# What is the heaviest possible water using oxygen and hydrogen isotopes, and what might it's physical properties be? [closed]

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.

• 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. Mar 21 at 9:16
• $\ce{^3H2^{21}O}$ would be tough to synthesize and study since the half-life of $\ce{^{21}O}$ is only 3.4 seconds.
– MaxW
Mar 21 at 9:58
• @MaxW - better than a few picoseconds for $^{26}$O and a few $10^{-23}$ seconds for $^{7}$H Mar 22 at 22:00
• @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. Mar 23 at 12:43

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".