Often writing a half equation to show reduction or oxidation is straight forward – all we need to do is work out whether electrons have been gained or lost by looking at the charges on the ions …
However with polyatomic / compound ions we usually need to balance the change in numbers of oxygen and hydrogen atoms as well as adding electrons. We start by assigning oxidation states to the relevant element …
Occasionally the reaction takes place under alkaline conditions, in which case balancing the half equation is a little more confusing …
The next step is putting your half equations together to write fully balanced redox equations – you can check out some examples and lots of practice questions here 😎.