Ionic half-equations show either the species that are oxidisedAl → Al3+ + 3e-or the species that are reducedCu2+ + 2e- → CuThey can be combined to give the overall ionic equation by balancing the equations so they have the same number of electrons2Al → 2Al3+ + 6e-3Cu2+ + 6e- → 3Cuthen adding the two equations with the electrons cancelled out.2Al + 3Cu2+ → 2Al3+ + 3CuIn the overall equation the electrons cancel because electrons are transferred from one species to another so the number of electrons lost by one species is equal to the number of electrons gained by the other species. The total change in oxidation number of the species being oxidised is equal to the total change in oxidation number of the species being reduced.