When Rutherford began his studies at Cambridge, he was among the first 'aliens' (those without a Cambridge degree) allowed to do research at the university, and was additionally honoured to study under
J. J. Thomson.
Radioactivity Again under Thomson's leadership, Rutherford worked on the conductive effects of
X-rays on gases, which led to the discovery of the
electron, the results first presented by Thomson in 1897. Hearing of
Henri Becquerel's experience with
uranium, Rutherford started to explore its
radioactivity, discovering two types that differed from X-rays in their penetrating power. Continuing his research in Canada, in 1899 he coined the terms "
alpha ray" and "
beta ray" to describe these two distinct types of
radiation. In 1898, Rutherford accepted the Macdonald Chair of Physics at
McGill University in Montreal, Canada, on Thomson's recommendation. From 1900 to 1903, he was joined at McGill by the young chemist
Frederick Soddy (
Nobel Prize in Chemistry, 1921) for whom he set the problem of identifying the
noble gas emitted by the radioactive element
thorium, a substance which was itself radioactive and would coat other substances. Once he had eliminated all the normal chemical reactions, Soddy suggested that it must be one of the inert gases, which they named
thoron. This substance was later found to be
220Rn, an isotope of radon. In 1907, he returned to Britain to take the
Langworthy Professorship at the
Victoria University of Manchester. In Manchester, Rutherford continued his work with alpha radiation. In conjunction with
Hans Geiger, he developed zinc sulfide
scintillation screens and
ionisation chambers to count alpha particles. By dividing the total charge accumulated on the screen by the number counted, Rutherford determined that the charge on the alpha particle was two. In late 1907, Ernest Rutherford and
Thomas Royds allowed alphas to penetrate a very thin window into an evacuated tube. As they
sparked the tube into discharge, the spectrum obtained from it changed, as the alphas accumulated in the tube. Eventually, the clear spectrum of helium gas appeared, proving that alphas were at least ionised helium atoms, and probably helium nuclei. In 1910 Rutherford, with Geiger and mathematician
Harry Bateman published their classic paper describing the first analysis of the distribution in time of radioactive emission, a distribution now called the
Poisson distribution.
Model of the atom s passing through the
plum pudding model of the atom undisturbed.
Bottom: Observed results: a small portion of the particles were deflected, indicating
a small, concentrated charge. Diagram is not to scale; in reality the nucleus is vastly smaller than the electron shell. Rutherford continued to make ground-breaking discoveries long after receiving the Nobel prize in 1908. Rutherford was inspired to ask Geiger and Marsden in this experiment to look for alpha particles with very high deflection angles, which was not expected according to any theory of matter at that time. Such deflection angles, although rare, were found. Reflecting on these results in one of his last lectures, Rutherford was quoted as saying: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." It was Rutherford's interpretation of this data that led him to propose the
nucleus, a very small,
charged region containing much of the atom's mass. In 1912, Rutherford was joined by
Niels Bohr (who postulated that electrons moved in specific orbits about the compact nucleus). Bohr adapted Rutherford's nuclear structure to be consistent with
Max Planck's quantum hypothesis. The resulting
Bohr model was the basis for
quantum mechanical atomic physics of
Heisenberg which remains valid today.
Discovery of the proton Together with
H.G. Moseley, Rutherford developed the
atomic numbering system in 1913. Rutherford and Moseley's experiments used
cathode rays to bombard various elements with streams of electrons and observed that each element responded in a consistent and distinct manner. Their research was the first to assert that each element could be defined by the properties of its inner structures – an observation that later led to the discovery of the
atomic nucleus. It was not until 1919 that Rutherford expanded upon his theory of the "positive electron" with a series of experiments beginning shortly before the end of his time at Manchester. He found that nitrogen, and other light elements, ejected a proton, which he called a "hydrogen atom," when hit with α (alpha) particles. Rutherford returned to the Cavendish Laboratory in 1919, succeeding J. J. Thomson as
Cavendish Professor of Physics, a position he held until his death in 1937. During his tenure, Nobel prizes were awarded to
James Chadwick for discovering the neutron (in 1932),
John Cockcroft and
Ernest Walton for an experiment that was to be known as "splitting the atom" using a
particle accelerator, and
Edward Appleton for demonstrating the existence of the
ionosphere.
Development of proton and neutron theory In 1919–1920, Rutherford continued his research on the "hydrogen atom" to confirm that alpha particles break down nitrogen nuclei and to affirm the nature of the products. This result showed Rutherford that hydrogen nuclei were a part of nitrogen nuclei (and by inference, probably other nuclei as well). Such a construction had been suspected for many years, on the basis of atomic weights that were integral multiples of that of hydrogen; see
Prout's hypothesis. Hydrogen was known to be the lightest element, and its nuclei presumably the lightest nuclei. Now, because of all these considerations, Rutherford decided that a hydrogen nucleus was possibly a fundamental building block of all nuclei, and also possibly a new fundamental particle as well, since nothing was known to be lighter than that nucleus. Thus, confirming and extending the work of
Wilhelm Wien, who in 1898 discovered the proton in streams of
ionised gas, in 1920 Rutherford postulated the hydrogen nucleus to be a new particle, which he dubbed the
proton. In 1921, while working with Niels Bohr, Rutherford theorised about the existence of
neutrons, (which he had christened in his 1920
Bakerian Lecture), which could somehow compensate for the repelling effect of the positive charges of
protons by causing an attractive
nuclear force and thus keep the nuclei from flying apart, due to the repulsion between protons. The only alternative to neutrons was the existence of "nuclear electrons", which would counteract some of the proton charges in the nucleus, since by then it was known that nuclei had about twice the mass that could be accounted for if they were simply assembled from hydrogen nuclei (protons). But how these nuclear electrons could be trapped in the nucleus, was a mystery. In 1932, Rutherford's theory of
neutrons was proved by his associate
James Chadwick, who recognised neutrons immediately when they were produced by other scientists and later himself, in bombarding
beryllium with alpha particles. In 1935, Chadwick was awarded the Nobel Prize in Physics for this discovery.
Induced nuclear reaction and probing the nucleus In Rutherford's four-part article on the "Collision of α-particles with light atoms" he reported two additional fundamental and far reaching discoveries. Blackett was awarded the Nobel prize in 1948 for his work in perfecting the high-speed cloud chamber apparatus used to make that discovery and many others. == Personal life and death ==