In about 1800 John Dalton pictured atoms as tiny solid spheres. He thought they were indivisible, which means they could not be split into anything smaller, and that each element had its own type of atom.
In 1897 J J Thomson discovered the electron, a tiny negatively charged particle far smaller than an atom. This showed that atoms can be divided. Thomson suggested the plum pudding model: a ball of positive charge with negative electrons spread through it.
Later, Rutherford's team fired alpha particles at thin gold foil. Most went straight through, a few were deflected through large angles and a very few bounced back. This showed that the positive charge and most of the mass are in a tiny nucleus at the centre, with the rest of the atom empty space. The plum pudding model was replaced by the nuclear model. Bohr then suggested that electrons orbit the nucleus in shells at fixed distances, and Chadwick later discovered the neutron, which explained the rest of the nuclear mass.
