One of the key moments in scientists’ quest to comprehend the structure of matter came when they realized that not all elements are stable. The nuclei of many heavy elements—uranium, radium, and plutonium, to name a few—are unstable, spontaneously decaying into other nuclei and releasing energy in the process. This radioactivity can occur in any of three ways: alpha decay, beta decay, and gamma decay. In the first, an alpha particle (the nucleus of a helium atom, which consists of two protons and two neutrons) comes shooting out of the nucleus at high speed. In the second, an energetic beta particle (an electron or its antiparticle, a positron) is emitted. And in the third, which usually follows immediately after an alpha or beta decay, a high-energy gamma-ray photon radiates from the nucleus.
The amount of energy released in radioactive decay depends on the difference in mass between the original and final nucleus multiplied by the speed of light squared, as expressed by Einstein’s famous E=mc2 equation.