All ordinary matter is made up of combinations of chemical elements, each with its own atomic number. Additionally, elements may exist in different isotopes (each isotope of an element differs in the number of neutrons in the nucleus). A particular isotope of
a particular element is called a nuclide. Some nuclides are inherently unstable. This transformation may include radioactive decay, (either by emission of particles, usually electrons –beta decay-, positrons or alpha particles) or by spontaneous fission, and electron capture.
While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a “daughter” nuclide, or decay product. In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the formation of a stable daughter nuclide. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain.
In general, the half-life of a nuclide depends solely on its nuclear properties; it is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field. In general, the half-life of any nuclide is essentially a constant. Therefore, in any material containing a radioactive nuclide, the proportion of the original nuclide to its decay product(s) changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclide(s) into a material to the present.