7.01 What Do You Know About "The Atomic Models"? & 7.02 Models of the Atom & 7.03 Atomic Theory & 7.04 Atoms and Molecules

How and why might a model be changed, modified, or discarded?

Models may be modified as new data is applied. For example, Thomson's model was modified by Rutherford, who added his data to the knowledge of the atom. Rutherford's model of the atom was modified by the Bohr Model. Models that do not fit new insights may have to be modified or totally discarded. For example, Thomson's plum pudding model did not explain the experimental data of Rutherford. Models may be retained if they consistently explain or agree with new experimental data. For example, the concept of positive and negative charge has been retained throughout all models.

How have spectra influenced the choice of atomic models?

The presence of the spectral lines along with Planck's constant suggests energy levels. Bohr used this data in developing his model.

Tutorial

The law of conservation of matter states that matter cannot be created or destroyed during a chemical reaction. However, the basis of the atomic theory is the notion that all matter can be broken down into tiny, invisible particles called atoms. That is, suppose you took a piece of something, anything you can think of, and cut it in half and then in half again, and then into smaller and smaller pieces and smallest pieces. You would eventually get to a smallest "piece" and it would not be possible to make it any smaller without destroying the properties of the material—and if you did destroy its properties, it would no longer be the material you started with. Atoms are so small that they are invisible to the naked eye and to ordinary microscopes. For many centuries, it was commonly accepted that atoms were indivisible; they could not be broken into particles smaller than themselves. This notion of the atom as the fundamental structure of matter actually began with the ancient Greeks, and for centuries, people did not think it possible to find any particle smaller than an atom. In the 18th century, John Dalton refined and restated the ancient Greek idea of atoms and gave us what has become known as Dalton's Atomic Theory. It was only around the turn of the 20th century that scientists began to understand that atoms were themselves made up of small bits of matter. In 1897, J.J. Thomson showed that all kinds of matter contained tiny charged particles called electrons. These electrons, besides carrying a charge, had a mass so tiny as to be almost negligible when compared to the mass of the atoms of which they were a part. So Thomson proposed a theory that the electrons were distributed within the massive, oppositely charged atom-like plums in a plum pudding—or chocolate chips in ice cream. About the same time, Antoine Henri Becquerel (1903), in France, had found that elements containing uranium somehow emitted (gave off) relatively massive charged particles called alpha particles. Ernest Rutherford and his associates (one was a man named Geiger) did a famous experiment called the alpha scattering experiment (ca. 1910). Rutherford "aimed" alpha particles at a thin sheet of metal (sometimes referred to as the "Gold Foil Experiment") and measured where the particles hit a screen behind the metal target. His detection screen formed a semicircle around the target. The beam of particles spread out slightly, mostly going straight through the target. However, Geiger found that a few of the alpha particles were deflected by more than 180 degrees; that is, they actually bounced back! Ernest Rutherford(1871-1937) was so astonished by this result that he said it was as if he had fired a cannonball at a piece of tissue paper and the cannon ball had bounced back from the paper. Ernest Rutherford became a Nobel laureate for his pioneering work in nuclear physics. As a result of these experiments, the Rutherford, or nuclear, model of the atom was accepted. This model placed almost all of the mass of the atom in a small, dense nucleus of positive charge, surrounded by negatively charged electrons that were thought to be orbiting the nucleus at relatively large distances from it. Many other experiments enabled us to understand more about the atom's structure. In 1932 Chadwick established the presence in the nucleus of the neutron, a particle with no charge (said to be "neutral") but having approximately the same mass as the proton (the name given to the positively charged nuclear particles). An atomic model that replaced Rutherford's model was introduced by Neils Bohr. In 1913, Bohr perfected the Rutherford theory of the atom by an early use of the quantum theory. An electron orbiting the nucleus is held in this orbit by the Coulomb Force. According to classical electromagnetic theory, an accelerating charge should radiate energy and thus spiral into the nucleus. Bohr resolved this, stating the electron can orbit certain fixed orbits, when in a particular orbit, it does not radiate energy. Energy is radiated when the electron jumps to a lower level and energy is absorbed when the electron jumps to a higher level. Today we know that the atom has a very small, massive nucleus containing protons and neutrons and that electrons surround this nucleus at very large distances from it. The arrangements and motion of the electrons are very complicated and require a sophisticated mathematical model to understand. Sometimes, for simplicity, we use the orbital model of the atom, which postulates a small dense nucleus surrounded by electrons that travel around the nucleus in much the same manner as the planets travel about the sun. In this model, the nuclear mass is the sum of the masses of the protons and neutrons contained within it; the mass of each electron is about 1/1800th of the mass of a proton (this orbital model of the atom is the one you see sketched when you see the logo of the Atomic Energy Commission). The quantum mechanical model of the atom is the model that is mathematically complex. It concludes that the electrons do not follow orbits like planets, but are rather present about the nucleus as a "cloud." It is known that electrons have certain defined amounts of energy. The energy "content" and consequent interaction of electrons can explain many phenomena, from how light comes about to how water freezes to how a cake bakes. When we think about the vastly different examples of matter and all their properties—think of iron and steel, water, ice, helium, rubber balloons, DNA, green leaves, coal and diamonds—it is amazing to know that the atomic theory can explain all these.That explanation boils down to a fundamental description of all forms of matter as being made up of atoms having, for each element, a given number of protons and neutrons in the nucleus and a certain number and arrangement of electrons about the nucleus.

