Question

Why only outermost orbitals combine? Why not inner orbitals?

Answer 1

The combination of orbital takes place according to the energy level since the energy level of inner orbital is much higgher than outermost orbital so only outermost orbital combine.

Answer 2

only the outermost orbitals can combine becoz it's easier for the outermost then the inner.
Let's see an example..take an empty bottle and start putting balls in it. Then when u want to remove that balls then at first u have to remove the outermost ball .