Who discovered the atom?
By most standards, John Dalton is credited with having been the first to propose (and somewhat prove) that all matter was made of atoms. He did this in the early 1800s in his experiments calculating the atomic weights of gaseous elements. Isaac Newton also proposed such a theory in the 1600s although the technology in his time didn’t allow him to experimentally prove his theory. Even before Newton was the ancient Greek philosopher Democritus who had a similar proposal.
The first scientist to significantly “prove” the existence of atoms was Rutherford who “invented the cloud chamber and used it to show that when thin gold foil is bombarded by helium nuclei (alpha particles), the particles are occasionally deflected by a very large angle, but usually pass straight through. This gave rise to the realization that the gold was composed of atoms, with a tiny nucleus at the middle which could occasionally collide with an alpha particle and send it flying.” [source: newton.dep.anl.gov]