Question

Why were Rutherford's students surprised by the results of the gold foil experiment?

Answer 1

The majority of alpha particles were not repelled, but passed through the gold foil.

Explanation:

Rutherford's group set out to confirm the Thompson 'Plum Pudding' Model of the atom. That is, the Thompson atom was postulated to be a spherical field of positive charge with electrons embedded (suspended) in the volume like plums in a gelatin pudding. If the postulate was right, then the alpha particles (charged helium nuclei => `He^(+2)`) would be reflected away from the gold foil much like rubber balls bouncing off of a wall. However, the majority of alpha particles passed through the gold foil without being affected by the gold foil atoms. A small fraction of particles were deflected at obtuse angles while an even smaller fraction of the `alpha`-particles were deflected back toward the source. These observations led to postulating the 1st two part atom consisting of a small central dense positive nucleus surrounded by a diffuse electron cloud. The postulated structure is often referred to as the Rutherford 'Shell' Model of the atom.