An equation that shows only those species involved in the oxidation or the reduction half of a redox (reduction-oxidation) reaction.
The redox reaction between potassium manganate VII and iron(II) sulfate may be represented by the equation:
MnO4- + 8H+ + 5Fe2+ Mn2+ + 5Fe3+ + 4H2O
This shows all of the reacting species. However, if we consider only the particles being reduced then that part of the equation is:
MnO4- + 8H+ + 5e Mn2+ + 4H2O
And the species being oxidised are:
Fe2+ Fe3+ + e
The whole equation is obtained by multiplying through to equalise the electrons and then adding the two half-equations.