Rutherford's expectations before conducting his experiment were primarily based on Thomson's plum pudding model of the atom, which posited that the atom was a diffuse cloud of positive charge with electrons embedded within it, similar to plums in a pudding.
Rutherford anticipated that, due to this even distribution of positive charge and mass throughout the atom, most alpha particles would pass through the gold foil with little or no deflection. He believed that only a small fraction of the alpha particles might be deflected at very small angles due to occasional interactions with electrons, but that the overall impact of the positive charge would not impede their passage significantly.
Given this understanding, the correct expectation based on Thomson's model would be:
All the alpha particles would pass straight through the foil because of the even distribution of mass and charge throughout the atom.
However, Rutherford's experiment revealed that a small number of alpha particles were deflected at large angles, leading to the conclusion that atoms have a small, dense nucleus, fundamentally altering our understanding of atomic structure.