What is the oxidation number of Ni in NiO

+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)