Oxidation States: The Rules (AQA A-Level Chemistry): Revision Notes
1.9.2 Oxidation States: The Rules
Definition of Oxidation State
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.
Example: The oxidation state of (oxygen molecule) or (lithium atom) is 0.
Monatomic Ions
The oxidation state of a simple monatomic ion is equal to the charge on the ion.
Example: The oxidation state of is +1.
Compounds
In a neutral compound, the sum of the oxidation states of all atoms must equal 0.
Example: In sodium chloride , has an oxidation state of +1
has an oxidation state of -1
Therefore
Polyatomic Ions
In a polyatomic ion, the sum of the oxidation states of all atoms must equal the overall charge of the ion.
Example: In the sulphate ion , oxygen typically has an oxidation state of -2, and the total for four oxygens is -8.
Therefore, sulphur must have an oxidation state of +6 to balance the charge (i.e., ).
Oxygen
- Oxygen typically has an oxidation state of -2 in most compounds.
- Exceptions:
- In peroxides (e.g., ), oxygen has an oxidation state of -1.
- In oxygen difluoride , oxygen has an oxidation state of +2.
Hydrogen
- Hydrogen usually has an oxidation state of +1.
- Exception: In metal hydrides (e.g., ), hydrogen has an oxidation state of -1.
Group 1 and Group 2 Elements
- Group 1 elements (e.g., ) always have an oxidation state of +1 in their compounds.
- Group 2 elements (e.g., ) 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.
Example: In , 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.
Example: is referred to as copper(II), with an oxidation state of +2.
Examples of Finding Oxidation States
From Formulae
Example: Find the oxidation state of zinc in
Oxygen has an oxidation state of -2, and hydrogen is +1.
The overall charge of is 0.
Therefore:
Thus, zinc has an oxidation state of +2 in
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.