How do you find the oxidation number of an element?
Answer
The oxidation number of an element is found by applying standard rules and setting the sum of all oxidation numbers equal to the overall charge of the species.
The working
Free elements
The oxidation number of any element in its free (uncombined) state is zero, e.g. O₂, H₂, Na, Fe are all 0.
Fixed values
Assign known values first: Group 1 metals = +1, Group 2 = +2, hydrogen = +1 (−1 in hydrides), oxygen = −2 (−1 in peroxides), fluorine = −1.
Ions
For a monatomic ion, the oxidation number equals its charge; for example, Na⁺ is +1 and S²⁻ is −2.
Set up the equation
The sum of all oxidation numbers in a neutral molecule is 0, and in a polyatomic ion it equals the ion's charge. Solve for the unknown element.
Example
In H₂SO₄: 2(+1) + x + 4(−2) = 0, giving x = +6 for sulphur.
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