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Reading about heavy water, I have found that the chemical formula for it can be written 2H2O or D2O.

Deuterium is an isotope of hydrogen that is found in large quantities in water, more than one atom per ten thousand hydrogen atoms has a deuterium nucleus. The isotope is denoted “2H” or “D”, and is normally known as “heavy hydrogen”. Deuterium is used in a number of conventional nuclear reactors in the form of heavy water (D2O), and it will probably also be used as fuel in fusion reactors in the future (Source: ScienceDaily).

Encyclopædia Britannica says the formula can be 2H2O. Is this correct?

2H (or 2H) = D is interesting to me as D is not in the periodic table of elements.

I thought that formulae should be formed using the symbols within the periodic table. What are the rules surrounding these chemical formulae?

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    $\begingroup$ Yes the Encyclopædia Britannica is correct. The 2 exponent before the H means that the atomic mass number is 2. Since H has one proton, that means there is one neutron also. // Deuterium is often denoted with a D, and Tritium with a T. The bare $\ce{^4He}$ nucleus is also known as an $\alpha$ particle. No other element has multiple symbols for isotopes. $\endgroup$ – MaxW Apr 8 at 8:06
  • $\begingroup$ @MaxW - So is it more correct to use superscript 2 or is it down to style requirements in articles? $\endgroup$ – Chris Rogers Apr 8 at 8:18
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    $\begingroup$ If i writing about using H isotopes for some purpose then using D and T is fine. If I were just discussing various isotopes (eg $\ce{^{23}Na}$) in general then I'd use the $\ce{^2H}$ and $\ce{^3H}$ forms for consistency. $\endgroup$ – MaxW Apr 8 at 8:30
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According to this entry in the IUPAC Gold book

https://goldbook.iupac.org/html/D/D01648.html

the symbol for Deuterium is H together with the isotopes notation.

However deuterium and tritium are routinely indicated by D and T respectively. Their use is dictated by the framework. Besides nuclear reactions, when only straightforward chemical properties* are considered, it is convenient to use a single letter rather than H and super/sub-scripts.

In organic nomenclature, while writing names for molecules, the presence of deuterium is signalled by the symbol (d).

*note that isotopic effect might however affect chemical properties at fine levels. The toxicity of heavy water is a notable striking example.

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