Question

What caused a large deflection of alpha particles in Rutherford's experiment?

Answer 1

See explanation below

Explanation:

First of all, so there is no confusion, the large deflection refers to the angle of deflection; the majority of the particles went straight through. Alpha particles, as you may know, are positively charged particles. They most passed through the gold foil with no reflection, allowing Rutherford to know that (a) an atom is mostly empty space.

But some of the particles were deflected at large angles; these reflection patterns couldn't be explained with billiard balls (they bounce off each other in different directions, depending on where one ball hits another), so Rutherford turned to Coulomb's law about charges. He then was able to use the law to infer this: The atom has a small, dense, positively charged nucleus at its core.