How ionic compounds named?

You need to indicate the charge of the cation when you are dealing with most transition metals or big metals like Pb or Bi. Unlike group 1 or group 2 elements, transition metals are typically capable of forming cations with many different charges. To give an extreme example, molybdenum can form cations with charges of +6, +5, +4, +3, and +2. So if someone were to say "molybdenum chloride", it could be any one of five compounds: MoCl2, MoCl3, MoCl4, MoCl5, MoCl6 To differentiate them you need to indicate the charge of the molybdenum: Molybdenum (II) chloride = MoCl2 Molybdenum (III) chloride = MoCl3 Molybdenum (IV) chloride = MoCl4 Molybdenum (V) chloride = MoCl5 Molybdenum (VI) chloride = MoCl6