More on oxidation states

This post gives you more worked examples of how to calculate oxidation states.

OXIDATION STATES

The oxidation state of an element in a compound, molecule or ion is essentially the number of electrons it is released into bonding compared with the uncombined element. You are expected to learn and use the rules for assigning oxidation states. Using the rules of assignment below you can use them to work out the oxidation state of an element in a compound or ion.

Element	Oxidation state	Example
Hydrogen, H	Usually +1	HCl
Group 1 metals	Always +1	NaCl
Group 2 metals	Always +2	MgCl2
Aluminium, Al	Always +3	AlCl3
Oxygen, O	Usually -2	MgO
Fluorine, F	Always -1	NaF

 To calculate the oxidation state of Sulphur in H₂SO₄

H = +1, O= -2 and net charge = 0

So (2 x +1) + S + (4 x -2) = 0

So 2 + S – 8 = 0

So -6 + S = 0

S=+6

To calculate the oxidation state of Nitrogen in N₂O

O= -2 and net charge = 0

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

So 2N – 2 = 0

So 2N = 2

N=+1

 

To calculate the oxidation state of Chlorine in ClO₃^- 

O= -2  and net charge = -1

So Cl + (3 x -2) = -1

So Cl + (– 6) = -1

So Cl = 6 – 1

Cl=+5