Why did Rutherford expect the alpha particles to be only scattered a bit by the gold nucleus?
In the plum pudding model, negative charge is distributed throughout the positively charged nucleus. Although an alpha particle (which has positive charge) would be deflected significantly by a large positive nucleus, the distributed electrons would serve to make the whole atom effectively neutral with only small local deviations from neutrality. The purpose of the experiment was to determine the distribution of charge within the nucleus. Since Rutherford assumed the charge to be relatively evenly distributed, he expected alpha particles to to be only slightly deflected by the small local deviations in charge within the gold nuclei. Importantly, Rutherford expected all of the alpha particles to show some deflection. The significant finding was that most of the particles were not deflected at all, but a small number were deflected a large amount.
