Isotopes
Water has several isotopes, especially hydrogen, though oxygen too has isotopes. Known oxygen isotopes range in mass number from 12 to 28. Oxygen has 3 stable isotopes: 16O, 17O, and 18O.
16O is the most abundant: 99.762%. 16O is the principal form by stellar evolution, as 16O can be made by stars that were initially fueled entirely by hydrogen.
17O and 18O are produced later in the stellar life cycle. 17O comes by burning hydrogen into helium during the CNO cycle.
18O is common in the helium-rich zones of stars. 18O is typically produced when 14N (nitrogen), abundantly derived from CNO combustion, captures a 4He (helium) nucleus.
Hydrogen has 2 stable isotopes: protium and deuterium. Protium has a nucleus of a single proton. Deuterium (aka heavy hydrogen) has a nucleus comprising a proton and a neutron. Both protium and deuterium sport a single electron.
Water is typically a mixture of protium and deuterium, naturally varying by source. More than 99.98+% of the hydrogen in ocean water is protium.
Recently evaporated ocean water, including rainwater, river water, and snow, tends to have the lighter isotopes of hydrogen and oxygen. Such waters evaporate faster than heavier varieties.
Heavy water, properly termed deuterium oxide (D2O), has a richer deuterium content. 0.0156% of the hydrogen in ocean water is deuterium.
Rats avoid heavy water by its smell. Humans are generally unaware of the difference (from ordinary water), other than heavy water sometimes has a sweet flavor, or causes a burning sensation.
Conversely, light water is deuterium depleted. Light water has been found beneficial for mammals with cancer.
There are differences in the bonds between light and heavy water, at the atomic and molecular levels.
The distance between deuterium and oxygen nuclei in D2O is 3% shorter than the distance between hydrogen and oxygen in an H2O molecule. Conversely, the hydrogen bonds between one molecule of water and another are 4% longer in heavy water than light water.
Despite their differences, light and heavy water behave much alike. Their melting temperatures differ by less than 4 °C, and they boil at even closer temperatures. This owes to competing nuclear quantum effects (NQEs) being offset along different molecular axes.
One NQE is associated with motion perpendicular to the plane of a water molecule. Another NQE relates to motion parallel to the hydrogen bond.
The difference in nuclear quantum effects between H2 and D2 is that one NQE increases while the other lessens. Hence, a small quantum net effect between the different hydrogen forms, and there is similarity in phase transitions.
Another oddity of water is its lack of chemical purity. Water is never just H2O. A small fraction of any body of H2O splits into positively charged hydrons: hydrogen ions (H+), which are protons without bonded electrons, and negatively charged hydroxyl groups (OH–).
Hydrons latch onto water molecules, forming hydronium ions (H3O+). So-called “pure water” at room temperature balances equal numbers of positive hydronium ions and negative hydroxyl groups, creating a neutral solution (pH = 7).
