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
- Assign the fixed oxidation numbers to the non-metal partners (O, Cl, I, …).
- Solve for the metal: e.g. in MnO₂, x+2(−2)=0⇒x=+4.
- 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 …