Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, FRS (30 August 1871 – 19 October 1937) was a New Zealand chemist and physicist who became known as the father of nuclear physics. In early work he discovered the concept of radioactive half life, proved that radioactivity involved the transmutation of one chemical element to another, and also differentiated and named alpha and beta radiation. He was awarded the Nobel Prize in Chemistry in 1908 "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances".

Rutherford performed his most famous work after he received this prize. In 1911, he postulated that atoms have their positive charge concentrated in a very small nucleus, and thereby pioneered the Rutherford model, or planetary, model of the atom, through his discovery and interpretation of Rutherford scattering in his gold foil experiment. He is widely credited with first splitting the atom in 1917, and leading the first experiment to "split the nucleus" in a controlled manner by two students under his direction, John Cockcroft and Ernest Walton in 1932.

Ernest Rutherford was the son of James Rutherford, a farmer, and his wife Martha Thompson, originally from Hornchurch, Essex, England. James had emigrated from Perth, Scotland, "to raise a little flax and a lot of children". Ernest was born at Spring Grove (now Brightwater), near Nelson, New Zealand. His first name was mistakenly spelled Earnest when his birth was registered.

He studied at Havelock School and then Nelson College and won a scholarship to study at Canterbury College, University of New Zealand, where he was president of the debating society, among other things. After gaining his BA, MA and BSc, and doing two years of research at the forefront of electrical technology, in 1895 Rutherford travelled to England for postgraduate study at the Cavendish Laboratory, University of Cambridge (1895–1898), and he briefly held the world record for the distance over which electromagnetic waves could be detected.

In 1900 he married Mary Georgina Newton (1876–1945); they had one daughter, Eileen Mary (1901–1930), who married Ralph Fowler.
He was knighted in 1914. In 1916 he was awarded the Hector Memorial Medal. In 1919 he returned to the Cavendish as Director. Under him, Nobel Prizes were awarded to Chadwick for discovering the neutron (in 1932), Cockcroft and Walton for an experiment which was to be known as splitting the atom using a particle accelerator, and Appleton for demonstrating the existence of the ionosphere. He was admitted to the Order of Merit in 1925 and in 1931 was created Baron Rutherford of Nelson, of Cambridge in the County of Cambridge, a title that became extinct upon his unexpected death in hospital following an operation for an umbilical hernia (1937). Since he was a peer, British protocol at that time required that he be operated on by a titled doctor, and the delay cost him his life. He is interred in Westminster Abbey, alongside J.J. Thomson, and near Sir Isaac Newton.

During the investigation of radioactivity he coined the terms alpha and beta in 1899 to describe the two distinct types of radiation emitted by thorium and uranium. These rays were differentiated on the basis of penetrating power.

In 1898 Rutherford was appointed to the chair of physics at McGill University in Montreal, Canada, where he did the work that gained him the Nobel Prize in Chemistry in 1908. In 1900 he gained a DSc from the University of New Zealand, and from 1900 to 1903 he was joined at McGill by the young Frederick Soddy (Nobel Prize in Chemistry, 1921) and they collaborated on research into the transmutation of elements. Rutherford had demonstrated that radioactivity was the spontaneous disintegration of atoms. He noticed that a sample of radioactive material invariably took the same amount of time for half the sample to decay — its "half-life" — and created a practical application using this constant rate of decay as a clock, which could then be used to help determine the age of the Earth, which turned out to be much older than most of the scientists at the time believed.

In 1903, Rutherford realized that a type of radiation from radium discovered (but not named) by French chemist Paul Villard in 1900, must represent something different from alpha rays and beta rays, due to its very much greater penetrating power. Rutherford gave this third type of radiation its name also: the gamma ray.

In 1907 Rutherford moved to Britain to take the chair of physics at the University of Manchester. Continuing to work with alpha radiation, in conjunction with Hans Geiger he developed zinc sulfide scintillation screens and ionization chambers to count alphas. By dividing the total charge they produced by the number counted, Rutherford decided that the charge on the alpha was two. In late 1907 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 were trapped. Eventually, the clear spectrum of helium gas appeared, proving that alphas were at least ionized helium atoms, and probably helium nuclei.

In Manchester, along with Hans Geiger and Ernest Marsden he carried out the Geiger–Marsden experiment in 1909, which demonstrated the nuclear nature of atoms. It was his interpretation of this experiment that led him to formulate the Rutherford model of the atom in 1911 — that a very small positively-charged nucleus was orbited by electrons. In 1919 he became the first person to transmute one element into another when he converted nitrogen into oxygen through the nuclear reaction ¹⁴N + α → ¹⁷O + p. In 1921, while working with Niels Bohr (who postulated that electrons moved in specific orbits), Rutherford theorized about the existence of neutrons, which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keeping the nuclei from breaking apart. Rutherford's theory of neutrons was proved in 1932 by his associate James Chadwick, who in 1935 was awarded the Nobel Prize in Physics for this discovery.