Oxidation Number – IGCSE Chemistry Definition

Oxidation numbers (also called oxidation states) are used to track electron transfer in redox reactions. On the 0620 syllabus (Supplement), oxidation numbers appear most often as Roman numerals in compound names (e.g. iron(III) oxide) and in determining whether oxidation or reduction has occurred.

The 0620 definition

An oxidation number is a number assigned to an atom that indicates its degree of oxidation in a compound or ion. An increase in oxidation number = oxidation. A decrease in oxidation number = reduction.

Rules for assigning oxidation numbers

Rule	Examples
Elements in their natural state = 0	Fe = 0, O₂ = 0, Na = 0
Simple ions = charge of the ion	Na⁺ = +1, Cl⁻ = -1, Fe²⁺ = +2, O²⁻ = -2
Oxygen is usually -2	In H₂O, MgO, CO₂, each O = -2
Hydrogen is usually +1	In H₂O, HCl, each H = +1
Sum of oxidation numbers = overall charge	In a neutral compound, sum = 0; in an ion, sum = charge

Using Roman numerals in names

When a metal can form more than one ion, Roman numerals specify which one:

Name	Metal ion	Oxidation number of metal
Iron(II) chloride, FeCl₂	Fe²⁺	+2
Iron(III) chloride, FeCl₃	Fe³⁺	+3
Copper(I) oxide, Cu₂O	Cu⁺	+1
Copper(II) sulfate, CuSO₄	Cu²⁺	+2
Manganese(IV) oxide, MnO₂	Mn⁴⁺	+4
Lead(II) bromide, PbBr₂	Pb²⁺	+2

Tracking redox using oxidation numbers

If the oxidation number of an atom increases during a reaction, it has been oxidised (lost electrons).

If the oxidation number decreases, it has been reduced (gained electrons).

Example: 2Na + Cl₂ → 2NaCl

• Na: 0 → +1 (increase = oxidised)
• Cl: 0 → -1 (decrease = reduced)

Example: CuO + H₂ → Cu + H₂O

• Cu: +2 → 0 (decrease = reduced)
• H: 0 → +1 (increase = oxidised)

Worked exam question

In the reaction: Fe₂O₃ + 3CO → 2Fe + 3CO₂. (a) State the oxidation number of iron in Fe₂O₃. (1) (b) State the oxidation number of iron in Fe. (1) (c) Has iron been oxidised or reduced? Explain. (1)

Mark scheme

(a) +3 [1] (each O = -2, three oxygens = -6, two irons share +6, so each Fe = +3)

(b) 0 [1] (iron is an element in its natural state)

(c) Reduced [1]; because the oxidation number decreased from +3 to 0 / iron gained electrons

Common exam mistakes

• Confusing the sign of the oxidation number. Metals in compounds have positive oxidation numbers. Oxygen is usually -2.
• Forgetting that elements have an oxidation number of 0. Fe, O₂, Na, and Cl₂ all have oxidation number 0.
• Mixing up the Roman numeral naming convention. Iron(III) means the iron has a +3 oxidation number, not that there are 3 iron atoms.