ASChemistryAQARevision NotesPhysical ChemistryOxidation, Reduction & Redox EquationsOxidation States: The Rules

Oxidation States: The Rules (AQA AS Chemistry): Revision Note

Exam code: 7404

Written by: Stewart Hird

Reviewed by: Caroline Carroll

Updated on 5 August 2026

Oxidation States

Oxidation State Rules

A few simple rules help guide you through the process of determining the oxidation state of any element
Remember, you are determining the oxidation state of a single atom

Oxidation Rules Table

Rule	Example
Uncombined elements have an oxidation state of 0	H₂ Cl₂ Na
Some elements have the same oxidation state in their compounds with some exceptions	Group 1 elements are always +1 Group 2 elements are always +2 Aluminium is always +3 Hydrogen is always +1 except in metal hydrides where it is -1 Fluorine is always -1 Oxygen is always -2 except in peroxides where it is -1 and OF₂, where it is +2 Chlorine is always -1 except in compounds with F and O where it has a positive value
The sum of all of the oxidation states in a compound is equal to 0	In NaCl: Na= +1 Cl = -1 Sum of oxidation states= 0
The oxidation state of an element in a monoatomic ion is always equal to its charge	Zn²⁺= +2 Fe³⁺ = +3 Cl^-= -1
The sum of the oxidation states in an ion is equal to the charge on the ion	In SO₄^2- S= +6 O= (-2 x 4) = -8 Sum of oxidation states= -2
In a compound, the more electronegative element is given the negative oxidation state	In F₂O F is more electronegative so is (-1 x 2) = -2 O = +2 Sum of oxidation states= 0

Molecules or Compounds

In molecules or compounds, the sum of the oxidation states on the atoms is zero

Oxidation State in Molecules or Compounds

Elements	H in H₂ = 0	Both are the same and must add up to 0
Compounds	C in CO₂ = +4	1 x (+4) and 2 x (-2) = 0
Compounds	O in CO₂ = -2	1 x (+4) and 2 x (-2) = 0

Since CO₂ is a neutral molecule, the sum of the oxidation states must be zero
For this, one element must have a positive oxidation state and the other must be negative

How do you determine which is the positive one?

The more electronegative species will have the negative value
Electronegativity increases across a period and decreases down a group
O is further to the right than C in the periodic table so it has the negative value

How do you determine the value of an element’s oxidation state?

From its position in the periodic table and/or
The other element(s) present in the formula
The oxidation states of all other atoms in their compounds can vary
By following the oxidation state rules, the oxidation state of any atom in a compound or ion can be deduced
The position of an element in the periodic table can act as a guide to the oxidation state

Oxidation States & the Periodic Table

Metals	Non-metals
Have positive values in compounds	Mostly negative based on their usual ion e.g. Cl usually -1
Value is usually the Group number e.g. Al is +3
Where there are several possibilities, the values go no higher than the Group number e.g. Sn can be +2 or +4 Mn can be +2, +4, +6, +7	Can have values up to their Group number e.g. Cl can be +1, +3, +5, +7

Metals

Non-metals

Have positive values in compounds

Mostly negative based on their usual ion

e.g. Cl usually -1

Value is usually the Group number

e.g. Al is +3

Where there are several possibilities, the values go no higher than the Group number

e.g. Sn can be +2 or +4
Mn can be +2, +4, +6, +7

Can have values up to their Group number

e.g. Cl can be +1, +3, +5, +7

Test your understanding on the following examples:

Worked Example

Deducing oxidation states

Give the oxidation state of the elements in bold in these compounds or ions:

a. P₂O₅

b. SO₄^2-

c. H₂S

d. Al₂Cl₆

e. NH₃

f. ClO₂^-

Answers

Electrochemistry Table 2_Oxidation Numbers, downloadable AS & A Level Chemistry revision notes

Are oxidation states always whole numbers?

The answer is yes and no
When you try and work out the oxidation state of sulfur in the tetrathionate ion S₄O₆^2-you get an interesting result!

The oxidation state of sulfur in S₄O₆^2-is a fraction

The fact that the oxidation state comes out to +2.5 does not mean it is possible to get half an oxidation state
This is only a mathematical consequence of four sulfur atoms sharing +10 oxidation state
Single atoms can only have integer oxidation states, because you cannot have half an electron!

Examiner Tips and Tricks

Although there is a slight technical difference between the terms, oxidation state and oxidation number can be used interchangeably in your exams and examiners will know what you are talking about!

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