An atom is made up of a nucleus surrounded by electrons. The nucleus contains protons and neutrons. Isotopes are atoms that have the same number of protons, but different numbers of neutrons. The nuclei of some isotopes are unstable. They emit radiation and break down to form smaller nuclei.
An early model of the atom was the plum pudding model. It was disproved by Rutherford’s scattering experiment and replaced by the nuclear model.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science