1.9.2 Oxidation States: The Rules

Definition of Oxidation State

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The oxidation state (or oxidation number) of an atom represents its hypothetical charge if all bonds in a compound were considered completely ionic. This concept helps track electron transfers in redox reactions and determine whether oxidation or reduction has occurred.

Rules for Assigning Oxidation States

Uncombined Elements

The oxidation state of any element in its uncombined form (i.e., not bonded to any other element) is 0.

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Example: The oxidation state of $O_2$ (oxygen molecule) or $Li$ (lithium atom) is 0.

Monatomic Ions

The oxidation state of a simple monatomic ion is equal to the charge on the ion.

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Example: The oxidation state of $Li^+$ is +1.

Compounds

In a neutral compound, the sum of the oxidation states of all atoms must equal 0.

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Example: In sodium chloride $NaCl$ , $Na^+$ has an oxidation state of +1

$Cl^-$ has an oxidation state of -1

Therefore $+1 + (-1) = 0$

Polyatomic Ions

In a polyatomic ion, the sum of the oxidation states of all atoms must equal the overall charge of the ion.

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Example: In the sulfate ion $SO_4^{2-}$ , oxygen typically has an oxidation state of -2, and the total for four oxygens is -8.

Therefore, sulfur must have an oxidation state of +6 to balance the charge (i.e., $+6 + (-8) = -2$ ).

Oxygen

Oxygen typically has an oxidation state of -2 in most compounds.
Exceptions:
- In peroxides (e.g., $H_2O_2$ ), oxygen has an oxidation state of -1.
- In oxygen difluoride $OF_2$ , oxygen has an oxidation state of +2.

Hydrogen

Hydrogen usually has an oxidation state of +1.
Exception: In metal hydrides (e.g., $NaH$ ), hydrogen has an oxidation state of -1.

Group 1 and Group 2 Elements

Group 1 elements (e.g., $Na$ ) always have an oxidation state of +1 in their compounds.
Group 2 elements (e.g., $Mg$ ) always have an oxidation state of +2.

Electronegativity

In binary compounds (compounds of two elements), the more electronegative element is assigned the negative oxidation state.

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Example: In $HCl$ , chlorine is more electronegative and has an oxidation state of -1.

While hydrogen has +1.

Transition Metals and p-Block Elements

Transition metals and p-block elements can have variable oxidation states. Their oxidation state in a given compound is usually indicated using Roman numerals.

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Example: $Cu^{2+}$ is referred to as copper(II), with an oxidation state of +2.

Examples of Finding Oxidation States

From Formulae

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Example: Find the oxidation state of zinc in $Zn(OH)_2$

Oxygen has an oxidation state of -2, and hydrogen is +1.

The overall charge of $Zn(OH)_2$ is 0.

Therefore:

x + 2(+1) + 2(-2) = 0

x = +2

Thus, zinc has an oxidation state of +2 in $Zn(OH)_2$

From Systematic Names

Transition metals can have multiple oxidation states, and these are shown using Roman numerals.

Copper(II) means copper has an oxidation state of +2.
Copper(III) means copper has an oxidation state of +3.

Oxidation States: The Rules Simplified Revision Notes for A-Level AQA Chemistry

1.9.2 Oxidation States: The Rules

Definition of Oxidation State

Rules for Assigning Oxidation States

Uncombined Elements

Monatomic Ions

Compounds

Polyatomic Ions

Oxygen

Hydrogen

Group 1 and Group 2 Elements

Electronegativity

Transition Metals and p-Block Elements

Examples of Finding Oxidation States

From Formulae

Example: Find the oxidation state of zinc in $Zn(OH)_2$

From Systematic Names

500K+ Students Use These Powerful Tools to Master Oxidation States: The Rules For their A-Level Exams.

Other Revision Notes related to Oxidation States: The Rules you should explore

Oxidation States: The Rules Simplified Revision Notes for A-Level AQA Chemistry

1.9.2 Oxidation States: The Rules

Definition of Oxidation State

Rules for Assigning Oxidation States

Uncombined Elements

Monatomic Ions

Compounds

Polyatomic Ions

Oxygen

Hydrogen

Group 1 and Group 2 Elements

Electronegativity

Transition Metals and p-Block Elements

Examples of Finding Oxidation States

From Formulae

Example: Find the oxidation state of zinc in Zn(OH)2Zn(OH)_2Zn(OH)2​

From Systematic Names

500K+ Students Use These Powerful Tools to Master Oxidation States: The Rules For their A-Level Exams.

Other Revision Notes related to Oxidation States: The Rules you should explore

Example: Find the oxidation state of zinc in $Zn(OH)_2$