James Chadwick discovered that there is a fundamental particle in the atom that has no electrical charge and has a mass approximately equal to that of the proton.

The word “atom” comes from the Greek atomos, meaning “indivisible,” but discoveries in the late nineteenth century indicated that the atom has a divisible and very complex structure. By 1914, Ernest Rutherford, an English physicist, had developed a model of the atom based on his own work as well as on the work of many scientists before him. In this model, nearly all the mass of the atom and all the positive electrical charge are concentrated in an extremely small part of the atom, the nucleus. Atomic nucleus Rutherford estimated the diameter of the nucleus to be one ten-thousandth of the diameter of the atom. Consequently, the electrons associated with the atom occupied a much larger volume than the nucleus and carried all the negative electrical charge, but had very little mass. He named the carrier of the positive charge in the nucleus the “proton” Protons (for the Greek word for “first”). [kw]Chadwick Discovers the Neutron (Feb., 1932)
[kw]Neutron, Chadwick Discovers the (Feb., 1932)
Neutrons
Atoms;structure
[g]England;Feb., 1932: Chadwick Discovers the Neutron[07990]
[c]Science and technology;Feb., 1932: Chadwick Discovers the Neutron[07990]
[c]Physics;Feb., 1932: Chadwick Discovers the Neutron[07990]
Chadwick, James
Rutherford, Ernest
Joliot, Frédéric
Joliot-Curie, Irène
Bothe, Walther

James Chadwick.

(The Nobel Foundation)

At this point, then, there were two elementary particles: the proton, with a positive charge, and the electron, Electrons with a negative electrical charge of the same magnitude as the positive charge of the proton. The atom, therefore, had to be built up with only these two particles. Helium, for example, would have two protons and two electrons. However, because the mass of the helium atom was known to be four times the mass of the hydrogen atom (which has one proton and one electron), the nucleus of the helium atom needed two more protons to produce the appropriate mass. In order to keep the electrical charge of the nucleus equal to that of two protons, it was suggested there were also two electrons in the nucleus, which neutralized the charge of the additional two protons.

As early as 1920, Rutherford speculated that there might be another elementary particle with about the same mass as the proton, but with no charge. Perhaps it was somehow produced by the combination of a proton and an electron. In 1921, American chemist William Draper Harkins Harkins, William Draper named this hypothetical particle the neutron, because it was electrically neutral.

English physicist James Chadwick began his search for the neutron at the Cavendish Laboratory Cavendish Laboratory under Rutherford’s guidance. At first, his search was unsuccessful; however, he was not the only one searching. In 1930, Walther Bothe of Germany found that when beryllium and boron were bombarded by high-energy alpha particles, radiation with no electrical charge but with great penetrating power was produced. Only two years later, Irène Joliot-Curie and Frédéric Joliot reported that this radiation could cause protons to be ejected from paraffin. These scientists concluded that the radiation was a type of gamma ray—that is, electromagnetic energy of very high frequency.

When Chadwick read the account of the experiments performed by the French scientists, he immediately decided to examine this phenomenon further. He found that when boron and beryllium were bombarded by alpha particles from polonium, the mysterious radiation from these two substances could eject protons from any materials that contained hydrogen. He also discovered, from calculating the energy acquired by nitrogen atoms bombarded by the unknown radiation, that the radiation could not be gamma rays, as previously reported.

Chadwick then showed that his experimental results were completely consistent with the assumption that each proton ejected from the paraffin had undergone a collision with an unknown particle of approximately equal mass (very much like what happens when billiard balls collide). When Chadwick was unable to deflect this particle in a magnetic field, he concluded that it had no electrical charge. Given that it did not correspond to any previously known particle, it must be the long-sought neutron.

The discovery of the neutron, in 1932, marked the beginning of nuclear physics—that is, the study of the nuclear structure of the atom. It was readily seen that, as the nuclei of most elements are extremely stable, there were forces of attraction hitherto unknown between the “nucleons” Nucleons (the word coined to designate the particles in the nucleus). Until the discovery of the neutron, the only known forces in physics were those of gravity, electricity, and magnetism. After 1932, it was necessary to speak of a new kind of force: the nuclear force.

