Isotopes (from the Greek iso-, equal, and topos, place; in reference to isotopes of an element having the same position in the periodical table of elements) are forms of a given chemical element that have different atomic masses. The nuclei of isotopes of an element contain identical numbers of protons, and so the isotopes have the same atomic number. Each isotope has a different number of neutrons and thus has a different atomic mass.

Most elements have both stable and radioactive isotopes. Radioactive isotopes of an element are commonly used as tracers in medical, biological, and industrial studies to gain information about physical and mechanical processes. In geology and archaeology, radioactive isotopes are used to determine the age of a sample. Hydrologists find isotopes useful in their research in a variety of ways. They are described in detail on this site.

For additional general information on isotopes and hydrology, see the January/February 2003 issue of Southwest Hydrology focusing on "Tracking Groundwater with Isotopes." The issue includes articles by Brenda Ekwurzel, "Dating Groundwater With Isotopes," (see pdf version or html version) and by James Hogan, "Isotope Hydrology: Web and Print Resources," (see pdf version or html version).

Types of Isotopes:

• Stable Isotopes
• isotopic fractionation
• Radioactive Isotopes
• decay types
• types of radioactive isotopes (by origin)