Isotopes

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isotope , in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F. Soddy in explaining aspects of radioactivity; the first stable isotope (of neon) was discovered by J. J. Thomson . The nuclei of isotopes contain identical numbers of protons, equal to the atomic number of the atom, and thus represent the same chemical element, but do not have the same number of neutrons. Thus isotopes of a given element have identical chemical properties but slightly different physical properties and very different half-lives, if they are radioactive (see half-life ). For most elements, both stable and radioactive isotopes are known. Radioactive isotopes of many common elements, such as carbon and phosphorus, are used as tracers in medical, biological, and industrial research. Their radioactive nature makes it possible to follow the substances in their paths through a plant or animal body and through many chemical and mechanical processes; thus a more exact knowledge of the processes under investigation can be obtained. The very slow and regular transmutations of certain radioactive substances, notably carbon-14, make them useful as "nuclear clocks" for dating archaeological and geological samples. By taking advantage of the slight differences in their physical properties, the isotopes may be separated. The mass spectrograph uses the slight difference in mass to separate different isotopes of the same element. Depending on their nuclear properties, the isotopes thus separated have important applications in nuclear energy. For example, the highly fissionable isotope uranium-235 must be separated from the more plentiful isotope uranium-238 before it can be used in a nuclear reactor or atomic bomb .

isotopes Forms of elements with the same chemical properties, differing in atomic mass because of differing numbers of neutrons in the nucleus. Thus, hydrogen has three isotopes, of atomic masses 1, 2, and 3, generally written as ¹H, ²H (deuterium), and ³H (tritium). ¹H is the most abundant isotope of hydrogen; ²H is stable, while ³H is radioactive.

Radioactive isotopes are unstable, and decay to stable elements, emitting radiation in the process. This may be α‐radiation, β‐radiation (electrons), γ‐radiation, or X‐rays, depending on the isotope. The time taken for half the radioactivity to decay is the half‐life of the isotope, and can vary from a fraction of a second, through several days to years (e.g. the half‐life of ³H is 12½ years, that of ¹⁴C is 5200 years).

Stable isotopes can be detected only by their different atomic mass. Since they emit no radiation, they are safe for use in labelled compounds given to human beings. Examples of stable isotopes commonly used in nutrition research include ²H, ¹³C, ¹⁵N, and ¹⁸O.

isotope One or more atomic variants of a chemical element which have the same number of protons in their nucleus but different numbers of neutrons. Most elements have several stable isotopes; in addition, a few elements have natural radioactive isotopes (radioisotopes) which are unstable. These radioactive nuclei (the parent isotopes) disintegrate spontaneously into different atoms, often of a different element (the daughter isotopes). The ratio of the parent and daughter isotopes is used in radioactive age dating.

isotope One of two or more atoms of the same element that have the same number of protons in their nucleus but different numbers of neutrons. Hydrogen (1 proton, no neutrons), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons) are isotopes of hydrogen. Most elements in nature consist of a mixture of isotopes.