molecule.Ball-and-stick models specifically indicate the covalent bonds between atoms, while space-filling models portray overall molecular shape but don't explicitly show covalent bonds. Chemists normally represent a molecule by giving its structural formula. Some elements even exist as molecules rather than as individual atoms. An ionic bond is going to be a more compact, attraction as charges are exchanged.
More often, a large majority of molecules react as expected, but other processes, or side reactions, also occur.
Thus, the amount of product actually formed, called the yield of the reaction, is usually less than the amount predicted by calculations.The amount of product actually formed in a reaction divided by the amount theoretically possible and multiplied by 100% is the reaction's percent yield.
Atomic Weights
Thus, the mass in grams of a single atom is much too small a number for convenience, so chemists use a unit called the unified atomic mass unit (u) (also known as a dalton [Da] in biological work). One unified atomic mass unit is defined as exactly 1/12 the mass of an atom of C12 and is equal to 1.660 539×10−24 g. Electron mass is negligible, meaning that protons and neutrons both have a mass of almost exactly 1 u.Thus, the mass of a specific atom in unified atomic mass units—called the atom's atomic mass—is numerically close to the atom's mass number. An element's atomic weight is the weighted average of the isotopic masses of the element's naturally occurring isotopes. Can be calculated by mass times abundance, and summing those up. Atomic weights are extremely useful because they are conversion factors between numbers of atoms and masses; that is, they allow us to count a large number of atoms by weighing a sample of the substance.
Sig Figs
Zeros in the middle of a number are like any other digit; they are always significant. Thus, 4.803 cm has four significant figures. Zeros at the beginning of a number are not significant; they act only to locate the decimal point. Thus, 0.006 61 g has three significant figures. (Note that 0.006 61 g can be rewritten as 6.61×10−3 g or as 6.61 mg.) Zeros at the end of a number and after the decimal point are always significant. The assumption is that these zeros would not be shown unless they were significant. Thus, 55.220 K has five significant figures. (If the value were known to only four significant figures, we would write 55.22 K.) Zeros at the end of a number and before the decimal point may or may not be significant. We can't tell whether they are part of the measurement or whether they just locate the decimal point. Thus, 34,200 m may have three, four, or five significant figures. Often, however, a little common sense is helpful. A temperature reading of 20 °C probably has two significant figures rather than one, since one significant figure would imply a temperature anywhere from 10 °C to 30 °C and would be of little use. Similarly, a volume given as 300 mL probably has three significant figures. On the other hand, a figure of 93,000,000 mi for the distance between the Earth and the Sun probably has only two or three significant figures.
Rutherford found that alpha (α) radiation
consists of a stream of particles that are repelled by a positively charged electrode, attracted by a negatively charged electrode, and have a mass-to-charge ratio which identifies them as helium nuclei, 4/2He2+. Alpha particles thus consist of two protons and two neutrons. Because the emission of an α particle from a nucleus results in a loss of two protons and two neutrons, it reduces the mass number of the nucleus by 4 and reduces the atomic number by 2. Alpha emission is particularly common for heavy radioactive isotopes.
Gamma (γ) radiation
is unaffected by either electric or magnetic fields (Figure 20.1) and has no mass. Like visible light, ultraviolet rays, and X rays, γ radiation is simply electromagnetic radiation of very high energy. Gamma radiation almost always accompanies α and β emission as a mechanism for the release of energy, but it is often not shown when writing nuclear equations because it changes neither the mass number nor the atomic number of the product nucleus.
nuclear reactions differ from chemical reactions
one element is converted into another 1/1 H + 2/1 H --> 3/2 He In contrast, the identities of the atoms remain the same in a chemical reaction; only the bonds between atoms change.
Conversions and Dim. Analysis
quantity described in one unit is converted into an equivalent quantity with a different unit by multiplying with a conversion factor that expresses the relationship between units
Stoichiometry
refers to the chemical arithmetic needed to relate amounts of reactants and products in a chemical reaction. In any balanced chemical equation, the coefficients tell the number of formula units, and thus the number of moles, of each substance in the reaction. You can then use molar masses as conversion factors to calculate reactant and product masses.
Chemists use the same symbols
represent chemistry on both a small-scale, microscopic level and a large-scale, macroscopic level and tend to not distinguish between what is happening on each of the two levels, which can be very confusing to newcomers to the field. On the microscopic level, chemical symbols represent the behavior of individual atoms and molecules. Atoms and molecules are much too small to see, but we can nevertheless describe their microscopic behavior. On the macroscopic level, formulas and equations represent the large-scale behaviors of atoms and molecules that give rise to visible properties. A chemical formula or equation can be read either on the macroscopic level or on the microscopic level.
Now, look at the atoms on each side of the reaction arrow. Although we haven't explicitly stated it yet, chemical equations are always written so that they are balanced
that is, the numbers and kinds of atoms on both sides of the reaction arrow are the same. This requirement is a consequence of the law of mass conservation-mass is neither created nor destroyed
In a nuclear equation,
the element symbols represent only the nuclei of atoms rather than the entire neutral atoms, so the subscript represents only the number of nuclear charges (protons). An emitted electron is written as 0/-1e, where the superscript 0 indicates that the mass of an electron is essentially zero when compared to that of a proton or neutron, and the subscript indicates that the charge is −1. The equation is balanced because the total number of neutrons and protons, collectively called nucleons, or nuclear particles, is the same on both sides of the equation and the number of charges on the nuclei and on any elementary particles (protons and electrons) is the same on both sides. A nuclear equation is balanced when the sum of the mass numbers of reactants equals the sum of the mass numbers of the products. Likewise, the reaction is balanced when the sum of the atomic numbers of the reactants equals the sum of the atomic numbers of the products.
Nuclear chemistry
the study of properties and reactions of atomic nuclei, is a topic of high societal importance nuclear reactions - change atomic nucleus Some people consider nuclear energy a "clean" source of electricity because it does not emit air pollutants or the greenhouse gas carbon dioxide. However, drawbacks of nuclear power include the need for a long-term waste disposal plan and the potential for serious accidents such as the Chernobyl plant in Ukraine in 1986 and the Fukushima Daiichi plant in Japan in 2011. Although radiation from nuclear accidents is a public health concern, nuclear medicine uses radioactive isotopes to diagnose and treat conditions ranging from appendicitis to cancer.