Chemical Formulae and the Periodic Table of Elements 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).


Encyclop&aelig;dia 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?
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.