Isotope

Article By:

Horen, Daniel J. Nuclear Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.357000

One member of a (chemical-element) family of atomic species which has two or more nuclides with the same number of protons (Z) but a different number of neutrons (N). Because the atomic mass is determined by the sum of the number of protons and neutrons contained in the nucleus, isotopes differ in mass. Since they contain the same number of protons (and hence electrons), isotopes have the same chemical properties. However, the nuclear and atomic properties of isotopes can be different. The electronic energy levels of an atom depend upon the nuclear mass. Thus, corresponding atomic levels of isotopes are slightly shifted relative to each other. A nucleus can have a magnetic moment which can interact with the magnetic field generated by the electrons and lead to a splitting of the electronic levels. The number of resulting states of nearly the same energy depends upon the spin of the nucleus and the characteristics of the specific electronic level. See also: Atomic structure and spectra; Hyperfine structure; Isotope shift

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