isotope
From Glossary of Meteorology
Chemical term describing alternative forms of the same element, differing by the number of neutrons in the nucleus.
Isotopes of the same element only differ in their masses and usually have very similar chemical properties. Small differences in chemical reaction rates may be associated with the differing masses— the kinetic isotope effect; such effects are usually of the order of 1% or less but can be as large as a factor of 7 for transfer of hydrogen/deuterium.
