isotopologue

In chemistry, isotopologues (also spelled isotopologs) are molecules that differ only in their isotopic composition. They have the same chemical formula and bonding arrangement of atoms, but at least one atom has a different number of neutrons than the parent.

An example is water, whose hydrogen-related isotopologues are: "light water" (HOH or ), "semi-heavy water" with the deuterium isotope in equal proportion to protium (HDO or {{chem2|^{1}H^{2}HO}}), "heavy water" with two deuterium atoms ( or {{chem2|^{2}H2O}}); and "super-heavy water" or tritiated water ( or {{chem2|^{3}H2O}}, as well as {{chem2|HTO [^{1}H^{3}HO]}} and {{chem2|DTO [^{2}H^{3}HO]}}, where some or all of the hydrogen is the radioactive tritium isotope). Oxygen-related isotopologues of water include the commonly available form of heavy-oxygen water ({{chem2|H2^{18}O}}) and the more difficult to separate version with the {{chem2|^{17}O}} isotope. Both elements may be replaced by isotopes, for example in the doubly labeled water isotopologue {{chem2|D2^{18}O}}. Altogether, there are 9 different stable water isotopologues, and 9 radioactive isotopologues involving tritium, for a total of 18. However only certain ratios are possible in mixture, due to prevalent hydrogen swapping.