Isotopologues are chemical species that differ only in the isotopic composition of their molecules or ions. Simply, the isotopologue of a chemical species has at least one atom with a different number of neutrons.
An example is water, where some of its hydrogen-related isotopologues are: "light water" (HOH or H2O), "semi-heavy water" with the deuterium isotope in equal proportion to protium (HDO or 1H2HO), "heavy water" with two deuterium isotopes of hydrogen per molecule (D2O or 2H2O), and "super-heavy water" or tritiated water (T2O or 3H2O), where the hydrogen atoms are replaced with tritium isotopes. Oxygen-related isotopologues of water include the commonly available form of heavy-oxygen water (H218O) and the more difficult to separate version with the 17O isotope. Both elements may be replaced by isotopes, for example in the doubly-labeled water isotopologue D218O.
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