When alpha particles are sent towards a thin metal foil, most of them go straight through the foil because :

An electron is 7294 times less heavy than the alpha particle.
Atom has mostly empty space.
A nucleus is a region where a relatively small mass is concentrated.
An atom's mass is concentrated in its nucleus, with the rest of the atom being empty.
As a result, when delivered through metal foil, positively charged alpha particles pass right through the foil.