Rutherford's Gold Foil

In 1909 Geiger and Marsden, working for Rutherford, fired alpha particles (small, fast and positive) at a thin gold foil. In a plum pudding atom, all of them should pass straight through.

Most did. A few bent sharply, and about 1 in 8000 bounced straight back.

The gold foil experiment: most alpha particles pass straight through

Rutherford's answer (1911): all the positive charge and almost all the mass sit in a tiny nucleus. The rest is mostly empty space with the electrons. An atom is about $10^{-10}\ \mathrm{m}$ across, its nucleus about $10^{-14}\ \mathrm{m}$.