5.1 In an isotopically modified compound, the isotope used is indicated by its mass number placed as a superscript on the left of the symbol of the element concerned. Deuterium and tritium are written 2H and 3H respectively. The carbon atom in a ring or in a simplified angular-chain representation is explicitly designated when its mass number is shown:

5.2 When atomic symbols in formulae are drawn without square brackets (as above) the compounds are assumed to be isotopically substituted, i.e. the atom concerned is completely replaced by the nuclide shown. To indicate isotopic labelling (partial replacement of the atom by the nuclide shown), atomic symbols in formulae should be in square brackets: