Through the Gold Foil Experiments he performed during his graduate studies, Ernest Rutherford discovered that atoms held a nucleus. These experiments and the findings were published in 1911. During this experiment, Rutherford's associate Hans Geiger initiated the beginnings of the Geiger counter as well.
Rutherford discovered that when narrow beams of alpha particles were fired at a thin film of mica or metal, that same beam broadened. The experiment led to the discovery that 1 in 20,000 of these particles turned at a 90-degree angle on occasion. This discovery led Rutherford to the idea that an atom was not just a cloud of floating electrons, but had a positively charged center that held most of its mass. Rutherford named this mass a nucleus. His discovery showed that not only was part of the atom positively charged, but also heavy by atomic standards. He surmised that the like charges in the atoms cause them to deflect from each other like magnets with the same charge repel each other. Rutherford also gathered that the atom's nucleus was relatively small compared to the total mass of the atom. The logical conclusion was that JJ Thompson, who had hypothesized that atoms were clouds of electrons, was partially right since the atom took up such a small area of the total mass.