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I am told that in a chemical equation the metal comes first and then the non metal. for example MgO, ZnSO4,etc. But when both the elements are non metals or metals, the one with the lower atomic number would come first. for example:- H2O,CO2,HF,etc. But why is it that hydroxide is OH- and not HO-? Also why is methane CH4 and not H4C?