In Rutherford's Gold Foil experiment, he bombarded gold with alpha particles and discovered the ....

electron?

the atom was mostly space.

http://www.rsc.org/chemsoc/timeline//pages/1911.html

nucleus?

that would fit.

Actually, Rutherford's Gold Foil experiment did not involve the discovery of the electron. The experiment was conducted by Ernest Rutherford in 1909 in order to better understand the structure of the atom.

In this experiment, Rutherford bombarded a thin piece of gold foil with alpha particles, which are positively charged particles. According to the prevailing model of the atom at that time, called the Thomson model or the "plum pudding" model, it was believed that the atom consisted of a positively charged sphere with negatively charged electrons embedded within it.

Rutherford expected the alpha particles to pass through the gold foil with only slight deflections due to the positive charge of the atom being evenly spread out. However, to his surprise, some of the alpha particles were deflected at large angles and even bounced back in the direction they came from.

Based on these unexpected results, Rutherford proposed a new model of the atom, known as the nuclear model or the Rutherford model. According to this model, most of the mass of the atom and its positive charge is concentrated in a small, dense nucleus at the center. The electrons then orbit around the nucleus at a distance.

So, in summary, Rutherford's Gold Foil experiment did not involve the discovery of the electron, but rather led to the discovery of the atomic nucleus and the development of a new model of the atom. The discovery of the electron had actually been made by J.J. Thomson in 1897 using cathode ray tube experiments prior to Rutherford's work.