Rutherford's Gold Foil Experiment: How the Nucleus Was Found

“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.”

That’s how Ernest Rutherford later described the result of an experiment carried out in his laboratory in Manchester. A few alpha particles, fired at a sheet of gold thinner than a hair, had bounced straight back. Nothing in the accepted model of the atom could explain it. Rutherford’s explanation, published in 1911, gave us the picture of the atom we still build on: a tiny, dense nucleus surrounded by electrons.

The scientist

Ernest Rutherford was born in 1871 in New Zealand, the fourth of twelve children of a farming family. A scholarship took him to Cambridge in 1895, where he worked under J.J. Thomson, the discoverer of the electron. He then spent nine years at McGill University in Canada, where he studied radioactivity. There he identified and named alpha and beta radiation and, with Frederick Soddy, showed that radioactive decay transforms one element into another. That work earned him the 1908 Nobel Prize in Chemistry, which amused him, as he considered himself a physicist.

In 1907, he moved to the University of Manchester, where he gathered a talented team, including the German physicist Hans Geiger (later famous for the Geiger counter).

The tool: alpha particles

Rutherford knew that alpha particles were emitted by radioactive elements such as radium, and that they were:

• positively charged (charge +2)
• heavy (about four times the mass of a hydrogen atom; they’re helium nuclei)
• fast (thousands of kilometres per second)

They made perfect probes for exploring the inside of atoms: heavy and fast enough to punch through thin materials, and charged so they’d be deflected by any concentrated charge. See alpha, beta and gamma radiation.

The experiment

Starting around 1908, Geiger and a 20-year-old undergraduate, Ernest Marsden, set up the experiment:

1. A radioactive source emitted a narrow beam of alpha particles in a vacuum.
2. The beam struck a sheet of gold foil only about 0.0004 mm thick, a few hundred to a few thousand atoms thick. Gold was chosen because it can be hammered into extremely thin sheets.
3. Around the foil was a zinc sulfide screen. Each time an alpha particle hit it, it produced a tiny flash of light, a scintillation.
4. Working in a darkened room, the experimenters observed the screen through a microscope and counted the flashes by eye at different angles, sometimes for hours at a time.

What the plum pudding model predicted

At the time, the accepted model was J.J. Thomson’s plum pudding model: an atom was a sphere of spread-out positive charge with electrons embedded in it. See Thomson’s plum pudding model.

If that were true, the positive charge would be diluted throughout the atom, and its electric field would be weak everywhere. The fast, heavy alpha particles should pass through the gold with, at most, tiny deflections of a fraction of a degree.

What actually happened

• Most alpha particles went straight through the foil, undeflected or nearly so.
• Some were deflected through small angles.
• A very small fraction, around 1 in 8,000 in one of Geiger and Marsden’s measurements, were deflected through more than 90°, and some bounced almost straight back.

Rutherford had suggested looking for large-angle scattering almost as an afterthought, not expecting to find any. The result was a genuine shock.

Rutherford’s explanation (1911)

Rutherford spent many months thinking through the mathematics. In 1911, he published his conclusions:

1. Most of the atom is empty space. That’s why most alpha particles passed straight through.
2. The positive charge and nearly all the mass are concentrated in a tiny central region, which he called the nucleus. Only a particle passing very close to this concentrated charge would feel a strong enough repulsion to be turned back.
3. The nucleus is extremely small. Since so few particles were strongly deflected, the target must be tiny. Rutherford estimated the nucleus was at least 10,000 times smaller than the atom.
4. Electrons occupy the space around the nucleus, balancing its positive charge.

Rutherford also derived a formula predicting how many alpha particles should be scattered at each angle, depending on the nuclear charge and the alpha particles’ energy. In 1913, Geiger and Marsden tested it carefully with foils of different metals and thicknesses, and the results matched Rutherford’s predictions. They also showed the nuclear charge was roughly half the atomic weight, an early hint towards the idea of atomic number.

A sense of scale

If an atom were the size of a football stadium, the nucleus would be about the size of a small pea at the centre spot, yet it would hold more than 99.9% of the stadium’s mass. That’s why most alpha particles never came close enough to be deflected. See inside the atomic nucleus and how small is an atom?

What the nuclear model couldn’t explain

Rutherford’s model had a serious problem. According to classical physics, electrons orbiting a nucleus would continuously radiate energy and spiral into the nucleus within a tiny fraction of a second. Atoms shouldn’t be stable.

In 1913, Niels Bohr, who had worked in Rutherford’s Manchester laboratory, fixed this by proposing that electrons occupy fixed energy levels, where they don’t radiate. See the Bohr model.

The nuclear model also didn’t explain what the nucleus was made of. That came later:

• Rutherford identified the proton around 1917–1920. See the proton.
• His colleague James Chadwick discovered the neutron in 1932. See the neutron.
• Henry Moseley, another member of the Manchester group, showed in 1913 that each element’s nuclear charge increases by one from element to element: the atomic number. See atomic number explained.

Why this experiment still matters

• It established the nuclear atom, the basis of all later atomic models.
• It introduced a powerful method: learn about structure by firing particles at a target and studying how they scatter. The same idea underlies modern particle physics, from the discovery of quarks inside protons to experiments at the Large Hadron Collider.
• It’s a model example of the scientific method: a prediction based on an accepted model, an unexpected result, and a new model that explains everything.

Rutherford went on to lead the Cavendish Laboratory in Cambridge from 1919 until his death in 1937. Element 104, rutherfordium, is named after him. See elements named after scientists.

Key takeaways

• Geiger and Marsden, under Rutherford, fired alpha particles at thin gold foil and counted where they went.
• Most passed straight through, but about 1 in 8,000 bounced back at large angles, which the plum pudding model couldn’t explain.
• Rutherford concluded in 1911 that the atom is mostly empty space with a tiny, dense, positive nucleus.
• The nuclear model couldn’t explain why electrons don’t spiral inwards; Bohr’s model addressed this in 1913.
• The experiment’s method, probing structure through scattering, is still central to physics today.