J.J. Thomson proposed the plum pudding model of the atom in 1897. Ernest Rutherford's team carried out the alpha particle scattering experiment in 1909. (a)...

(a) Thomson's plum pudding model

In this model, the atom is a sphere of uniform positive charge [1], with negatively charged electrons embedded throughout it, like plums (or raisins) scattered in a pudding. [1]

The positive charge was thought to be spread evenly across the entire volume of the atom, not concentrated in one place. The electrons sat within this positive "cloud," and their negative charges balanced the positive charge, making the atom overall neutral.

(b) Key observation that could not be explained

A small number of alpha particles bounced back (were deflected through angles greater than 90 degrees). [1]

In the plum pudding model, the positive charge is spread thinly across the whole atom. There would be nothing dense or concentrated enough to cause a massive, highly charged alpha particle to reverse direction. The fact that some alpha particles bounced straight back was, as Rutherford said, "as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

(c) Rutherford's nuclear model

The atom has a small, dense, positively charged nucleus at its centre. [1] The electrons orbit the nucleus at a relatively large distance, and the atom is mostly empty space. [1]

The nucleus contains virtually all the mass of the atom in a tiny volume (about 10,000 times smaller in diameter than the atom). This explains why most alpha particles pass straight through (they travel through empty space) and why a few are deflected sharply (they approach the concentrated positive charge closely).

(d) Limitation of the nuclear model

At the time Rutherford proposed his model, the neutron had not yet been discovered, so he could not fully explain the mass of the nucleus using protons alone. [1]

The mass of the nucleus was roughly double what would be expected from protons alone. The neutron was only discovered later by Chadwick in 1932, resolving this discrepancy.