Revisitation of the Heitler-London theory for H-2 shows that the exchange-overlap component of the bond energy has a strength which is maximum at the bond length. Making the usual assumptions typical of elementary Huckel theory including overlap it is similarly found that in the valence bond description of the chemical bond covalent bonding is strongest for atomic orbitals maximizing the strength of the complete exchange-overlap component of the bond energy. (C) 2005 Elsevier B.V. All rights reserved.