Chemical bonding, the 8 --- N rule and defect reactions

The atomic coordination is 2, and the top of the valence band is formed from the lone pair p state electrons, with the bonding p states deeper in the valence band. 

Similar diagrams can be constructed for other elements. For example, group V elements bond in p configurations, with s electrons forming the non-bonding pairs. It follows that the optimum number, Z, of covalent bonds for elements is. 

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