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Oxidation Number

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Oxidation Number

Summary: An oxidation number is a value assigned to an atom in a compound or polyatomic ion that represents its degree of oxidation or reduction. It indicates the number of electrons an atom has gained, lost, or shared during bond formation. Oxidation numbers follow a set of IGCSE rules and are used to determine whether a chemical species has been oxidised or reduced in a reaction. An increase in oxidation number signifies oxidation (loss of electrons), while a decrease signifies reduction (gain of electrons). Roman numerals in compound names, such as iron(II) or iron(III), directly correspond to the oxidation number of the metal ion.

Tags: chemistry oxidation-number redox igcse oxidation-states

Created: 2026-07-13

An oxidation number (also called oxidation state) is a number assigned to an atom in a compound or polyatomic ion that indicates the degree to which that atom has been oxidised or reduced relative to its elemental state. The concept provides a systematic way to keep track of electron transfer in chemical reactions, even when the bonding is covalent rather than ionic. Oxidation numbers are not true ionic charges in covalent molecules, but they serve as a bookkeeping tool that allows chemists to identify redox processes. The assignment follows a consistent set of rules: atoms in their elemental form always have an oxidation number of zero; the oxidation number of a monatomic ion equals its charge; in a neutral compound the sum of all oxidation numbers is zero; and in a polyatomic ion the sum equals the overall charge of the ion. By comparing the oxidation numbers of each atom before and after a reaction, one can determine whether a redox reaction has occurred (i.e. whether any atom has changed oxidation number) and identify which species have been oxidised or reduced.

Definition and Purpose

An oxidation number is a numerical value that reflects the apparent charge an atom would have if all bonds were treated as purely ionic. In IGCSE chemistry, oxidation numbers are primarily used to:

  • Identify redox reactions (reactions involving both Oxidation and Reduction)
  • Determine which species has been oxidised and which has been reduced
  • Write and interpret chemical formulas
  • Understand the nomenclature of transition metal compounds (e.g. iron(II) chloride vs iron(III) chloride)

The concept is closely tied to OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons).

Rules for Assigning Oxidation Numbers

The Cambridge IGCSE Chemistry syllabus specifies four key rules for assigning oxidation numbers. These rules are applied in the following order:

Rule 1: Elements in Their Uncombined State Have an Oxidation Number of 0

Any element that exists in its standard elemental (uncombined) state has an oxidation number of zero. This applies to:

  • Monatomic elements: Fe (iron), Cu (copper), Na (sodium) — each has oxidation number 0
  • Diatomic molecules: H2, O2, N2, Cl2, F2, Br2, I2 — each atom has an oxidation number of 0
  • Polyatomic elemental forms: P4, S8 — each atom has an oxidation number of 0

Hence, in Cl2(g), each chlorine atom has an oxidation number of 0.

Rule 2: The Oxidation Number of a Monatomic Ion Equals Its Charge

For a simple (monatomic) ion, the oxidation number is exactly equal to the charge on the ion. The sign and magnitude are the same.

Examples:

IonChargeOxidation Number
Na++1+1
Mg2++2+2
Al3++3+3
O2--2-2
Cl--1-1
S2--2-2

This rule is intuitive: for a simple ion, the oxidation number is simply the charge it carries.

Rule 3: The Sum of Oxidation Numbers in a Neutral Compound Is 0

In a neutral chemical compound (having no overall charge), the sum of the oxidation numbers of all atoms must equal zero.

Example: In H2O, hydrogen is typically +1 and oxygen is typically -2. Verification: 2(+1) + (-2) = 0.

This rule allows us to deduce an unknown oxidation number when the others are known. For instance:

In FeCl3: each Cl is -1 (Rule 2 applied to Cl- in its ionic form). Let the oxidation number of Fe be x.

  • x + 3(-1) = 0
  • x = +3

Under the IUPAC naming convention, this compound is called iron(III) chloride.

Rule 4: The Sum of Oxidation Numbers in a Polyatomic Ion Equals the Charge on the Ion

For a polyatomic ion (an ion composed of multiple atoms), the sum of the oxidation numbers must equal the overall charge of the ion.

Example: In SO42- (sulfate ion), oxygen is typically -2. Let the oxidation number of sulfur be x.

  • x + 4(-2) = -2
  • x - 8 = -2
  • x = +6

Another example: In NO3- (nitrate ion), let the oxidation number of N be x.

  • x + 3(-2) = -1
  • x - 6 = -1
  • x = +5

Common Oxidation Numbers to Memorise

For IGCSE, it is essential to know the most common oxidation numbers of key elements and groups. These serve as a starting point when applying Rule 3 and Rule 4 to deduce unknown oxidation numbers.

Atom / GroupCommon Oxidation NumberNotes
Group I (Li, Na, K, etc.)+1Always +1 in compounds
Group II (Be, Mg, Ca, etc.)+2Always +2 in compounds
Group III (Al)+3Aluminium is always +3
Hydrogen (H)+1Except in metal hydrides (e.g. NaH) where it is -1 (beyond IGCSE)
Oxygen (O)-2Except in peroxides (e.g. H2O2) where it is -1 (beyond IGCSE)
Group VII (F, Cl, Br, I)-1Usually -1 in halides; fluorine is always -1
Transition metalsVariablee.g. Fe can be +2 or +3; Cu can be +1 or +2

Oxidation and Reduction in Terms of Oxidation Number

At the IGCSE level, redox processes are defined in three equivalent ways. The oxidation number definition is one of the most powerful for identifying redox reactions that do not involve obvious transfer of oxygen or hydrogen.

Oxidation

Oxidation is defined as an increase in oxidation number. This corresponds to a loss of electrons by the species being oxidised.

