But there is a difference in the way binary molecular and binary ionic compounds are named. Because there are often many different molecular binary compounds for the same two elements, numerical prefixes (mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca, undeca, and dodeca) are usually used with molecular compounds. The prefixes are usually omitted in ionic compound names, because the names of the cations and anions are usually all you need to know to work out the stoichiometry of the compound. For example, the molecular compounds SF4 and SF6 are sulfur tetrafluoride and sulfur hexafluoride; but the ionic compounds FeF2 and FeF3 are iron(II) fluoride and iron(III) fluoride.
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