Lesson 5 of 11 · 7 min
Stock notation and redox by oxidation number
NCERT §7.3
Iron forms two oxides, FeO and Fe₂O₃, with the same two elements. A Roman numeral tells them apart at a glance.
The lesson in notes
In short
Metals that show more than one oxidation number have it written as a Roman numeral in brackets after the symbol (Stock notation, after Alfred Stock): Au(I)Cl and Au(III)Cl₃, Sn(II)Cl₂ and Sn(IV)Cl₄, Fe(II)O and Fe₂(III)O₃, Mn(IV)O₂.
Oxidation is an increase in the oxidation number of an element; reduction is a decrease.
An oxidising agent contains an element whose oxidation number falls; a reducing agent contains an element whose oxidation number rises.
A redox reaction is one in which oxidation numbers of the reacting species change. If no oxidation number changes, the reaction is not redox.
In CH₄ + 4Cl₂ → CCl₄ + 4HCl carbon goes from −4 to +4, so methane is oxidised even though no electron is fully transferred.
In 2Cu₂O + Cu₂S → 6Cu + SO₂, copper falls from +1 to 0 and sulphur rises from −2 to +4. Cu₂S is both the reducing agent (through S) and partly reduced (through Cu).