text: In chemistry, isotopologues 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 ), "heavy water" with two deuterium isotopes of hydrogen per molecule ( or ), and "super-heavy water" or tritiated water ( or , as well as  and , where some or all of the hydrogen atoms are replaced with the radioactive tritium isotope). Oxygen-related isotopologues of water include the commonly available form of heavy-oxygen water () and the more difficult to separate version with the  isotope. Both elements may be replaced by isotopes, for example in the doubly labeled water isotopologue . All taken together, 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.
  The atom(s) of the different isotope may be anywhere in a molecule, so the difference is in the net chemical formula. If a compound has several atoms of the same element, any one of them could be the altered one, and it would still be the same isotopologue. When considering the different locations of the same isotopically modified element, the term isotopomer, first proposed by Seeman and Paine in 1992, is used.
  Isotopomerism is analogous to constitutional isomerism of different elements in a structure. Depending on the formula and the symmetry of the structure, there might be several isotopomers of one isotopologue. For example, ethanol has the molecular formula . Mono-deuterated ethanol, , is an isotopologue of it. The structural formulas  and  are two isotopomers of that isotopologue.