ATOMIC PHYSICS

What is an Atom?

Atomic physics was making rapid strides during the second decade of the twentieth century. By the turn of the twentieth century, there was general acceptance among physicists of the molecular theory of matter. Molecules were believed to be composed of still smaller units of matter, atoms. However, it was also becoming clear that the "unsplittable" atoms were composed of even smaller parts. While the properties of electrons were beginning to be understood, the structure of the atom itself remained a mystery.

Rutherford's Experiments

Various theories of atomic structure were ventured, but the one given the most credence was the model proposed by Professor Ernest Rutherford of England. From about 1906 Rutherford had been firing alpha particles (positively charged particles consisting of two protons and two neutrons that are emitted by several radioactive substances) at sheets of matter in hopes that the record of how they passed through or were deflected by the sheets would help to suggest a picture of the atom. In 1908 Rutherford fired alpha particles at a sheet of gold only two thousand atoms thick (a thickness of l/50,000th of an inch). He found that most of the alpha particles passed through the gold and were recorded on a photographic plate behind it. This seemed to indicate that the atoms were mostly empty space (we now know that the size of a proton is only l/100,000th of the diameter of an atom). A comparatively small number of alpha particles, Rutherford noticed, were deflected from the main stream at a substantial angle.

The Nuclear Atom

By 1911 Rutherford had collected enough data to put forward his revolutionary theory of the nuclear atom. In a paper delivered on 7 March 1912 in Manchester, England, Rutherford described his discoveries, and soon thereafter he explained his theory before American physicists at Princeton University's Physics Colloquium. He argued that the center of the atom was composed of protons (from the Greek word for "first") and that it was surrounded by electrons. In Rutherford's view, the atom was "built up like a solar system on an extremely small scale. The positive electricity is concentrated into a very small nucleus, which takes the place of the sun, and the negative electrons revolve around this like planets. It seems probable that they are arranged in rings, like the rings of Saturn."

The Electron's Charge

Another pioneer in the study of the atom was American physicist Robert A. Millikan, who set out in 1906 to find the absolute charge of the electron. His earliest efforts involved charting the course of water droplets as they fell through the air while the pull of a charged plate above them countered the force of gravity. The evaporation of the droplets resulted in poor results, and in 1911 he switched to oil droplets. By passing X rays through the chamber in which the oil droplets were raining down, Millikan was able to attach extra electrons to the falling droplets. When a droplet absorbed electrons the charged plate above had a noticeably greater effect. The descent of such droplets was slowed, and if the positive charge above was great enough the droplets even rose. The change was due, Millikan reasoned, to the additional electrons. By calculating the effects of gravity pulling the droplet down and the positive plate pulling it upward he was able to determine the charge of a single electron. (Millikan's result closely approximates the now accepted value of sixteen-quintillionths of a coulomb.) For this discovery Millikan was awarded the Nobel Prize in physics in 1923.

The Puzzling Electroscope

The electroscope used in the early twentieth century to detect the presence of radiation posed an intriguing problem for physicists. At the time electroscopes consisted of two thin wafers of gold joined at the upper end and suspended inside a jar so that they were separated by a small distance. When the gold wafers were electrically charged from an outside source they would repel each other (since they had the same charge) and form an inverted V. When radiation penetrated the glass of the jar, scientists discovered, it would carried off the electric charge and the wafers would slowly fall back to their original place. Yet even when scientists kept all known radiation sources away from the jar the gold plates would eventually return to their original state. An undetermined source of radiation was apparently entering the electroscope.

Cosmic Rays

The Austrian physicist Victor Hess believed that the Earth itself might be the source of the radiation. In 1911 he made ten balloon flights with an electroscope in hopes of achieving a height at which the hypothesized radiation from the Earth would become negligible. To his amazement, the gold wafers, instead of remaining apart for a longer duration, collapsed toward each other much more rapidly. The radiation affecting them was coming not from the Earth but from space. At the suggestion of Millikan the newly discovered rays from space were dubbed "cosmic rays." Hess won the Nobel Prize in physics in 1936 for his discovery.

The Genesis of Quantum Physics

Based on Rutherford's theory, and the earlier work of Max Planck, the 1910s witnessed the genesis of quantum physics. The first of the famous Solvay Congresses, attended by many American physicists, on quantum physics was held in 1911 in Brussels. In 1912 the Danish physicist Niels Bohr explained how, in Rutherford's atomic nuclear theory, the electron orbiting the central proton did not simply spiral downward into the nucleus. He proposed the notion that the orbital momentum of the electron is quantized, and that radiation is emitted when electrons jump between valences around the nucleus. Bohr, in Europe, became director of the new Institute for Theoretical Physics in 1916, and many American scientists traveled to the institute to discuss Bohr's work. In 1913 the English scientist Frederick Soddy further excited American chemists and physicists with his theory of isotopes. Soddy showed that atoms of different atomic weights could act in chemically identical ways. Excitement over atomic theory electrified the American scientific community in the 1910s, and by 1919 American Irving Langmuir was deeply involved in studying the structure of electronic valances around the atomic nucleus. Yet it would remain for those who followed to unlock the atom's secrets further.

THE ATOMS AND THE GREEKS

Atom is a word derived from ancient Greek. The philosophers Leucippus and Democritus had conceived of the universe as being constructed from tiny bits of hard matter, of differing shapes, that could not be broken into parts. Since the ancient Greek prefix for "not" is "a" and the word for "splittable" is "tomoi," these pre-Socratic natural philosophers called these tiny building blocks "atomoi." Modern scientists simply adopted the word in describing the tiny bits of fundamental matter from which, they believed, all things were made.

Source:

Philip Wheelwright, ed., The Presocratics (Indianapolis, Ind.: Bobbs-Merrill. 1966).

Sources:

Isaac Asimov, Understanding Physics (New York: Walker, 1966);

Barbara Lovett Cline, The Questioners (New York: Crowell, 1965);

Arthur H. Compton, "What is Matter Made Of?," Scientific American (15 May 1915): 451-452;

Daniel J. Kevles, The Physicists: The History of a Scientific Community in Modern America (New York: Knopf, 1971);

Emilio Segre, From X-Rays to Quarks: Modern Physicists and Their Discoveries (San Francisco: Freeman, 1980);

James S. Trefil, From Atoms to Quarks: An Introduction to the Strange World of Particle Physics (New York: Scribners, 1980).