Example: 2Na(s) + Cl2(g) → 2NaCl(s)

  • Na in its elemental form has an oxidation number of 0
  • Na in NaCl has an oxidation number of +1 (as Na+)
  • The oxidation number of Na has increased from 0 to +1: Na has been oxidised

A species that is oxidised is called a Reducing Agent.

Reduction

Reduction is defined as a decrease in oxidation number. This corresponds to a gain of electrons by the species being reduced.

Continuing the above example:

  • Cl in Cl2 has an oxidation number of 0
  • Cl in NaCl has an oxidation number of -1 (as Cl-)
  • The oxidation number of Cl has decreased from 0 to -1: Cl has been reduced

A species that is reduced is called an Oxidising Agent.

Identifying Redox Reactions

A reaction is classified as a redox reaction if there is a change in oxidation number for any atom across the reaction. If no atom changes oxidation number, the reaction is not a redox reaction.

Worked example: Is the following reaction a redox reaction?

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

AtomReactants (oxidation numbers)Products (oxidation numbers)Change?
Na+1 (in NaOH)+1 (in NaCl)No
O-2 (in NaOH)-2 (in H2O)No
H+1 (in NaOH and HCl)+1 (in H2O)No
Cl-1 (in HCl)-1 (in NaCl)No

No atom changes oxidation number, so this neutralisation reaction is not a redox reaction. This is a useful confirmation that acid-base neutralisation reactions are generally not redox processes.

By contrast, in: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

  • Zn changes from 0 to +2 (oxidised)
  • Cu changes from +2 to 0 (reduced) This is a redox reaction.

Roman Numerals and Oxidation Numbers

In IGCSE nomenclature, Roman numerals in brackets immediately after a metal name indicate the oxidation number of that metal. This system (Stock nomenclature) is essential for naming compounds of transition metals, which can exhibit variable oxidation states.

FormulaSystematic NameOxidation Number of MetalIon
FeCl2Iron(II) chloride+2Fe2+
FeCl3Iron(III) chloride+3Fe3+
Cu2OCopper(I) oxide+1Cu+
CuOCopper(II) oxide+2Cu2+
MnO2Manganese(IV) oxide+4Mn4+
PbO2Lead(IV) oxide+4Pb4+

Note that the Roman numeral is not the number of atoms of that element in the formula; it is specifically the oxidation number of the metal ion.

To determine which Roman numeral to use, apply Rule 3 (sum in compound = 0) using the known oxidation numbers of the other elements.

Worked Examples

Example 1: Finding the Oxidation Number of Sulfur in H2SO4

  1. Identify the known oxidation numbers: H is +1, O is -2.
  2. Write an equation for the sum:
    • 2(+1) + x + 4(-2) = 0 (Rule 3)
    • 2 + x - 8 = 0
    • x = +6
  3. Answer: The oxidation number of S in H2SO4 is +6.

Example 2: Finding the Oxidation Number of Mn in MnO4-

  1. Identify the known oxidation numbers: O is -2.
  2. Write an equation for the sum:
    • x + 4(-2) = -1 (Rule 4, polyatomic ion charge is -1)
    • x - 8 = -1
    • x = +7
  3. Answer: The oxidation number of Mn in MnO4- is +7.

Example 3: Identifying Oxidation and Reduction

Consider: 2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g)

AtomReactantsProductsChangeProcess
Fe+3 (in Fe2O3)0 (elemental Fe)Decrease by 3Reduction
C0 (elemental C)+4 (in CO2)Increase by 4Oxidation
O-2 (in Fe2O3)-2 (in CO2)No changeNeither

Iron has been reduced (oxidation number decreased), and carbon has been oxidised (oxidation number increased). This confirms that the extraction of iron in the blast furnace is a redox process.

Visual Summary

The relationship between oxidation number changes and redox processes can be summarised on a number line:

Reduction (gain of electrons)
← ← ← ← ← ← ← ←
... -3   -2   -1    0   +1   +2   +3 ...
                  → → → → → → → →
              Oxidation (loss of electrons)
  • Moving right on the number line = oxidation (increase in oxidation number)
  • Moving left on the number line = reduction (decrease in oxidation number)

Sources

  • Cambridge IGCSE Chemistry 0620 syllabus, Section 6.4: Redox
  • Cambridge IGCSE Chemistry Coursebook (4th edition), Richard Harwood and Ian Lodge, Cambridge University Press
  • Redox Reactions
  • Oxidation
  • Reduction
  • OIL RIG

Common Misconceptions

MisconceptionCorrect Understanding
”The oxidation number of oxygen is always -2.”In most compounds oxygen is -2, but in peroxides (e.g. H2O2) the oxidation number of oxygen is -1. For IGCSE, assume oxygen is -2 unless told otherwise.
”The oxidation number of hydrogen is always +1.”In most compounds hydrogen is +1, but in metal hydrides (e.g. NaH) the oxidation number of hydrogen is -1. For IGCSE, assume +1 unless told otherwise.
”A Roman numeral in a name tells you how many atoms of that element are in the formula.”The Roman numeral indicates the oxidation number of the metal ion, not the number of atoms. For example, in copper(II) oxide (CuO), the “(II)” means Cu has an oxidation number of +2, not that there are two copper atoms.
”If oxidation numbers do not change, it is still a redox reaction because ions are present.”A reaction is only redox if at least one atom changes oxidation number. Reactions such as precipitation, acid-base neutralisation, and double displacement do NOT involve any change in oxidation number and are therefore not redox reactions.
”A positive oxidation number means the atom has gained protons.”Oxidation numbers reflect the relative electron distribution, not the number of protons (which never changes in chemical reactions). A positive oxidation number means the atom has fewer electrons assigned to it than in its elemental state.

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