Answer:
+2
Explanation:
Nickel (II) oxide, or NiO, is an ionic compound. Recalling a few rules of determining the oxidation number of substances:
1. Oxidation numbers of elements in ionic compounds are usually their own ionic charge
2. The total oxidation number of a neutral compound should be 0
And,
3. The oxidation number of oxygen in most cases is -2.
Therefore, since O in NiO has a oxidation number of -2, and nickel (II) ion usually has a charge of 2+, the oxidation number of Ni in NiO is +2 (always remember to put the positive sign in front of the number)
