Rutherford's classic experiment consisted in bombarding a thin gold foil with particles (helium nuclei). He expected that some particles would be slightly deflected by the positive charge inside the atoms. Most went through without much change, some were deflected, and surprisingly, some came straight back. This led Rutherford to conclude that the positive charge must be concentrated in a very small volume called the nucleus and not across the entire atom, with a typical radius of the order of m.