With the discovery of the neutron came a much clearer understanding of atomic structure, specifically the structure of the nucleus. German physicist Werner Heisenberg Heisenberg, Werner proposed that envisioning the nucleus of an atom as being constructed of neutrons and protons resolved a number of difficulties. These included the problem of the missing mass of the helium atom—the answer is that two neutrons make up the additional mass. Neutrons also provide an explanation for isotopes, Isotopes which are atoms of the same element that have different atomic masses. Clearly, the additional mass is caused by the presence of more neutrons in the nucleus.

In 1939, it was discovered that when uranium atoms are bombarded by neutrons, they undergo fission. Shortly thereafter, physicists showed that this fission Nuclear fission would release considerable energy. When it was also discovered that the fission process produced additional neutrons, scientists realized that a chain reaction of great power was possible. Consequently, the discovery of the neutron ushered in the atomic age. Neutrons
Atoms;structure

• Asimov, Isaac. Mass and Energy: The Neutron, the Structure of the Nucleus. Vol. 2 in Worlds Within Worlds: The Story of Nuclear Energy. Washington, D.C.: U.S. Atomic Energy Commission, Office of Information Services, 1972. Brief discussion on the history of atomic energy by a renowned science writer begins with the theory of relativity and concludes with the use of neutrons to bombard uranium. Nonmathematical and nontechnical presentation, with many fine illustrations and photographs.
• Brown, Andrew. The Neutron and the Bomb: A Biography of Sir James Chadwick. New York: Oxford University Press, 1997. First full-length biography of Chadwick places his work within the larger context of his life and times. Features numerous extracts from his personal correspondence with other scientists. Includes index.
• Crowther, J. G. The Cavendish Laboratory, 1874-1974. New York: Science History Publications, 1974. An excellent account for readers interested in the historical context in which the development of atomic theory took place. Describes how close Chadwick’s contemporaries in France and Germany came to discovering the neutron.
• Goldhaber, Maurice. “With Chadwick at the Cavendish.” Bulletin of the Atomic Scientists 13 (December, 1982): 12-13. Lively brief account of Chadwick and some of his coworkers at the Cavendish Laboratory by a distinguished German physicist who worked with Chadwick. Of interest for its view of the personal side of scientific activity, as it shows how scientific ideas arise and how scientists are motivated and influenced by one another.
• Hughes, Donald J. The Neutron Story. Garden City, N.Y.: Doubleday, 1959. Excellent historical perspective provided by an author who was directly acquainted with some of the physicists associated with the development of nuclear physics. The physics is presented in nontechnical language without passing over significant details. Includes index.
• Oliphant, Mark. “The Beginning: Chadwick and the Neutron.” Bulletin of the Atomic Scientists 15 (December, 1982): 14-18. Description of Chadwick’s work by a close associate of Chadwick at Cavendish Laboratory. Demonstrates how scientists work closely with others concerned with the same problems.
• Piel, Gerard. The Age of Science: What Scientists Learned in the Twentieth Century. New York: Basic Books, 2001. An overview of the scientific achievements of the twentieth century. Chapter 3 discusses Chadwick’s work. Includes many illustrations and index.
• Smyth, Henry De Wolf. Atomic Energy for Military Purposes. 1945. Reprint. Stanford, Calif.: Stanford University Press, 1990. The first published account of the Manhattan Project—the story of the development of the atomic bomb during World War II. As the bomb’s production depended on the possibility of a chain reaction, the role of the neutron was crucial to the success of the project. Contains numerous helpful appendixes and an index.

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Bohr Uses Quantum Theory to Identify Atomic Structure

Rutherford Describes the Atomic Nucleus

Rutherford Discovers the Proton

Cockcroft and Walton Split the Atom