Variable-Charge Metals
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Many transition metals and some main-group metals form more than one stable cation charge, unpredictable from their periodic-table position alone.
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Iron forms both Fe2+ and Fe3+, and copper forms both Cu+ and Cu2+, each pair stable in different compounds.
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A single formula like "iron chloride" is genuinely ambiguous here — FeCl2 and FeCl3 are different compounds needing different names.