The alpha particle is emitted by certain radioactive elements as they decay to a stable element. It consists of two protons and two neutrons; it is positively charged. The element that undergoes "alpha decay" changes into a new element whose atomic number is down two and atomic mass is down four from the original element. Alpha decay occurs when a nucleus has so many protons that the strong nuclear force is unable to counterbalance the strong repulsion of the electrical force between the protons. Because of its mass, the alpha particle travels relatively slowly (less than 10% the speed of light), and it can be stopped by a thin sheet of aluminum foil.

When Henri Becquerel first discovered the property of radioactivity in 1896, he did not know that the radiation consisted of particles as well as energy. Ernest Rutherford began experimenting to determine the nature of this...