The concept of an atom — the point at which matter cannon longer be cut — dates back to the ancient Greeks, with the word itself derived from the Greek word “atomos” meaning “indivisible.” Until 1897, scientists believed that atoms had no internal structure and were the smallest units of matter. That was before the discovery of a small negatively charged particle — the electron — by Joseph John Thomson.
In 1904, J.J Thomspon suggested that these particles were embedded in a positively charged substance much like fruit dispersed in plum pudding in his appropriately named plum-pudding model of the atom.
This model was overturned by the Geiger-Marsden experiment, also known as the gold foil experiment or the α-particle scattering experiments, pioneered by Ernest Rutherford and conducted by his protégés, Ernest Marsden and Hans Geiger.
Firing α-particles — which we now know are identical to a helium-4 nucleus — emitted by a radioactive source at a thin sheet of gold foil Rutherford reasoned that if the plum-pudding model of the atom was correct these traveling particles would experience the tiniest of deflections. This is due to the fact that an α-particle is about 7,000 times more massive than an electron.
The 1911 experiments showed that occasionally α-particles experienced a large deflection. While only one in 20,000 alpha particles had been deflected 45° or more this was enough to spark a major rethink of the atom and unveiled the presence of the atomic nucleus.
Rutherford compared the results to firing a 15-inch shell at a sheet of tissue paper and having it bounce back directly at you!
This revealed that the majority of the matter in an atom was concentrated at its center. Rutherford proposed a model of the atom with electrons orbiting a massive positively charged nucleus.
This model in time would be overturned, but it represented a vital step in discovering the proton and the neutron and unveiling atomic structure.