It has been shown that the pairing theory of orbital hybridization accounts satisfactorily for the variations which are observed in the properties of C-H bonds. Since heteropolar effects are in the opposite directions to the effects described, it is concluded that hybridization influences the properties of predominantly covalent bonds to a greater extent than do differences in electronegativity. The extent of second-order hybridization in the molecules CH, NH and OH has been dealt with in the light of this analysis. The consequences of the electronegativity concept have been examined on the basis of a generalized atomic orbital approximation. In particular, variations of the electronegativity of the carbon, nitrogen and oxygen atoms in different states of hybridization have been analyzed. The idea of 'lone' electrons has been formalized and the results of this definition have been discussed. Finally, the effects of orbital hybridization on the dipole moments and nuclear quadrupole coupling constants of molecules have been considered.