Rutherford/Geiger–Marsden gold foil experiment: what did they really think about alpha particles? Most tellings of the experiment where alpha particles were fired at a thin gold foil point out the following:


*

*atomic model being tested was the plum pudding model: negatively charged particles (electrons) embedded in a diffuse volume of counteracting positive charge

*because the positive charge is diffuse in the gold foil, the massive positively charged alpha particle should tear right through it with minimal deflection

*while almost all alpha particles passed through, a small minority were deflected strongly, some even knocked backward

*this result was better explained by positive charge being concentrated in a
small volumes separated by empty space


But thinking about it more, this seems like an oversimplification. For one, there's the implication that the alpha particle itself is dense, and some tellings even quote Rutherford's analogy of an artillery shell being fired at tissue paper and occasionally bouncing back. However, if the plum-pudding model were applied to the positively charged alpha particle, then the positive charge would be spread in a diffuse volume. Using the analogy, it'd be firing tissue paper at tissue paper. In that case, why DIDN'T they expect particles to be deflected?
 A: The plum-pudding analogy is only an analogy, and it breaks down when subjected to such close scrutiny. They knew the electrical charges and the masses involved, and the forces produced by those charges, and they did the maths. If the positive charges were spread out through the whole volume of the atom then the alpha particles (with known charge, mass and velocity) wouldn’t deflect by very much, regardless of their own volume. The big deflections only occur because the positive charge of an atom is concentrated into a very small part of its total volume. 
A: Before Rutherford conducted this experiment, physicists had already gained some ideas of the charge, mass and radius of electrons (e.g. Thomson and Millikan) as well as those of alpha particles. The knowledge of alpha particle came from the study of natural radiation--the alpha decay, which was also the source of alpha particles in Rutherford's experiment. So if you know the charges and masses of electrons and alpha particles, and you want to know the mass distribution of the positively charged part of matters, you can shoot a beam of alpha particles to know.
I hope that answers your question.
