John Dalton described atoms as solid spheres, and said that different-sized spheres made up the different elements.

J J Thomson found that atoms contain even smaller, negatively charged particles - electrons.

He proposed a new theory in which the atom was a ball of positive charge, with discrete electrons throughout it. It was known as the 'plum pudding model’.

The 'plum pudding model' was a theory proposed by J J Thompson in which the atom is a ball of positive charge, with discrete electrons throughout it. Like a 'pudding' with bits of ‘plum’ spread throughout it. 

Experiment: Ernest Rutherford and his student conducted the alpha particle scattering experiments where they fired positively charged alpha particles at an extremely thin sheet of gold.

Results: Rather than all of the alpha particles passing straight through the sheet of gold as you would expect from the plum pudding model, some were deflected to the side, and a small number were even deflected backwards.

Proposed by Ernest Rutherford.

The nuclear model of the atom is the idea that each atom is composed of a small positively charged nucleus at the centre, surrounding by a ‘cloud’ of negative electrons.

The electrons should be attracted to the positive nucleus, so the atom should just collapse.

Niels Bohr suggested that electrons orbit the nucleus in shells, which are a fixed distance from the nucleus.

This was in contrast to Ernest Rutherford's theory in which the electrons were spread in a ‘general cloud’ around the nucleus.

James Chadwick discovered neutral particles called neutrons.