Cambridge Physics - Discovery of the Nucleus

'The Bohr Model'
Rutherford found the positive charge is all inside a central nucleus much smaller than the atom. Niels Bohr explained how electrons could orbit this nucleus.

7. The Bohr Atom

Rutherford proposed that the atom contained a massive nucleus containing all of its positive charge, and that the much lighter electrons were outside this nucleus. The nucleus had a radius about ten thousand times smaller than the radius of the atom, only ten femtometres, or one hundred thousand billionth of a metre!

Scattering at large angles occured when the alpha particles came near to a nucleus. The reason that most alpha particles were not scattered at all was that they were passing through the relatively large 'gaps' between nuclei.

Rutherford revised Thomson's 'plum pudding' model, showing how electrons could orbit a positively charged nucleus, like planets orbiting a sun. In 1915 Niels Bohr adapted Rutherford's model by saying that the orbits of the electrons were quantised, meaning that they could exist only at certain distances from the nucleus.