Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for this. In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil.

He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about 1000 atoms thick.α-particles are doubly-charged helium ions. Since they have a mass of 4µ, the fast-moving α-particles have a considerable amount of energy. 

It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms. Since the α-particles were much heavier than the protons, he did not expect to see large deflections. But, the α-particle scattering experiment gave totally unexpected results .