Question

why do most of the a-particle pass straight through the gold foil

Answer 1

In Rutherford's gold foil experiment, the particles of which most passed straight through the gold foil were alpha particles, which are positively charged. The few alpha particles that were deflected back or to one side were being repelled by nuclei, which are also positively charged.

Answer 2

it was one of his experimental observation which meant that much of the space in the atom is empty.. therefore the rays travelled straight without no deflection....