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Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons. Such atoms are called isotopes, atoms of the same element having different masses. Thus isotopes are atoms of the same element that have different masses due to differences in the number of neutrons they contain. The name comes from the Greek phrase ‘isos topos’, meaning “same place”: because they have the same atomic number, isotopes of the same element occupy the same position on the periodic table. Because the atomic number of any element is established, sometimes isotopes are represented simply with the mass number, thus: 93Ag. They may also be designated with a subscript notation indicating the number of neutrons, so that this information can be obtained at a glance without having to do the arithmetic. For the silver isotope shown here, this is written as Isotopes can also be indicated by simple nomenclature: for instance, carbon-12 or carbon-13.

Many isotopes are stable, meaning that they are not subject to radioactive decay, but many more are radioactive. The latter, also known as radioisotopes, play a significant role in modern life. Carbon-14, for instance, is used for estimating the age of objects within a relatively recent span of time—up to about 5,000 years—whereas geologists and other scientists use uranium-238 to date minerals of an age on a scale with that of the Earth. Concerns over nuclear power and nuclear weapons testing in the atmosphere have heightened awareness of the dangers posed by certain kinds of radioactive isotopes, which can indeed be hazardous to human life. However, the reality is that people are subjected to considerably more radiation from non-nuclear sources. Radioactivity is a term used to describe a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation.

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