Early Ideas - John Dalton (1803)
It was thought that matter was made of atoms which were indivisible spheres.
Plum Pudding Model - J.J Thompson
J.J Thompson carried out experiments and discovered the electron which led to the plum pudding model which like before was that matter was made of atoms which were indivisible spheres. But the plum pudding model was a positively charged sphere with electrons embedded in it.
Nuclear Model
Ernst Rutherford
He carried out the Alpha Scattering Experiment which led him to come up with the nuclear model that the mas of an atom is concentrated at the centre in a positively charged nucleus.
Niels Bohr
He discovered that electrons orbit the nucleus in shells/energy levels. Further experiments led to the discovery that the nucleus was composed of smaller positively charged protons.
James Chadwick
He found evidence of particles with mass but no charge in the nucleus named neutrons.
Details
An atom has a radius of ~