Why weren't all the alpha particles deflected in Rutherford experiment?

Answer 1

While most alpha particles passed straight through the foil. A small % of them were deflected at very large angles, some even backscattered. Because alpha particles have about 8000x the mass of an electron and impacted the foil at very high velocitiesIn order for the alpha particles to be deflected by significant amounts, they must pass close to one or more nuclei in the foil. Since nuclei occupy only a very small fraction of the the volume of an atom, and the foil was very thin so it was not very many atoms thick, the likelihood of such close encounters was small and only a small fraction of the alpha particles were deflected by large angles.