Rutherford's Atomic Nucleus (1911): The Gold Foil Experiment

Rutherford’s discovery of the atomic nucleus was announced in 1911 by the physicist Ernest Rutherford at the University of Manchester. Building on experiments by Hans Geiger and Ernest Marsden, in which alpha particles fired at thin metal foil occasionally bounced back, he concluded that almost all of an atom’s mass is packed into a tiny, positively charged centre. The finding reshaped physics and chemistry and opened the road to quantum theory and nuclear energy.

Background: what was an atom?

For most of the 19th century, atoms were thought of as tiny, indivisible balls, an idea that went back in spirit to the Greek thinker Democritus. That picture broke down at the end of the century. In 1896 Henri Becquerel discovered radioactivity, showing that some atoms emit radiation, and in 1897 J. J. Thomson identified the electron, a particle far smaller than any atom.

If atoms contained negatively charged electrons, they also had to contain positive charge to stay neutral overall. In 1904 Thomson proposed what became known as the “plum pudding” model: electrons embedded in a diffuse, evenly spread sphere of positive charge, like fruit in a pudding.

Rutherford and alpha particles

Ernest Rutherford (1871–1937) was born in New Zealand and had studied under Thomson at Cambridge. At McGill University in Montreal he showed, with Frederick Soddy, that radioactivity involves the transformation of one element into another, work that earned him the 1908 Nobel Prize in Chemistry. In 1907 he moved to the Victoria University of Manchester.

Rutherford’s favourite tool was the alpha particle, a fast, positively charged particle given off by radioactive elements such as radium. He later showed that alpha particles are the nuclei of helium atoms. Because alpha particles are heavy and fast, they were ideal probes for the inside of matter.

The Geiger-Marsden experiment

At Manchester, the German physicist Hans Geiger studied how alpha particles were scattered as they passed through thin foils of metal. In 1909 Rutherford suggested that Ernest Marsden, a 20-year-old undergraduate, check whether any alpha particles were deflected through large angles.

The apparatus was simple. A radioactive source sent a narrow beam of alpha particles at a very thin metal foil, often gold, which can be beaten extremely thin. Around the foil, a screen coated with zinc sulphide gave off a faint flash, or scintillation, wherever an alpha particle struck it. Working in a darkened room, the experimenters counted the flashes through a microscope. Their eyes needed time to adjust to the darkness before each session, and the counting was slow and tiring, so observers worked in short shifts. Later, Geiger’s electrical counter would make this kind of work much easier.

Most alpha particles passed almost straight through. But a small number were deflected by more than 90 degrees, back towards the source. In their 1909 paper, Geiger and Marsden reported that, for a platinum foil, about 1 in 8,000 alpha particles was turned back in this way. On Thomson’s model this should essentially never have happened.

Rutherford later recalled his astonishment in a well-known lecture, saying it was as incredible “as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

The nuclear model of 1911

Rutherford spent more than a year working out what the results meant. He concluded that a large deflection could only be caused by a single, very strong encounter with a concentrated charge, not by many small nudges from a spread-out one.

He presented his idea to the Manchester Literary and Philosophical Society on 7 March 1911 and published it in the Philosophical Magazine in May 1911. In his model, the atom’s positive charge and almost all its mass were concentrated in a tiny central region, surrounded at a great distance by electrons. Using mathematics, he predicted exactly how the number of scattered particles should vary with angle, foil thickness and particle speed.

The model was strikingly empty. If the atom were the size of a large sports stadium, its nucleus would be roughly the size of a small marble at the centre, yet it would hold almost all the mass. Rutherford was cautious about the details: his paper left open whether the central charge was positive or negative and said little about how the electrons were arranged. Its importance was not widely recognised at first, and few physicists outside Manchester paid much attention in 1911.

Rutherford initially spoke of a “central charge”. The term “nucleus” for this centre came into use in his writings soon afterwards, by 1912. Rutherford also estimated from the scattering data that the central charge was roughly proportional to an element’s atomic weight. In 1913 his former student Henry Moseley used X-rays to show that each element has a characteristic nuclear charge, the atomic number, which gave the periodic table a firm physical basis.

Confirmation and the Bohr model

Geiger and Marsden spent the next two years testing Rutherford’s formula with different metals, foil thicknesses and angles. Their results, published in 1913, matched his predictions closely.

The nuclear atom had a serious problem, however. Classical physics said that electrons circling a nucleus should radiate energy and spiral inwards, making atoms unstable. In 1913 the Danish physicist Niels Bohr, who had worked with Rutherford in Manchester, solved this by applying ideas from quantum theory: electrons could occupy only certain fixed orbits. The Rutherford-Bohr model became the familiar textbook picture of the atom.

Key people

• Ernest Rutherford designed the research programme and interpreted the results. He later became director of the Cavendish Laboratory in Cambridge.
• Hans Geiger led the experimental work and later developed the Geiger counter for detecting radiation.
• Ernest Marsden carried out the key observations as a student, and later became an important scientist in New Zealand.
• J. J. Thomson discovered the electron and proposed the plum pudding model that the experiment overturned.
• Niels Bohr combined Rutherford’s nucleus with quantum ideas in 1913.

Consequences and legacy

The nucleus became the centre of a new science. In 1917 Rutherford found that firing alpha particles into nitrogen produced hydrogen nuclei, which he named protons, the first deliberate splitting of one element into another; he published the result in 1919. James Chadwick, one of his colleagues at Cambridge, discovered the neutron in 1932.

These discoveries led directly to the discovery of nuclear fission in 1938, and from there to nuclear power and nuclear weapons. Scattering experiments of the same basic kind, firing particles at targets to reveal hidden structure, remain central to particle physics, from the discovery of quarks to modern colliders. In the late 1960s, for example, experiments at the Stanford Linear Accelerator Center scattered electrons off protons and found evidence of smaller particles inside them, much as Rutherford’s alpha particles had revealed the nucleus inside the atom.

The element rutherfordium, number 104, is named after him. Rutherford died in 1937 and is buried in Westminster Abbey, near Isaac Newton.

Timeline

• 1897: J. J. Thomson identifies the electron.
• 1904: Thomson proposes the plum pudding model of the atom.
• 1907: Rutherford becomes professor of physics at Manchester.
• 1909: Geiger and Marsden report alpha particles deflected through large angles.
• 1911, 7 March: Rutherford presents his nuclear model in Manchester.
• 1911, May: The model is published in the Philosophical Magazine.
• 1913: Geiger and Marsden confirm Rutherford’s predictions; Bohr publishes his model.
• 1919: Rutherford reports the first artificial nuclear transformation.
• 1932: Chadwick discovers the neutron.

Why it matters

Rutherford’s nucleus showed that the atom is mostly empty space around a tiny, dense core. That simple picture underlies modern chemistry, nuclear physics and quantum mechanics, and it remains one of the clearest examples of an experiment overturning an accepted theory.