At the start of the twentieth century, what the inside of an atom looked like was a puzzle. The most common view was J.J. Thomson's "plum pudding" model: a structure in which positive charge is smeared diffusely across the whole atom, with electrons sprinkled through the cloud. New Zealand-born Ernest Rutherford (1871–1937) designed an elegant experiment to test this model.

His team in Manchester (Hans Geiger and Ernest Marsden) fired fast, positively charged alpha particles from radioactive sources at a very thin gold foil. If "plum pudding" were right, the particles should pass through almost undeflected — a diffuse cloud could not seriously deflect them. Most results were like this; but a small minority deflected at large angles, and some bounced straight back. Rutherford described it: "It was the most incredible event of my life — as if you fired a shell at a sheet of tissue paper and it came back and hit you."

The only explanation was this: nearly all of an atom's positive charge and mass was gathered in a very small, very dense central region — the nucleus. Those that bounced back were the few particles that struck this tiny nucleus almost head-on. Rutherford published this model in 1911: the atom is mostly empty, with a minuscule nucleus at its centre and electrons roaming a wide void around it. Enlarge an atom to the size of a stadium and the nucleus is a pea at the centre.

This discovery gave rise to Niels Bohr's 1913 model of electrons in definite orbits, and then to quantum mechanics; the nucleus itself became the subject of nuclear physics, energy, and weapons. Rutherford also showed that radioactivity is one element transforming into another, revealing the true face of a modern alchemy.