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Radioactive decay is the process in which an unstable atom's nucleus will lose energy via radiation, until its structure eventually breaks down through the loss of subatomic particles (such as neutrons or electrons), causing it to transmutate into another element or isotope. Decay can come in several forms, such as alpha decay, where the nucleus emits an alpha particle of two neutrons and two protons, or beta decay, where one electron or positron is emitted. Decay will continue until the atom reaches a stable state, and the atoms may transmutate several times before stabilizing, which is known as a decay chain. For example, a uranium-238's decay chain may see it transmutate into 14 different isotopes of seven different elements until it eventually stabilizes as lead-206. Most elements have stable isotopes, which are those most commonly-found in nature, however there are 37 known elements with no stable isotopes and these are known as the "radioactive elements".

Half Life

Radioactive decay is completely random, it may take milliseconds or millions of years, and there is no way of predicting when an individual atom will break down - therefore, the concept of "half-life" is used to measure the rate of decay. Half-life refers to the length of time it takes for half of the atoms of a particular isotope to decay. However, half-life is a process of exponential decay, which means that after each half life period the equation is repeated - i.e.one half-life period will leave 50 percent of the original amount, two-half life periods will leave 25 percent, three half-lives will leave 12.5 percent, and so on. Hypothetically the process is infinite until the final atom transmutates.

While the figures shown refer to the half-lives of each element's most stable isotope, less stable isotopes may have shorter half lives. For example, the half-life of uranium-235 (that most-commonly used in reactors and bombs) is 703.8 million years, roughly six times shorter than uranium-238. Similarly, plutonium-239 (also more fissile) has a half life of 24,110 years, which is over 3,300 times shorter than that of plutonium-244.