AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The rules for assigning oxidation states.
Full specification wording
Oxidation, reduction and redox equations: The rules for assigning oxidation states.
Oxidation states are assigned by a fixed set of rules applied in order.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Oxidation states are assigned by a fixed set of rules applied in order. An uncombined element is zero. The states in a species sum to its overall charge, so zero for a neutral compound and the ionic charge for an ion. Group 1 is +1, Group 2 is +2 and fluorine is always −1. Hydrogen is +1 except in metal hydrides, where it is −1, and oxygen is −2 except in peroxides, where it is −1, and when bonded to fluorine. Work out the unknown element last, by difference. In sulfate, for example, four oxygens contribute −8 and the ion carries 2⁻, so sulfur must be +6. Signs are always written, and a value quoted without a sign is read as positive.
What earns the marks
- Using the rule that oxidation states sum to the overall charge on the species.
- Assigning the fixed values first, such as oxygen at −2 and hydrogen at +1.
- The correct value of the unknown element, written with its sign.
- Identifying an exception correctly, such as oxygen at −1 in a peroxide or hydrogen at −1 in a metal hydride.
Common mistakes and how to put them right
Setting the total to zero for a polyatomic ion instead of to the ionic charge.
Giving oxygen −2 in hydrogen peroxide, where it is −1.
Multiplying the charge of an ion by the number of atoms, so the sulfate ion is treated as 8⁻.
Omitting a minus sign, which is then read as a positive value.
Confusing the oxidation state of an element with the charge on a whole polyatomic ion.
Exam tips
- Always write the sign, even for a positive value; an unsigned number is taken to be positive.
- Set the calculation out as an equation, for example x + 4(−2) = −2, so the method is visible.
- Check the halogens: a halogen bonded to oxygen or to a more electronegative halogen is positive.
Try the lesson
The start of the Oxidation, reduction and redox equations: The rules for assigning oxidation states. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the oxidation state of sulfur in the compound H₂S?
Listen
Oxidation states are assigned by a fixed set of rules applied in order. An uncombined element is zero. The states in a species sum to its overall charge, so zero for a neutral compound and the ionic charge for an ion. Group 1 is +1, Group 2 is +2 and fluorine is always −1. Hydrogen is +1 except in metal hydrides, where it is −1, and oxygen is −2 except in peroxides, where it is −1, and when bonded to fluorine. Work out the unknown element last, by difference. In sulfate, for example, four oxygens contribute −8 and the ion carries 2⁻, so sulfur must be +6. Signs are always written, and a value quoted without a sign is read as positive. A strong answer earns marks for these points. using the rule that oxidation states sum to the overall charge on the species. assigning the fixed values first, such as oxygen at −2 and hydrogen at +1. the correct value of the unknown element, written with its sign. identifying an exception correctly, such as oxygen at −1 in a peroxide or hydrogen at −1 in a metal hydride. Exam tip: Always write the sign, even for a positive value; an unsigned number is taken to be positive.
Key words
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rules in order
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sum equals the charge
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fixed values and exceptions
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peroxides and metal hydrides
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See every definition
- rules in order
- Oxidation states are assigned by a fixed set of rules applied in order.
- sum equals the charge
- The states in a species sum to its overall charge, so zero for a neutral compound and the ionic charge for an ion.
- fixed values and exceptions
- Group 1 is +1, Group 2 is +2 and fluorine is always −1, with hydrogen and oxygen having stated exceptions.
- peroxides and metal hydrides
- Hydrogen is −1 in metal hydrides and oxygen is −1 in peroxides, where these differ from their usual values.
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