Q.Match the correct pair: Au(I)Cl

Stock Notation — Naming the Oxidation State

Many metals can exist in more than one oxidation state. Iron forms both FeO and

Fe₂O₃; copper forms Cu₂O and CuO; manganese runs all the way from +2 to +7. A name

like "iron chloride" or a bare formula leaves you doing detective work every time:

which iron is this?

The German chemist Alfred Stock proposed the fix that chemistry still uses: write

the metal's oxidation number as a Roman numeral in parentheses immediately after the metal's symbol (in a formula) or after its name (in words).

The Core Idea

Stock notation = formula or name + (Roman numeral for the metal's oxidation state).

• Aurous chloride → Au(I)Cl · auric chloride → Au(III)Cl₃
• Stannous chloride → Sn(II)Cl₂ · stannic chloride → Sn(IV)Cl₄
• Ferrous oxide → Fe(II)O · ferric oxide → Fe₂(III)O₃

The Roman numeral is the metal's oxidation number, worked out with the usual rules

(O = −2, halides = −1, H = +1, and the oxidation numbers of a neutral compound sum

to zero).

How to Write It

1. Assign the fixed oxidation numbers to the non-metal partners (O, Cl, I, …).
2. Solve for the metal: e.g. in MnO₂, $x + 2(-2) = 0 \Rightarrow x = +4$.
3. Insert the Roman numeral after the metal's symbol: Mn(IV)O₂ — or after its name: manganese(IV) oxide.

Why It Matters

Stock notation makes the oxidation state part of the compound's identity — which

immediately tells you whether a species is in its oxidised or reduced form. For

example, Hg₂(I)Cl₂ is the reduced form of Hg(II)Cl₂; Fe(II)O can be oxidised to

Fe₂(III)O₃ but Fe₂(III)O₃ cannot be oxidised further as an Fe(III) oxide. In redox

chemistry this labelling is exactly what lets you track who was oxidised and who was

reduced.

Common Mistakes …