Does the electronegativity scale apply to ionic crystals as to molecules? A theoretical study of the bonding character in molecular and crystalline alkaline-earth oxides based on dipole moments Original Research Article

The analysis of the dipole moments and of the bonding contributions from all electron Hartree-Fock wavefunctions clearly shows that in MO alkaline-earth diatomic molecules (M  Mg, Ca, Sr, Ba) the degree of ionic character increases as MgO < CaO < SrO < BaO. This is the trend predicted by the electronegativity scale. On the other hand, when the same molecules are embedded in the Madelung potential to provide a model of the corresponding crystalline materials the bonding becomes highly ionic with a small but increasing covalent character as one goes from MgO to BaO. This is opposite to the electronegativity scale which cannot be used in a straightforward way to predict the trend of bonding character in ionic solids.