Atomic Theory

One of the first attributes of atoms to be described was relative atomic weight. Although a single atom was too small to weigh, atoms could be compared to each other. The chemist Jons Berzelius assumed that equal volumes of gases at the same temperature and pressure contained equal numbers of atoms. He used this idea to The evolution of atomic theory. Illustration by Hans & Cassidy. Courtesy of Gale Group. compare the weights of reacting gases. He was able to determine that, for example, oxygen atoms were 16 times heavier than hydrogen atoms. He made a list of these relative atomic weights for as many elements as he knew. He devised symbols for the elements by using the first letter or first two letters of their Latin names, a system still in use today. The symbol for hydrogen is H, for oxygen is O, for sodium (natrium, in Latin) is Na, and so on. The symbols also proved useful in describing how many atoms combine to form a molecule of a particular compound. For example, to show that water is made of two atoms of hydrogen and one atom of oxygen, the symbol for water is H₂O. One oxygen atom can even combine with one other oxygen atom to produce a molecule of oxygen with the symbol O₂ .

As more and more elements continued to be discovered, it became convenient to begin listing them in symbol form in a chart. In 1869, Dmitri Mendeleev listed the elements in order of increasing atomic weight and grouped elements that seemed to have similar chemical reactions. For example, lithium (Li), sodium (Na), and potassium (K) are all metallic elements that burst into flame if they get wet. Similar elements were placed in the same column of his chart. Mendeleev began to see a pattern among the elements, where every eighth element on the atomic weight listing would belong to the same column. Because of this periodicity or repeating pattern, Mendeleev's chart is called the "Periodic table of the elements." The table was so regular, in fact, that when there was a "hole" in the table, Mendeleev predicted that an element would eventually be discovered to fill the place. For instance, there was a space for an element with an atomic weight of about 72 (72 times heavier than hydrogen) but no known element. In 1886, 15 years after its prediction, the element Germanium (Ge) was isolated and found to have an atomic weight of 72.3. Many more elements continued to be predicted and found in this way. However, as more elements were added to the periodic table, it was found that if some elements were placed in the correct column because of similar reactions, they did not follow the right order of increasing atomic weight. Some other atomic characteristic was needed to order the elements properly. Many years passed before the correct property was found.

As chemistry experiments were searching for and characterizing more elements, other branches of science were making discoveries about electricity and light that were to contribute to the development of atomic theory. Michael Faraday had done much work to characterize electricity; James Clerk Maxwell characterized light. In the 1870s, William Crookes built an apparatus, now called a Crookes tube, to examine "rays" being given off by metals. He wanted to determine whether the rays were light or electricity based on Faraday's and Maxwell's descriptions of both. Crookes's tube consisted of a glass bulb, from which most of the air had been removed, encasing two metal plates called electrodes. One electrode was called the anode and the other was called the cathode. The plates each had a wire leading outside the bulb to a source of electricity. When electricity was applied to the electrodes, rays appeared to come from the cathode. Crookes determined that these cathode rays were particles with a negative electrical charge that were being given off by the metal of the cathode plate. In 1897, J. J. Thomson discovered that these negatively charged particles were coming out of the atoms and must have been present in the metal atoms to begin with. He called these negatively charged subatomic particles "electrons." Since the electrons were negatively charged, the rest of the atom had to be positively charged. Thomson believed that the electrons were scattered in the atom like raisins in a positively-charged bread dough, or like plums in a pudding. Although Thomson's "plum-pudding" model was not correct, it was the first attempt to show that atoms were more complex than just homogeneous spheres.

At the same time, scientists were examining other kinds of mysterious rays that were coming from the Crookes tube that did not originate at its cathode. In 1895, Wilhelm Roentgen noticed that photographic plates held near a Crookes tube would become fogged by some invisible, unknown rays. Roentgen called these rays "x rays," using "x" for unknown as in mathematics. Roentgen also established the use of photographic plates as a way to take pictures of mysterious rays. He found that by blocking the x rays with his hand, for instance, bones would block the x rays but skin and tissue would not. Doctors still use Roentgen's x rays for imaging the human body.

Photographic plates became standard equipment for scientists of Roentgen's time. One of these scientists, Henri Becquerel, left some photographic plates in a drawer with uranium, a new element he was studying. When he removed the plates, he found that they had become fogged. Since there was nothing else in the drawer, he concluded that the uranium must have been giving off some type of ray. Becquerel showed that this radiation was not as penetrating as x rays since it could be blocked by paper. The element itself was actively producing radiation, a property referred to as radioactivity. Largely through the work of Pierre and Marie Curie, more radioactive elements were found. The attempts to characterize the different types of radioactivity led to the next great chapter in the development of atomic theory.

