1. Who discovered nucleus?
Ernest Rutherford discovered the nucleus in 1911 from the results he obtained in several experiments in which he bombarded a thin sheet of gold with alpha particles (a sub-microscopic particle with a positive charge) from a radioactive element.
2. What were the observations?
Rutherford observed, by means of a fluorescent screen, to what extent the particles with which he bombarded the sheet were scattered. Most of them traversed the metal sheet without changing direction; however, a few were reflected backwards with small angles.
This result was completely unexpected since at the moment the atom was considered, according to the Thomson model, as a positively charged diffuse sphere, where the negative particles (electrons) were inserted. According to Thomson's model, if an alpha particle collided with an atom, it would pass directly through. Furthermore, it could not be affected by the electric fields of the atom since, according to Thomson model, the electric fields were too weak to affect a transient alpha particle to any significant degree.
3. Conclusions.
When hitting the gold foil, alpha particles had found an electrostatic force much greater than that of Thomson's model, which in turn implied that the positive charge of the atom was concentrated in a volume much smaller than that Thomson imagined.
Using a mathematical analysis of the forces involved, Rutherford showed that the scattering was caused by a small positively charged nucleus, located in the center of the gold atom. In this way, he deduced that most of the atom is empty space, which explained why most of the particles that bombarded the gold foil, passed through it without deviating, thus discovering the atomic nucleus.