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Why is carbon dioxide written as C02 and not O2C?

This applies to other compounds as well such as water H20 not being 02H
water is H20 by you saying 02H makes no sense because the water molecule has 2 hydrogens, and one oxygen not the other way around...however you could write H20 as OH2 :smile:
well O2H would be a completely different compound, one which doesn't exist, so that would simply be wrong. Sometimes you can rejig the way chemicals are written, for example CH4 can be H4C, but it's just not standard (when drawing chemical structures it's sometimes necessary to do it, but otherwise never), and another chemist looking at your work will know what you mean but think you're a **** for not standardising your work.
Reply 3
I suspect the OP meant OH2 rather than O2H, a further example would be why CaCO3 and not CCaO3 or O3CaC etc?

Most formulae are written from left to right with increasing electronegativity values (if you haven't come across them, simplistically you write formulae starting with elements at the bottom left of the periodic table finishing with those at the top right).

Some formulae like ammonium nitrate, NH4NO3, are conventionally written as ions rather than molecular formulae (but that is because they aren't molecules).

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