Earnest Rutherford (1871-1937), the 4th of 12 children, became J.J. Thompson’s first graduate student at Cambridge University. He started his career studying x-rays but soon turned his attention towards the brand new field of radioactivity.
In his now famous gold foil experiment, Rutherford directed a beam of alpha particles from a radioactive sample of purified Radium toward a micro-thin piece of gold foil.
He placed a ZnS detector on the opposite side of the gold foil. The detector would release a tiny burst of light whenever it was struck with an alpha particle. Rutherford believed that the foil was made of atoms whose structure could best be described by J.J. Thompson’s “plumb pudding” model, meaning that the electrons distributed throughout the atom in a matrix of gel-like positive charge.
As a result, he expected the alpha particles to pass right through the gold sample. In fact, most of the particles did just that. They passed through as if there was no foil at all. However, a tiny fraction of the particles were actually deflected so that they struck the ZnS screen at an angle.
Hans Geiger, Rutherford’s student at the time, modified the experiment in a way that provided a history changing insight into the structure of the atom. By moving the detector to the same side of the foil as the alpha particle emitter, they found that an extremely tiny fraction of the particles actually bounced back!
Rutherford said that in his mind, it was as if he had shot an elephant gun at a piece of tissue paper and the bullet bounced back. Rutherford correctly surmised that the only thing that could cause the particles to bounce back would be something more massive than the particles themselves. He had discovered the tiny nucleus at the center of every atom!
The nucleus is where protons and neutrons are located. It’s size is very small compared to the size of the atom (which itself is very small!). If the atom were the size of a huge domed football stadium, the nucleus of the atom would be roughly the size of a housefly sitting on the 50 yard line.