Radioactivity is the process whereby unstable atomic nuclei release energetic subatomic particles. The word radioactivity is also used to refer to the subatomic particles themselves. This phenomenon is observed in the heavy elements, like uranium, and unstable isotopes, like carbon-14.

Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel, after which the SI unit for radiation, the Becquerel, is named. Becquerel discovered that uranium salts were able to blacken a photographic plate placed in the dark, even through a paper barrier. Subsequent experiments distinguished three distinct types of radiation -- alpha particles, beta particles, and gamma rays. These are positively charged, negatively charged, and neutral, respectively. In the United States, human exposure to radioactivity is measured in rads, where one rad represents 0.01 joule of energy absorbed per kilogram of tissue.

Radioactivity is a random process, meaning that it is physically impossible to predict whether or not a given atomic nucleus will decay and emit radiation at any given moment. Rather, radioactivity is quantified using half-life, which is the period of time it takes for half of the given nuclei to decay. Half-life applies to a sample of any size, from a microscopic quantity to all the atoms of that type in the universe. Half-life varies widely, from a couple seconds (Astatine-218) to billions of years (Uranium-238).

In small doses, radioactivity is a useful process that can be harnessed by man. For example, nuclear reactors exploit radioactivity to generate heat. Phosphorescent materials sometimes include small quantities of radioactive atoms. During pharmaceutical testing, drugs are sometimes laced with radioactive atoms so that they can be more easily traced as they move throughout the body.

In large doses, radioactivity is extremely dangerous. In the Ukraine, a nuclear reactor meltdown incident that occurred during the Cold War era continues to have deleterious effects on the local population to this very day. Many weapons have been designed and tested which use radioactivity to kill people in large numbers. As the level of our science and technology progresses, we can only hope that the peaceful applications of radioactivity will be developed more rapidly than applications for warfare.