In 1932, James Chadwick proved that the atomic nucleus contained a neutral particle which had been proposed more than a decade earlier by Ernest Rutherford.
As of 1918, with Ernest Rutherford’s discovery of the positively-charged proton, it surely seemed to many physicists as if a nearly complete model of the atomic structure was at hand. After all, there was the electron (discovered by J.J. Thompson in 1897) which was tiny and seemed to buzz a “solar-system”-like orbit around the atomic nucleus (discovered by Rutherford, Geiger, and Marsden in 1909), which contained the proton – a particle much denser and more massive than the tiny electron, but with an opposite charge.
With just these basic elements of the atomic structure, it was clear why atoms appeared to be neutral (the negative charge of the electron and the positive charge of the proton canceled out), it was beginning to grow more clear how chemical bonds form within the electron shells of the atom, and it was suddenly apparent why different atoms absorbed and emitted only very specific colors of light (spectroscopy), because the wavelength of light absorbed and emitted by the electrons were very specific to the sizes of their orbits, which varied depending on the type of atom.
While according to such practical logic, there didn’t seem to be any need for any additions to the atomic structure at this point, this didn’t stop Rutherford from contemplating, in 1920, the possibility of the existence of a neutrally-charged particle with a similar mass to that of a proton. This would help to keep the atom neutral, and to fix some disparity found between the atomic number of an atom and its atomic mass (for the atomic number is defined by the number of protons, where the atomic mass, which measures the mass of the nucleus, was generally higher).
The Neutron
In 1932, English Physicist James Chadwick, after a decade-long struggle to track down this tricky particle (all the methods available at the time were used only to detect charged particles), performed tests on a new type of radiation which had been baffling physicists for years, and which had previously been mistaken for “gamma rays” (a form of radiation consisting of high-energy photons).
The test, to simplify as much as possible, went like this:
A sample of Beryllium was bombarded with alpha particles (another type of naturally occurring radiation which are technically just ionized helium nuclei), which causes it to emit this mysterious radiation. It was then discovered by Irene Joliot-Curie (daughter of Marie and Pierre Curie) and her husband Frederic Joliot-Curie that this radiation, upon striking a proton-rich surface (paraffin was the preferred example), would discharge some of the protons, which could then be detected using a Geiger counter (a device that measures radiation).
This was the premise, and from here, Chadwick simply had to play detective and put all the pieces of the puzzle together. For instance, he could tell that the mysterious radiation in question was neutral due to the fact that it was not affected by proximity to a magnetic field, and, unlike standard gamma radiation, did not invoke the photoelectric effect (when photons, such as gamma rays, strike certain surfaces, they discharge electrons, which can be simply measured), but rather discharged protons, which meant that the particles had to be more massive than previously expected.
In the end, Chadwick finally solved the puzzle and officially discovered the neutron in 1932, thus vindicating Rutherford’s original theory (not that Rutherford needed any more accomplishments in his already prolific scientific career). For his efforts, Chadwick received the Nobel Prize in 1935.
The New Atomic Model
With the discovery of the neutron, the atomic model seemed more complete than ever. The overall charges remained the same, and now there no longer seemed to be a discrepancy between the atomic mass and the atomic number.
Of course, it is now known that neutrons play a much more important role within an atom than was originally thought. It would become clear to physicists in the decades after this neutral particle’s original discovery that it within the atomic nucleus, both the proton and the neutron seemed to possess equal importance in determining atomic stability, which was refined even further in the 1960’s in the science known as Quantum Chromodynamics, which espoused a quark-based theory of the nucleus.
Neutrons would also play an important role in the processes involved in creating nuclear explosions and nuclear energy, for it is by bombardment with high-energy neutrons that scientists first learned how to split an atom.
Further Reading:
Democritus and Ancient Atomic Theories
John Dalton and the First Modern Atomic Theory
Brownian Motion and the Proof of Atoms
J.J. Thompson and the Discovery of Electrons
Ernest Rutherford and the Discovery of Protons and the Atomic Nucleus
References:
Gribbin, J. (2002). The Scientists: A History of the Science Told Through the Lives of its Greatest Inventors. New York: Random House.
Glashow, S. L. (1988). Interactions - A Journey Through the Mind of a Particle Physicist and the Matter of this World. New york: Warner Books.
Asimov, I. (1966). Understanding Physics: 3 Volumes in 1. Barnes and Noble Books.
