Ernest Rutherford contributed to the atomic theory by discovering that the atom is mostly empty space.
He can to that conclusion because he fired alpha particles at gold foil, which was so thin that it was only around .00004 cm thick, and while almost all shot straight through, some actually bounced back!
He likened it to shooting a 15-inch round(bullet) at tissue paper, only to have it bounce right back at you! Based on this, he theorized that the atom is mostly empty, which is why a majority of the particles passed right through, but in the very center of the atom there is a super-dense structure called a nucleus that held a majority of the atom's mass. This super-dense mass would be more than massive enough to deflect the particle, should they collide.
A. The opportunity to establish the US as a Military power :)
