Question

In molecular orbital theory, why are the sign and magnitude of overlap between orbitals on adjacent atoms good indications of whether and how strongly the atoms are bonded?

Answer 1

In molecular orbital theory, maximum overlap of atomic orbital gives strong bonds. Say in case of sigma bonds which is head on overlap, the bond formed is very strong. On the other hand in case of pi-bonding, the lateral overlap of atomic orbital occurs, which results in weak pi-bonds (weaker than sigma bonds). Thus maximum overlap results in strong bonds. Bonding between two bonding pair of atomic orbitals (same sign) gives strong bond as opposed to no-bonding between atomic orbitals (opposite sign). This thus results in a strong or no-bond case.