The nuclei of elements exhibiting radioactivity are unstable and are found to be undergoing continuous disintegration (i.e., gradual breakdown). The disintegration proceeds at a definite rate characteristic of the particular nucleus; that is, each radioactive isotope has a definite lifetime. However, the time of decay of an individual nucleus is unpredictable. The lifetime of a radioactive substance is not affected in any way by any physical or chemical conditions to which the substance may be subjected.
The rate of disintegration of a radioactive substance is commonly designated by its half-life, which is the time required for one half of a given quantity of the substance to decay. Depending on the element, a half-life can be as short as a fraction of a second or as long as several billion years.
The product of a radioactive decay may itself be unstable and undergo further decays, by either alpha or beta emission. Thus, a succession of unstable elements may be produced, the series continuing until a nucleus is produced that is stable. Such a series is known as a radioactive disintegration, or decay, series. The original nucleus in a decay series is called the parent nucleus, and the nuclei resulting from successive disintegrations are known as daughter nuclei.
There are four known radioactive decay series, the members of a given series having mass numbers that differ by jumps of 4. The series beginning with uranium-238 and ending with lead-206 is known as the 4n+2 series because all the mass numbers in the series are 2 greater than an integral multiple of 4 (e.g., 238=4×59+2, 206=4×51+2). The 4n+1 series, which begins with neptunium-237, is not found in nature because the half-life of the parent nucleus (about 2 million years) is many times less than the age of the earth, and all naturally occurring samples have already disintegrated. The 4n+1 series is produced artificially in nuclear reactors.
Because the rates of disintegration of the members of a radioactive decay series are constant, the age of rocks and other materials can be determined by measuring the relative abundances of the different members of the series. All of the decay series end in a stable isotope of lead, so that a rock containing mostly lead as compared to heavier elements would be very old.
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