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](https://www.sciencedaily.com/releases/2009/05/090511181356.htm)).
> 
> 
> 


Encyclopædia Britannica [says the formula can be](https://www.britannica.com/science/deuterium) 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. 


 7