After the discovery in 1897 of the electron by J.J. Thompson, the idea that there might be some internal structure to atoms (which were previously thought to be truly fundamental and indivisible particles) began to gain steam. The first such model, credited to Thompson himself, was the so-called “plum pudding” model, which paints an amusing mental picture of an atom as a mass of positive matter, with little “plums” of negative electrons scattered throughout.

Rutherford, Geiger, Marsden, and the Atomic Nucleus

In 1909, New Zealand physicist Ernest Rutherford, along with Hans Geiger and Ernest Marsden at the University of Manchester, performed a famous experiment in order to put the plum pudding model of the atom to the test.

In this experiment, alpha particles (a type of naturally-occurring radiation consisting of positively charged helium atoms) were fired at a high velocity into a very thin sheet of gold foil. The trajectories of these particles after passing through the foil were then detected. If the plum pudding model of the atom was correct, then it was assumed that many of the particles passing through would have had their courses slightly altered by the charge within the atoms, though in no dramatic fashion.

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