Ernest Rutherford

Ernest Rutherford the New Zealand-born British physicist (1871-1937) regarded as the father of nuclear physics. He discovered the atomic nucleus through his analysis of alpha-particle scattering, achieved the first artificial splitting of an atom, and won the 1908 Nobel Prize in Chemistry for his work on the disintegration of the elements.

Early Life and Education

Ernest Rutherford was born on 30 August 1871 near Nelson, New Zealand, the fourth child of a farmer and a schoolteacher. He won a scholarship to Canterbury College in Christchurch, where he took degrees in mathematics and physics. In 1895 he was awarded an exhibition that took him to the Cavendish Laboratory at Cambridge, where he worked under J. J. Thomson on the ionisation of gases.

At Cambridge Rutherford quickly showed both experimental ingenuity and a taste for bold generalisation. In 1898 he became professor of physics at McGill University in Montreal, where he began the study of radioactivity that made him famous. In 1907 he returned to Britain as professor at the University of Manchester, and in 1919 he succeeded Thomson as Cavendish Professor at Cambridge. He was a physically robust man with a loud voice and inexhaustible energy, and he inspired strong loyalty in the young researchers who gathered around him.

Radioactivity and the Transmutation of Elements

Working with the chemist Frederick Soddy at McGill, Rutherford showed that radioactive atoms decay by emitting particles and that in doing so they change into different elements. This discovery of transmutation disproved the ancient belief that atoms were permanent and unchangeable, and it earned Rutherford the Nobel Prize in Chemistry in 1908, an award he found amusing since he regarded himself as a physicist. He also noticed that the amount of radioactivity of a substance fell away in a regular fashion, and he began to suspect that the process was governed by chance rather than by any external condition.

He distinguished the emissions of radioactive substances into alpha and beta rays, later identifying the alpha particle as a helium nucleus, and he formulated the law of radioactive decay, according to which a fixed fraction of any quantity of a radioactive substance decays in a given time. This exponential law became a fundamental tool of physics, geology and archaeology. It underlies the technique of radiometric dating, which allows the age of rocks and of the Earth itself to be estimated.

The Nuclear Atom

In 1909 Rutherford's assistants Hans Geiger and Ernest Marsden, following his suggestion, fired alpha particles at a thin gold foil and observed that a small number were deflected through very large angles. Rutherford was astonished, remarking that it was almost as incredible as if one had fired a shell at a piece of tissue paper and it had come back and hit you. The experiment had been designed to test the prevailing model of the atom, and its outcome demanded a completely new picture.

He interpreted the result by proposing that nearly all the mass and positive charge of the atom is concentrated in a tiny central nucleus, while the electrons orbit at a comparatively great distance. This nuclear model replaced Thomson's pudding picture and introduced the modern image of the atom as mostly empty space. It also raised the question of why the orbiting electrons did not radiate away their energy, a puzzle that led Niels Bohr to develop the first quantum model of the atom.

Artificial Disintegration and the Cavendish Years

In 1917, by bombarding nitrogen with alpha particles, Rutherford produced hydrogen nuclei and thereby achieved the first deliberate transformation of one element into another by human agency. He had effectively split the atom, an achievement that inaugurated the study of nuclear reactions. Later he predicted the existence of the neutron, which his student James Chadwick discovered in 1932, and of the isotope of hydrogen known as deuterium.

As director of the Cavendish Laboratory from 1919 until his death, Rutherford presided over a golden age of experimental physics. Under his guidance the laboratory produced decisive advances in nuclear physics, and his students and associates, including Chadwick, Cockcroft, Walton and Bohr, shaped the emerging field. He was a demanding but generous leader, known for his booming voice and his impatience with vague thinking. His habit of working late into the night and his delight in a surprising result set the tone for the whole laboratory.

Influence and Legacy

Rutherford's work established that the atom has a nucleus and that the elements can be transformed, discoveries that opened the way to nuclear energy, nuclear medicine and the understanding of stellar processes. He was raised to the peerage in 1931 as Baron Rutherford of Nelson and died on 19 October 1937, four years before the first controlled nuclear chain reaction. His ashes were interred in Westminster Abbey near the graves of Newton and Kelvin.

He is remembered as one of the greatest experimental physicists who ever lived, a man who trusted the evidence of experiments over comfortable assumptions and who was willing to accept conclusions that overturned established belief. His nuclear model and his demonstration of artificial transmutation are foundations of modern physics, and the institutions he led carried his influence far beyond his own lifetime. The unit of radioactivity and the chemical element rutherfordium are named in his honour, as is the scattering phenomenon that revealed the nucleus.

See also

• Atomic nucleus
• Alpha particle
• Rutherford scattering
• Nuclear transmutation