What is radioactivity, and when was it discovered?
Radioactivity (i.e., radioactive decay) is the process by which unstable atoms that do not have sufficient binding energy to hold their nuclei together emit ionizing radiation, and it occurs in exponential fashion. Alpha decay occurs when an alpha particle (identical to a helium nucleus) is emitted. Beta decay occurs when a beta particle (such as a positron) is emitted. Gamma decay occurs when a gamma ray is emitted during transition of an atomic nucleus to a lower energy state. Henri Becquerel discovered spontaneous radioactivity from uranium in 1896, and Pierre and Marie Curie discovered radium and polonium in 1898. As a result, all three received the Nobel Prize in Physics in 1903.