Answer
(i) Oxidation number of chlorine is -1
Oxidation number of magnesium is +2
(ii) Oxidation number of oxygen is -2
Oxidation number of iron is +2
(iii) Oxidation number of chlorine is -1
Oxidation number of mercury is +1
Work Step by Step
(i) Since Chlorine is a halogen, it normally has only one stable ion, Cl$^{-}$, making its oxidation number -1. Because MgCl$_{2}$ is uncharged, the magnesium has an oxidation number of +2, to balance the two Cl$^{-}$ charges.
(ii). The oxidation number of oxygen in almost all molecules is -2. To balance the molecule's charge, the oxidation number of iron must be +2.
(iii). As a halogen, the oxidation number of chlorine is -1. To balance the molecule's charge, the oxidation number of mercury must be +1.