In 1896, Ernest Rutherford, a student of J. J. Thomson, began studying radioactivity. By testing various elements and determining what kinds of materials could block the radiation from reaching a photographic plate, Rutherford concluded that there were two types of radioactivity coming from elements. He named them using the first two letters of the Greek alphabet, alpha and beta. Alpha radiation was made of positively charged particles about four times as heavy as a hydrogen atom. Beta radiation was made of negatively charged particles that seemed to be just like electrons. Rutherford decided to try an experiment using the alpha particles. He set up a piece of thin gold foil with photographic plates encircling it. He then allowed alpha particles to hit the gold. Most of the alpha particles went right through the gold foil. But a few of them did not. A few alpha particles were deflected from their straight course. A few even came straight backward. Rutherford wrote that it was as surprising as if one had fired a bullet at a piece of tissue paper only to have it bounce back. Rutherford concluded that since most of the alpha particles went through, the atoms of the gold must be mostly empty space, not Thomson's space-filling plum-pudding. Since a few of the alpha particles were deflected, there must be a densely packed positive region in each atom that he called the nucleus. With all the positive charge in the nucleus, the next question was the arrangement of the electrons in the atom.

In 1900, physicist Max Planck had been studying processes of light and heat, specifically trying to understand the light radiation given off by a "black-body," an ideal cavity made by perfectly reflecting walls. This cavity was imagined as containing objects called oscillators which absorbed and emitted light and heat. Given enough time, the radiation from such a black-body would produce a colored-light distribution called a spectrum that depended only on the temperature of the black-body and not on what it was made of. Many scientists attempted to find a mathematical relationship that would predict how the oscillators of a black-body could produce a particular spectral distribution. Max Planck found that correct mathematical relationship. He assumed that the energy absorbed or emitted by the oscillators was always a multiple of some fundamental "packet of energy" he called a quantum. Objects that emit or absorb energy do it in discrete amounts, called quanta.

At this same time, there was a physicist working with Thomson and Rutherford named Niels Bohr. Bohr realized that the idea of a quantum of energy could explain how the electrons in the atom are arranged. He described the electrons as being "in orbit" around the nucleus like planets around the sun. Like oscillators in a black-body could not have just any energy, electrons in the atom could not have just any orbit. There were only Hydrogen's isotopes: hydrogen, deuterium, and tritium. Illustration by Hans & Cassidy. Courtesy of Gale Group. certain distances that were allowed by the energy that an electron had. If an electron of a particular atom absorbed the precisely right quantum of energy, it could move farther away from the nucleus. If an electron farther from the nucleus emitted the precisely right quantum of energy, it could move closer to the nucleus. What the precisely right values were differed for every element. These values could be determined by a process called atomic spectroscopy, an experimental technique that looked at the light spectrum produced by atoms. An atom was heated so that all of its electrons were moved far away from the nucleus. As they moved closer to the nucleus, the electrons would begin emitting their quanta of energy as light. The spectrum of light produced could be examined using a prism. The spectrum produced in this way did not show every possible color, but only those few that matched the energies corresponding to the electron orbit differences. Although later refined, Bohr's "planetary model" of the atom explained atomic spectroscopy data well enough that scientists turned their attention back to the nucleus of the atom.

Rutherford, along with Frederick Soddy, continued work with radioactive elements. Soddy, in particular, noticed that as alpha and beta particles were emitted from atoms, the atoms changed in one of two ways: (1) the element became a totally different element with completely new chemical reactions, or (2) the element maintained the same chemical reactions and the same atomic spectrum but only changed in atomic weight.

He called atoms of the second group isotopes, atoms of the same element with different atomic weights. In any natural sample of an element, there may be several types of isotopes. As a result, the atomic weight of an element that was calculated by Berzelius was actually an average of all the isotope weights for that element. This was the reason that some elements did not fall into the correct order on Mendeleev's periodic table—the average atomic weight depended on how much of each kind of isotope was present. Soddy suggested placing the elements in the periodic table by similarity of chemical reactions and then numbering them in order. The number assigned to each element in this way is called the atomic number. The atomic numbers were convenient ways to refer to elements.

