The oxidation number of the hydroxide ion (OH–) is -1. This can be determined by examining the individual oxidation states of the elements present in the ion.
In OH–, oxygen typically has an oxidation state of -2, while hydrogen has an oxidation state of +1. When we sum these oxidation states, we get:
-2 (for oxygen) + 1 (for hydrogen) = -1
Since OH– is a charged ion, its overall oxidation number must equal the charge of the ion itself, which is -1 in this case. This indicates that the net charge caused by the combination of these oxidation states results in the overall charge of the hydroxide ion.