Valence Electrons

Valence electrons are the electrons in an atom's outermost energy level. These are the electrons involved in bonding with other atoms. Remember that the elements in the periodic table are arranged in order of increasing atomic number. The number of valence electrons also increases as you move from left to right across a row. All of the elements in a column on the periodic table have the same number of valence electrons. For example, all of the elements in column 1 on the periodic table have one valence electron. The number of valence electrons in an atom determines how that atom reacts with other atoms. Therefore, elements with the same number of valence electrons (that is, elements in the same column on the periodic table) tend to have similar properties. For example, the alkali metals in the first column all react similarly with water. These similarities in properties are so important that several groups of elements are known specifically for their properties. For example, the elements in the last column—the noble gases—are all very unreactive. In contrast, the alkali metals (column 1) and the halogens (column 17) are among the most reactive elements. Column 1 elements: Alkali metals all have 1 valence electron and are extremely reactive. Column 2 elements: Alkaline earth metals all have 2 valence electrons. They are harder than Alkali metals and not as reactive. They react with other substances in a predictable manner. Column 16 elements: Chalcogens all have 6 valence electrons and are non-metals and semi-metals. Column 17 elements: Halogens all have 7 valence electrons. Halogens are non-metals and are highly reactive. Column 18 elements: Noble gases have 8 valence electrons (except helium which has 2 total electrons) and are chemically unreactive.

Practice #3 Analyze the following atom: Fluorine 1) Number of protons? 2) Number of neutrons? 3) Number of electrons? 4) Number of nucleons? 5) Is this atom an ion?

1) 9 2) 11 3) 10 4) 20 5) Yes, there are nine protons and ten electrons.

What does the term quantum mean?

A quantum means a discrete amount of something is involved. For example, people come in a discrete unit called a person. Light comes in discrete units of energy. Light quanta are called photons.

The Theory

According to the Atomic Theory, all atoms are made up of three kinds of particles—the proton, the neutron, and the electron—and from these three particles, all matter is created. For consistency, we will use this legend to identify the three types of particles: -proton -neutron -electron The two parts of the atom are the nucleus, which contains the protons and neutrons, collectively called nucleons, and the electron orbits, which contain the electrons.

Elements and Compounds

Atoms make up all the matter around you, from steel and air to lemonade and ice cream. With all the different substances in the world, you might think that there must be many different elements. This is not the case. There are just over 100 distinct elements known today, 90 of which can be found in nature. Just as 26 letters of the alphabet make up all the words in the dictionary, a small number of elements bond together to form an almost endless array of chemical compounds. Atoms very rarely exist as individual particles. Most substances you can think of are made up of combinations of atoms that are held together by chemical bonds. These bonds form because most atoms are more stable when they are bonded to other atoms. An *ionic compound* is formed when an electron from one atom is lost to another and the two ions form an ionic bond. They are held together by their opposite electrical charges, but they remain as individual ions. A *molecular compound*, or molecule, forms when atoms share some of their electrons in a covalent bond. The two atoms, often both nonmetals, overlap their valence energy levels and are held together by a mutual attraction to the shared electrons.

