After a cursory internet search, it appears to me that Hydrogen-3 (a part of Tritiated Water) is the heaviest stable* hydrogen isotope. Oxygen-21 is the heaviest isotope of Oxygen with a half-life of seconds, and Oxygen-16, -17, and -18 are the heaviest stable isotopes (again, this is based on an internet search and may not be accurate).

So could (3H2)(21O) conceivably be a thing? If not, what is the heaviest water possible, and what would some of its theoretical physical properties be? Would the mass overcome surface tension? Would there be stronger or weaker hydrogen bonds? And so on.

  • $\begingroup$ No, water with $^{21}\rm O$ is not much of a thing, and the heaviest realistically possible water would look just like the ordinary heavy water (which in turn you won't tell from ordinary water without instruments), only a bit heavier and radioactive. $\endgroup$ Mar 21, 2021 at 9:16
  • $\begingroup$ $\ce{^3H2^{21}O}$ would be tough to synthesize and study since the half-life of $\ce{^{21}O}$ is only 3.4 seconds. $\endgroup$
    – MaxW
    Mar 21, 2021 at 9:58
  • 1
    $\begingroup$ @MaxW - better than a few picoseconds for $^{26}$O and a few $10^{-23}$ seconds for $^{7}$H $\endgroup$
    – Jon Custer
    Mar 22, 2021 at 22:00
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    $\begingroup$ @IvanNeretin - one of my favorite short papers on isotopes, "Concerning the Taste of Heavy Water" by Urey and Failla, Science 81 (2098) p. 273 (Mar. 15, 1935). Blind taste test of distilled vs pure heavy water. No difference detected. $\endgroup$
    – Jon Custer
    Mar 23, 2021 at 12:43

1 Answer 1


The heaviest water obtained with stable isotopes would be $\ce{D_2^{ 18}O}$. This compound is not well-charactetized itself, but Ref. [1] indicates that the melting point is only about 0.28°C above that of ordinary water. The implication is that overall physical properties of $\ce{D_2^{ 18}O}$ are close to those of heavy water with oxygen-16($\ce{D_2^{ 16}O}$); apart from density the isotopic effect of $\ce{^{18}O}$ versus $\ce{^{16}O}$ is much less than that of deuterium versus protium/"hydrogen".


  1. F. Steckel and S. Shapiro, "Physical Properties of Heavy Oxygen Water. Part 1. Density and Thermal Expansion", Trans. Faraday Soc. 1963, 59, 331-343.
  • $\begingroup$ I can see how an increase in electron delocalization could increase van der Waals forces, but it seems that the lowering of electron energy levels you describe for the hydrogen would have the opposite effect on hydrogen bonding (weakening it), since this would tend to make hydrogen's electron more attracted to its nucleus. $\endgroup$
    – theorist
    Mar 21, 2021 at 18:16
  • $\begingroup$ Is it possible to obtain \ce{T_2^{ 18}O} ? Any idea what it would be like? Edit: I have no idea how to format the way you do, lol $\endgroup$
    – Jay Glenn
    Mar 21, 2021 at 20:41

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