Meanwhile, Thomson had continued his work with the Crookes tube. He found that, not only were cathode rays of electrons produced, but so were positive particles. After much painstaking work, he was able to separate the many different kinds of positive particles by weight. Based on these measurements, he was able to determine a fundamental particle, the smallest positive particle produced, called a proton. Since these were being produced by the atoms of the cathode and since Rutherford showed that the nucleus of the atom was positive, Thomson realized that the nucleus of an atom must contain protons. A young scientist named Henry Moseley experimented with bombarding atoms of different elements with x rays. Just as in atomic spectroscopy, where heat gives electrons more energy, x rays give protons in the nucleus more energy. And just as electrons give out light of specific energies when they cool, the nucleus emits x rays of a specific energy when it "de-excites." Moseley discovered that the energy of the emitted x rays for every element followed a simple mathematical relationship. The energy depended on the atomic number for that element, and the atomic number corresponded to the number of positive charges in the nucleus. So the correct ordering of the periodic table is by increasing number of protons in the atomic nucleus. The number of protons equals the number of electrons in a neutral atom. The electrons are responsible for the chemical reactions. Elements in the same column of the periodic table have similar arrangements of electrons with the highest energies, and this is why their reactions are similar.

Only one problem remained. Electrons had very little weight, 1/1,836 the weight of a proton. Yet the protons did not account for all of the atomic weight of an atom. It was not until 1932 that James Chadwick discovered the existence of a particle in the nucleus with no electrical charge but with a weight slightly greater than a proton. He named this particle the neutron. Neutrons are responsible for the existence of isotopes. Two atoms of the same element will have the same number of protons and electrons but they might have different numbers of neutrons and therefore different atomic weights. Isotopes are named by stating the name of the element and then the number of protons plus neutrons in the nucleus. The sum of the protons and neutrons is called the mass number. For example, uranium-235 has 235 protons and neutrons. We can look on a periodic table to find uranium's atomic number (92) which tells us the number of protons. Then by subtracting, we know that this isotope has 143 neutrons. There is another isotope of uranium, ²³⁸U, with 92 protons and 146 neutrons. Some combinations of protons and neutrons are less stable than others. Picture trying to hold 10 bowling balls in your arms. There will be some arrangement where you might be able to manage it. Now try holding 11 or only nine. There might not be a stable arrangement and you would drop the bowling balls. The same thing happens with protons and neutrons. Unstable arrangements spontaneously fall apart, emitting particles, until a stable structure is reached. This is how radioactivity like alpha particles is produced. Alpha particles are made of two protons and two neutrons tumbling out of an unstable nucleus.

Hydrogen has three kinds of isotopes: hydrogen, ²H (deuterium), and ³H (tritium).

The atomic weights of the other elements were originally compared to hydrogen without specifying which isotope. It is also difficult to get single atoms of hydrogen because it usually reacts with other atoms to form molecules like H₂ or H₂O. So a different element's isotope was chosen for comparison. The atomic weights are now based on 12 C (carbon-12). This isotope has six protons and six neutrons in its nucleus. Carbon-12 was defined to be 12 atomic mass units. (Atomic mass units, abbreviated amu, are units used to compare the relative weights of atoms. One amu is less than 200 sextillionths of a gram.) Every other isotope of every other element is compared to this. Then the weights of a given element's isotopes are averaged to give the atomic weights found on the periodic table.

Until this point in the story of the atom, all of the particles comprising the atom were thought of as hard, uniform spheres. Beginning in 1920 with the work of Louis de Broglie, this image changed. De Broglie showed that particles like electrons could sometimes have properties of waves. For instance, if water waves are produced by two sources, like dropping two pebbles into a pond, the waves can interfere with each other. This means that high spots add to make even higher spots. Low spots add to make even lower regions. When electrons were made to travel through a double slit, with some electrons going through one slit and some through the other, they effectively created two sources. The electrons showed this same kind of interference, producing a pattern on a collection plate. The ability of electrons and other particles to sometimes show properties of particles and sometimes of waves is called wave-particle duality. This complication to the nature of the electron meant that Bohr's idea of a planetary atom was not quite right. The electrons do have different discrete energies, but they do not follow circular orbits. In 1925, Werner Heisenberg stated that the precise speed and location of an electron cannot both be known at the same time. This "Heisenberg uncertainty principle" inspired Erwin Schrödinger to devise an equation to calculate how an electron with a certain energy moves. Schrödinger's equation describes regions in an atom where an electron with a certain energy is likely to be but not exactly where it is. This region of probability is called an orbital. Electrons move so fast within these orbitals that we can think of them as blurring into an electron cloud. Electrons move from one orbital into another by absorbing or emitting a quantum of energy, just as Bohr explained.