How was the nucleus of an atom discovered?
How was the nucleus of an atom discovered? By means of the classic gold foil experiment of Marsden, Rutherford, and Geiger, 1909, at the University of Manchester.
Before this experiment, it was thought that matter had a plum pudding structure (see J. J. Thomson: "positive ions in a sea of electrons"). Marsden (under Rutherford's and Geiger's direction) fired alpha particles at a thin gold plate. (FYI, gold is extremely malleable: it can be hammered out to very thin surfaces, only a couple of atoms thick as it was in this experiment.)
Most of the alpha particles passed straight through the plate; some were deflected; a few of the alpha particles bounced straight back at the generator. This last result was completely unexpected given the thickness of the sheets. In Rutherford's words,
" It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you. "
Rutherford could only account for this phenomenon by proposing a central atomic nucleus, which contained all the positive charge, and most of the mass. Thus began the nuclear age: matter was composed a central, heavy nuclear core about which electrons circulated.