In the spring of 1909, in a darkened basement laboratory at the University of Manchester, a twenty-year-old student named Ernest Marsden sat hunched over a microscope, counting flashes. Each tiny scintillation on his zinc sulphide screen marked the arrival of a single alpha particle, fired from a radioactive source at a sheet of gold leaf so thin it was only a few hundred atoms thick. His supervisor, Hans Geiger, had trained him to log the faint sparks for hours at a stretch in the dark. Then their professor, Ernest Rutherford, wandered in with a suggestion that sounded close to a joke: why not see whether any of the particles bounced back?
By every theory of the day, none should have. The reigning picture of the atom belonged to J. J. Thomson, discoverer of the electron, who imagined a diffuse ball of positive charge with electrons dotted through it like fruit in a dessert — the celebrated "plum pudding" model. Such an atom was soft matter all the way through, and an alpha particle — a helium nucleus, thousands of times heavier than an electron and moving at a healthy fraction of the speed of light — ought to have punched straight through with barely a wobble. Rutherford, a New Zealand farmer's son who had already won the 1908 Nobel Prize in Chemistry for his work on radioactivity — he joked that his own instant transmutation from physicist to chemist was the fastest reaction he knew — expected the search to turn up nothing at all.
What Geiger and Marsden found instead became one of the most famous results in the history of physics. Most alpha particles did sail through the gold as predicted. But roughly one in eight thousand was deflected through more than ninety degrees, and a few came almost straight back at the source. Rutherford's astonishment, recalled in a lecture near the end of his life, gave science one of its best-loved lines: "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you."
The fly in the cathedral
Rutherford brooded on the result for the best part of two years before announcing, in 1911, that he knew what the atom must look like. The only thing that could hurl a massive, fast alpha particle backwards was a head-on encounter with something minuscule, heavy and intensely charged. Nearly all of the atom's mass, he argued, is concentrated in a central nucleus tens of thousands of times smaller than the atom as a whole. The standard comparison remains vertiginous: if an atom were scaled up to the size of a cathedral, its nucleus would be about the size of a fly in the nave. Everything else — which is to say, almost everything — is empty space. The table you are leaning on is, atomically speaking, a vacuum with occasional interruptions.
The nuclear atom arrived with one glaring flaw. By the classical physics of the day, electrons circling a nucleus should radiate away their energy and spiral inwards in a fraction of a second, and solid matter should be impossible. The rescue came in 1913 from a young Dane who had worked in Rutherford's own department, Niels Bohr, who bolted the new quantum ideas onto the nuclear model and confined electrons to fixed, permitted orbits. Between the Manchester experiments and Bohr's repairs, the modern atom was born.
Splitting the atom
Rutherford was far from finished. In experiments reported in 1919 he bombarded nitrogen with alpha particles and knocked hydrogen nuclei out of it, turning nitrogen into oxygen — the first artificially induced nuclear transmutation, popularly billed as "splitting the atom" — and in 1920 he gave the hydrogen nucleus its modern name, the proton. As director of Cambridge's Cavendish Laboratory he then presided over a golden age: James Chadwick discovered the neutron there in 1932, the same year Cockcroft and Walton split lithium nuclei with a particle accelerator. Geiger went on to develop the radiation counter that carries his name, and Marsden became a grandee of New Zealand science. Rutherford himself, ennobled as Baron Rutherford of Nelson, died in 1937 and was buried in Westminster Abbey close to Isaac Newton; element 104, rutherfordium, and his portrait on New Zealand's hundred-dollar note keep the score of his fame.
Yet everything runs back to that Manchester basement: a student counting sparks in the dark, a professor's idle question, and a handful of particles that refused to go the right way. The nuclear century — its power stations, its medicine, its bombs — was set in motion by one particle in eight thousand.
Quiz nuggets
- In the 1909 gold foil experiment, about 1 in 8,000 alpha particles bounced back through more than 90 degrees.
- Rutherford compared the backscattering to firing a fifteen-inch shell at tissue paper and having it rebound.
- The flashes were counted by Hans Geiger and Ernest Marsden, working under Rutherford at Manchester.
- Rutherford proposed the nuclear model of the atom in 1911, replacing J. J. Thomson's plum pudding model.
- Rutherford won the 1908 Nobel Prize in Chemistry and is buried in Westminster Abbey near Isaac Newton.