In Ernest Rutherford’s gold foil experiment, most positively charged particles passed through the foil, but a few particles were deflected...

Rutherford inferred from the results of his gold foil experiment that the positive charge and mass of an atom are concentrated in a very small area in the center, and that atoms are mostly empty space. This led to the first nuclear model of the atom with the protons in what's now called the nucleus and electrons outside the nucleus. 

Rutherford used a radioactive source to bombard a thin sheet of gold foil with positive...

Rutherford inferred from the results of his gold foil experiment that the positive charge and mass of an atom are concentrated in a very small area in the center, and that atoms are mostly empty space. This led to the first nuclear model of the atom with the protons in what's now called the nucleus and electrons outside the nucleus. 

Rutherford used a radioactive source to bombard a thin sheet of gold foil with positive alpha particles. A screen similar to a photographic film detected the particles. The paths of the particles could be determined by where they hit the screen. Since most alpha particles passed through undeflected, Rutherford concluded that most of the space of the atom did not contain positive charge or significant mass. Very few deflected straight back, indicating that they had been directed toward a positive area of the atom. (Positive charges repel each other.) This led to the conclusion that the positive area is a very small part of the atom. Some particles were deflected at other angles, as if they had interacted with the positive area but not hit it straight on. Rutherford concluded the regions deflecting the alpha particles must be not only positively charged, but also heavier than the rest of the atom in order to deflect the relatively large alpha particles.