In 1909, Ernest Rutherford theorised that the atom has its charge concentrated in a very small nucleus. Under his direction, Hans Geiger and Ernest Marsden conducted an experiment in 1911 to test his hypothesis.
They did this by looking at how alpha particles scattered when they were fired at a thin layer of gold foil. Gold was used for its ability to be hammered into very thin sheets, a few atoms thick.
They surrounded the gold foil with a circular fluorescent screen, which would flash when an alpha particle collided with it. A vacuum was created within the circle so that the alpha particles could travel without being stopped by the air. They then lined up an alpha source perpendicular to the gold foil so that the alpha particles would be fired at the foil. Using a microscope, Geiger and Marsden would then be able to accurately determine where the alpha particles were deflected to by the foil when they struck the screen.
At the time of the experiment, the atom was thought to be similar to a plum pudding which was a model proposed by J.J. Thompson, where the negative charges are the plums inside a positive pudding. If that model was correct, they would expect the alpha particles to not be deflected by large angles because the charge would be spread out evenly across the atom.
Over a period of months, they counted the number of alpha particles deflected at different angles. The majority of the alpha particles were deflected through very small angles but a small number (about 1 in 8000) were deflected through very large angles of about 150 degrees or more. From this, Rutherford concluded that the atom has a very small positively charged nucleus, and that the charge on the nucleus was responsible for the electromagnetic repulsive force on the positively charged alpha particle which caused it to change direction. The fact that only a very small number of particles underwent a large deflection suggested that the nucleus was much smaller in diameter than the atom. His second conclusion was that the nucleus contains nearly all the mass of the atom.
Through various calculations and other experiments using other elements, Rutherford was able to calculate the minimum distance that the alpha particle could approach the nucleus by, and from this determine approximately the size of the nucleus, which he calculated to be below 45 fm. From his work, the nuclear model of the atom was established.
I know I promised some more quantum computing, but unfortunately I have to go camping for four days… so yeah… I forgot to plan ahead… it’ll be here next week.