What are some characteristics of waves?

Characteristics of waves include frequency, wavelength, energy, and amplitude.

Practice #2 Analyze the following atom: Lithium 1) Number of protons? 2) Number of neutrons? 3) Number of electrons? 4) Number of nucleons? 5) Is this atom an ion?

1) 3 2) 4 3) 3 4) 7 5) No, the number of electrons is equal to the number of protons.

10 Questions

1) Matter and energy make up the physical universe. T 2) Electrons of atoms spin around the nucleus made up of protons and neutrons. T 3) Atoms are about the size of a drop of water. F 4) The force that binds atoms together is electricity. F 5) No particle is smaller than an atom. F 6) An atom with a unique number of protons is called an element. T 7) There are 92 elements found naturally in the universe. T 8) Atoms and molecules are the same thing. F 9) Almost all elements and substances can exist as a solid, a liquid, or a gas. T 10) Matter can be destroyed. F

Albert Einstein

Albert Einstein (1879 - 1955), a theoretical physicist, is often best known for his equation E=mc2. In 1901, he published four papers on the photoelectric effect, Brownian motion, special relativity, and the equivalence of mass and energy, that would give him world recognition. Though Einstein, of Jewish heritage, was not a professing Christian as Oxford University's John Brooke, professor of science and religion, comments, "Einstein was not a conventional theist" nor consistent in his views about religion during his life." Brooke adds that Einstein believed in "some kind of intelligence working its way through nature." Einstein did stand in awe of the universe and described a "cosmic religious feeling," but he also rejected "the God of theology who rewards good and punishes evil." (Ideas and Opinions by Albert Einstein (New York: Crown Publishers, 1954),pgs. 261-262). Another of Einstein's statements show his wonder of mathematics, and yet not recognizing that it comes from God of the Bible. "How can it be that mathematics, being after all a product of human thought which is independent of experience, is so admirably appropriate to the objects of reality?"

How is an alpha particle and the nucleus of an atom similar?

An alpha particle is a helium nucleus (two protons and two neutrons). Thus, an alpha has a positive charge, as does every nucleus.

Periodic Table

Atoms are arranged in a chart called the Periodic Table, which organizes the atoms according to number of protons, how they behave in reactions, and how many electron shells the atoms have. A vertical column on the Periodic Table is called a Group or Family. The atoms in each group show close chemical analogies, in valence, and chemical properties. A horizontal row is called a period; each atom in the row has the same number of electron shells. An element is composed of a single type of atom.

Practice #1 (picture on ipad for all practice problems) Examine the three atoms below of the element beryllium (Be) and decide which pairs are ions and which pairs are isotopes of each other.

Beryllium atom A has four neutrons; beryllium atoms B and C have five neutrons. Beryllium A and B are isotopes of each other, and beryllium A and C are isotopes of each other. Beryllium C is an ion of the beryllium atom because it has only three electrons, which do not balance the four protons that are contained in the nucleus of the beryllium atom.

What are some applications of atomic models?

Changes in the model of the atom have given us insights into observed phenomena such as spectra analysis and radiation. Understanding these models and the principles they represent enabled the development of devices used in research, as well as our everyday life. Likewise, they have aided us in our understanding of the Periodic Table and chemical bonding. The detection of waves depends on their frequencies. Human eyes detect light waves at intermediate frequencies. Photographic emulsions can detect x-rays that have high frequencies. Crystals can be ground to specific size and shape to detect a certain radio frequency. Smaller crystals vibrate at high frequencies, while larger crystals vibrate at lower frequencies. Accelerating electric charges radiate energy in the form of electromagnetic waves. Since charges in atoms are accelerating, all matter emits electromagnetic waves. Only certain of these waves will produce spectral lines that we can see; the others will require a special receiver. Modern instruments such as lasers are practical results of our increased understanding of atoms. Electrons moving from a higher energy level to a lower energy level release energy in the form of light. Technical advances, such as infrared spectrometer and scanners using nuclear magnetic resonance (NRO/MRI), are also possible due to increased understanding of atomic and nuclear models. As our understanding of the relationship of energy and momentum increases, we realize that it becomes more difficult to locate electrons. If we use light to locate an electron, we add energy and thus change its momentum and its position. Associating electrons with probability waves has enabled us to free ourselves from the constraints of classical mechanics. Probability theories tell us that something has a possibility of occurring. The probability may be low, but the possibility is there. Such possibilities have led to modern theories of semiconductors. Materials can be thought of as potential energy wells. Classical mechanics states that an electron lacking a finite amount of energy cannot leave the well. Wave mechanics allow for the possibility that the electron can be found outside the well even without the correct amount of energy. Tunneling is also possible with the wave mechanical model. Using probability waves, a particle or wave can penetrate, or tunnel through, barriers which would be insurmountable in classical physics. In addition, assuming a wave model implies that all concepts involved with waves will apply here; such concepts would include Doppler shifts and superposition, including both constructive and distractive interference, as well as diffraction and reflection.

