Rutherford’s Gold Foil Experiment
In a series of experiments from 1909 to 1911, Ernest Rutherford established that atoms
have nuclei. His discovery came by accident and as a total surprise. His experiment
consisted of firing alpha particles, which we will examine in more detail shortly, at a
very thin sheet of gold foil. Alpha particles consist of two protons and two neutrons:
they are relatively massive (about 8000 times as massive as an electron), positively
charged particles. The idea of the experiment was to measure how much the alpha
particles were deflected from their original course when they passed through the gold foil.
Because alpha particles are positively charged and electrons are negatively charged, the
electrons were expected to alter slightly the trajectory of the alpha particles. The
experiment would be like rolling a basketball across a court full of marbles: when the
basketball hits a marble, it might deflect a bit to the side, but, because it is much bigger
than the marbles, its overall trajectory will not be affected very much. Rutherford
expected the deflection to be relatively small, but sufficient to indicate how electrons are
distributed throughout the “plum pudding” atom.
To Rutherford’s surprise, most of the alpha particles were hardly deflected at all: they
passed through the gold foil as if it were just empty space. Even more surprising was that
a small number of the alpha particles were deflected at 180º, right back in the direction
they came from.
This unexpected result shows that the mass of an atom is not as evenly distributed as
Thompson and others had formerly assumed. Rutherford’s conclusion, known as the
Rutherford nuclear model, was that the mass of an atom is mostly concentrated in a
nucleus made up of tightly bonded protons and neutrons, which are then orbited by