Hello all. I'm having trouble finding anywhere but my (unclear) lecture notes about the total number of states in a band. I'm getting a little muddled between my different models and am just wondering if the number of states in a band is equal to the number of atoms in the solid. In this case, a set of monovalent atoms would fill half of the band and divalent atoms would fill the whole band. This would explain why Lithium, Sodium, Potassium etc all conduct, but I still don't understand why (e.g.) phosphorus (valency 5) is an insulator. Any ideas would be much appreciated!!