Isotopes

Each type of atom has a certain number of protons. For example, if the atom has one proton, it will be hydrogen. The number of neutrons can vary; below are three different atoms of hydrogen. The first atom of hydrogen has no neutrons, the second atom of hydrogen has one neutron, and the third atom of hydrogen has two neutrons. When you have the same number of protons but different numbers of neutrons, they are called isotopes; these are three isotopes of hydrogen. The different number of neutrons changes the atomic mass of the hydrogen atom, but both the atomic number and the chemical properties of the hydrogen atom remain the same. The chemical property of an atom is determined by the number and arrangement of the electrons.

Numbers Game

In the nucleus, where you find the protons and neutrons, most of the mass of the atom is located; protons and neutrons each have a mass of one atomic unit (AMU). Each proton has a charge equal to one positive elementary charge. The number of protons is called the Atomic Number (Z-number) and the number of protons + the number of neutrons is called the Atomic Mass Number (A-number). The Atomic Number (Z-number) identifies the atom and the Atomic Mass Number (A-number) gives you the number of nucleons present in the nucleus. The electrons orbiting the nucleus are equal to the number of protons in the nucleus when the atom is not charged. When the atom has more or fewer electrons than the number of protons, the atom is referred to as an ion. An ion is a charged particle. Each electron has one negative elementary charge. The mass of the electron is considered negligible mass when compared to the mass of a nucleon. Below is a helium atom: It has two protons, two neutrons, and two electrons. This means the atomic number (Z-number) is two and the atomic mass number (A-number) is four. Having two protons and two electrons means the atom is not an ion.

Physical and Chemical Changes

Matter cannot be created or destroyed. But matter can change its form, and atoms can be rearranged in chemical reactions. *Physical changes* are changes that do not result in the formation of a new substance. The shape, form, or appearance of a substance may have changed, but the substance is still the same. The substance is the same because chemical bonds have not been broken or formed, so the atoms of the molecules are still connected in the same way. A phase change is an example of a physical change. Water can be solid ice, a liquid, or it can evaporate and become a gas. Regardless of its form, the chemical structure of water never changes. It remains H2O. *Chemical changes* are usually referred to as chemical reactions. Chemical changes always produce new substances with properties that are typically very different from those of the reactants. At the molecular level, producing a new substance means that chemical bonds have been broken or formed so that the atoms in molecules are rearranged to form new compounds. For example, the rusting of iron metal is a chemical reaction where iron (Fe) and oxygen (O) combine together to form iron oxide. 4Fe + 3O2 → 2Fe2O3

Fundamentals

The fundamental parts of the atom are not the electrons, protons, and neutrons. Protons and neutrons are composed from quarks, and it is these particles that are the fundamental particles of the nucleons; electrons belong to the lepton family. The fundamental parts of an atom are the electron, up-quark, and down-quark. The charges of these quarks are measured in fractions as they compare to the charge of an electron (-1). The up-quark has a charge of +2/3, and the down-quark has a charge of - 1/3. Two up-quarks and one down quark make up a proton, while one up-quark and two down-quarks make up a neutron. Proton = up quark + up quark + down quark Charge of the proton: +1 = 2/3 + 2/3 - 1/3 Neutron = up quark + down quark + down quark Charge of the neutron: 0 = 2/3 -1/3 - 1/3

Some examples of atomic models.

Thomson's plum pudding model, Bohr's planetary model, and the electron cloud models are just a few possible examples.