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?