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Metal elements often exhibit multiple oxidation states, which determine the composition of their compounds. While the traditional "-ous" / "-ic" suffixes were once common, chemists now prefer the concise Roman‑numeral system that follows the metal’s name, clearly indicating its oxidation state.

Copper Chlorides

Copper can form two chloride salts: CuCl and CuCl₂. In CuCl the chloride ion carries a -1 charge, requiring copper to be +1. This compound is called copper(I) chloride (cuprous chloride), a white powder used in fireworks and as a dye. In CuCl₂, two chloride ions give a total -2 charge, so copper is +2. The resulting salt is copper(II) chloride (cupric chloride), blue‑green when hydrated; it is employed in fireworks, as a catalyst, and as a pigment.

Iron Oxides

Iron’s oxidation states manifest in several oxides:

• FeO – one oxygen ion (‑2) means iron is +2, named iron(II) oxide (ferrous oxide), abundant in the Earth's mantle.
• Fe₂O₃ – three oxygen ions (‑6) require two iron ions (+6 total), yielding iron(III) oxide (ferric oxide), commonly known as rust.
• Fe₃O₄ – four oxygen ions (‑8) are balanced by three iron ions (+8 total) in a mixed oxidation state (+3 and +2), called iron(II, III) oxide.

Tin Chlorides

Tin commonly adopts +2 and +4 oxidation states:

• SnCl₂ – two chloride ions (‑2) require tin +2, named tin(II) chloride (stannous chloride), a colorless solid used in textile dyeing, electroplating, and food preservation.
• SnCl₄ – four chloride ions (‑4) require tin +4, named tin(IV) chloride (stannic chloride), a colorless liquid at room temperature.

Mercury Bromides

Mercury forms two bromide salts:

• Hg₂Br₂ – two bromide ions (‑2) split between two mercury atoms, giving each a +1 oxidation state. This is mercury(I) bromide (mercurous bromide), used in acousto‑optic devices.
• HgBr₂ – a single mercury atom with a +2 state balances the two bromide ions. This is mercury(II) bromide (mercuric bromide), which is highly toxic.