Applications to metal oxides and other inorganic solids

Group IV elements and related III / V compounds have already been discussed. Now we will look at other compounds such as NaCl (I / VII compound) and MgO (II / VI compound).

They are white, insulating solids with negligible small electronic conductivities and the bonding is mainly ionic. Addition of dopants to these compounds often produces ionic rather than electronic conductivity. If NaCl is completely ionic you would get electronic configurations on the ions of;

Na⁺: 1s²2s²2p⁶
Cl^- : 1s²2s²2p⁶3s²3p⁶

Therefore the 3s, 3p valence shell of Cl- is full and that of Na+ is empty. The 3p orbitals on Cl- are able to overlap slightly and form a narrow valence band, consisting of anion orbitals only. The 3s, 3p orbitals on na+ may also overlap forming an empty conduction band consisting of only cation orbitals.

Since there is also a large bangap (~7eV), the band structure of NaCl is very similar to that of an insulator. Any promotion of electrons into the conduction band can be regarded as back transfer of charge from Cl- to Na+.

You would therefore expect some correlation between the elcectronegativity differences between the ions and the size of the band gap. A large electronegativity difference favours ionic bonding and in these cases back transfer should be difficult. This correlates with the observation that ionic solids do have large band gaps.