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?