Remember (from the chapter on Periodic Trends) the following rules for assigning oxidation numbers. The oxidation number of all elements in a reaction can usually be figured out from these rules:
- The oxidation number of any pure element (not in ionic form) is always zero.
- The oxidation numbers of all the atoms in a molecule must add up to zero.
- The oxidation numbers of the atoms in any ion must add up to the charge on the ion.
- Alkali metals in a compound always have an oxidation number of +1.
- Alkaline earth metals in a compound always have an oxidation number of +2.
- Oxygen atoms in a compound always have an oxidation number of -2, except in a peroxide (where oxygen has an oxidation number of -1) and in other rare instances.
Then, in going from the left side to the right side of the equation, if the oxidation number of an element is decreasing, that means that it must be gaining electrons. So, for example, if an element has an oxidation number of +5 on the left and +2 on the right, it must be gaining 3 electrons, because each electron has a charge of -1:
+5 +(-3) = +2
The oxidation number of some other element in the equation must, therefore, be increasing from left to right, meaning that it must be losing electrons. So, for example, if an element has an oxidation number of -1 on the left and +3 on the right, it must be losing 4 electrons, because negative one minus negative four equals positive three:
-1 -(-4)= +3