Isotopologues are molecules that differ only in their isotopic

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 composition. Simply, the isotopologue of a chemical species has at least one atom with a different number of neutron

Neutron

The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s than the parent.

An example is water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, where some of its hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

-related isotopologues are: "light water" (HOH or H₂O), "semi-heavy water" with the deuterium

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 isotope in equal proportion to protium (HDO or ¹H²HO), "heavy water

Heavy water

Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...

" with two deuterium isotopes of hydrogen per molecule (D₂O or ²H₂O), and "super-heavy water" or tritiated water

Tritiated water

Tritiated water is a form of water where the usual hydrogen atoms are replaced with tritium. In its pure form it may be called tritium oxide or super-heavy water. Pure T2O is corrosive due to self-radiolysis. Diluted, tritiated water is mainly H2O plus some HTO . It is also used as a tracer for...

 (T₂O or ³H₂O), where the hydrogen atoms are replaced with tritium

Tritium

Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

 isotopes. Oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

-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.

External links

• Fractional abundance of atmospheric isotopologues, SpectralCalc.com