M01 - M02 Atomic model according to Rutherford

The idea put forward by the Greek philosopher Democritus, that the atom (Greek: atomos , indivisible) was the smallest particle of which matter was made, gave way at the beginning of the twentieth century to the idea that atoms were themselves made of even smaller particles: a very small positively charged central nucleus surrounded by negatively charged electrons.

Rutherford’s scattering experiment (1909) played a crucial role in this development. His colleagues fired a beam of -particles (helium nuclei) at a film of gold foil and found that whilst the majority of particles passed straight through, a small fraction ( about 1 in 8000 ) were deflected through a large angle. To explain these observations, Rutherford suggested that since
-particles are positively charged such deflection could only be caused by the particles coming close to a concentrated region of positive charge.

Only a very small fraction of the -particles were deflected, so he concluded that the region of positive charge which caused the scattering would only occupy a small part of the atom. From this he deduced that the atoms in the metal foil consisted of a central positive nucleus composed of protons, where the mass of the atom was concentrated.

Other models which were current at that time (including Thomson’s “plum pudding” model, which saw the atom as a sphere of positive electricity in which negative electrons were embedded) could not account for the deflection of the -particles.