Numerical prefixes valency

For most compounds, in most circumstances, it is unnecessary to specify valency. It can always be dispensed with when no ambiguity results, as can the numerical prefixes. So one still has very many simple names such as sodium chloride, calcium oxide, deuterium oxide, barium chloride. 

Incidentally, valent (valency) is of Latin origin, and so should be used with Latin numerical prefixes— univalent, bivalent, tervalent (or trivalent), quadrivalent, quinquevalent, sexivalent, not the Greek mono-, di-, tetra-, penta-, or hexa-. 

In naming double salts, etc, all the cations are now to be cited first, in alphabetical order, except that H+ is named last. Then all the anions are listed, these also in their alphabetical order. Water considered to be co-ordinately bound to a specific ion is cited as aqua- (note the change from aquo- that was customary before 1965). Numerical prefixes can be omitted if the valencies are constant or stated. Examples are ... 

Other methods for indicating or implying the presence of an atom in a nonstandard valence state have been used, especially the use of the prefix hydro e.g. 108). Such methods are sometimes convenient for simple molecules, but they are difficult to apply generally. A more general method that has seen extensive use utilizes the italicized symbol for the element with a superscript Roman numeral to indicate the valence (e.g. 109). This method has been objected to, however, because of ambiguity the superscript Roman number is also used to indicate oxidation number in inorganic compounds, and italicized atomic symbols are customarily used as locants for substituents. The A convention is a modification of the principle of this method, and avoids the objection. It was made a Provisional Recommendation of lUPAC in 1981. 

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