Chemistry / Unit 2 Chem 0316 short lesson, about 20 seconds
Naming Ionic Compounds with Variable-Charge Cations (Stock System)
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The idea
A variable-charge metal's compound is named by adding its charge as a Roman numeral in parentheses right after the metal's name.
In practice
FeCl2 is named iron(II) chloride and FeCl3 is named iron(III) chloride, the Roman numeral distinguishing two otherwise identical-sounding compounds.
Where people slip
The Roman numeral states the metal ion's own charge, never the compound's subscript count or the nonmetal's charge.
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