How do the number of electrons in an energy level affect bond formation?

Concept:

When an atom has incomplete number of electron in its outermost orbit then it has great tendency to react with another atom which satisfies their octate either by sharing or by transferring their electrons. The involved electrons are called valence electrons. These electrons will effect the energy level because of the transition of these electrons from one energy level to another energy level.  

In case of electrovalent compound, the valance electron complete their octate by transferring their valence electrons while in the covalent compound, they complete their octate by the sharing of their valence electrons.

Hence, the valence electron of the atom effect the energy levels by the transition from one state to another state in the bond formation.