+2 is the oxidation number of Ca in CaO
- Calcium has 20 electrons, and its electronic configuration can be written as 2,8,8,2.
- Thus, Calcium has 2 electrons in its outermost orbit, or it can be said that Calcium has 2 valence electrons.
- Ca wants to give away these two valence electrons to its neighbouring Oxygen atom in CaO.
- Calcium completes its octet by giving away the two valence electrons and attains the nearest inert gas argon’s electronic configuration.
- Thus, Calcium now has 18 electrons, and hence its oxidation number becomes +2.