688 



BELL SYSTEM TECHNICAL JOURNAL 



with the ionic method rather than the band method because of the 

 more immediate physical interpretation of the former. 



We may remark that in the discussion of metalHc sodium, it was not 

 necessary to consider the potential energy of the nuclei and the closed 



u ^ •« an^ ® '° '2 '"* '^ '^ 20 



LATTICE CONSTANT IN ANGSTROMS 



Fig. 18 — Energy bands for sodium chloride versus lattice constant. 



shell electrons as was done in NaCl. This is because sodium metal is 

 not ionic — although we think of it as consisting in part of Na+ ions, 

 the electrostatic forces between them are suppressed by the shielding 

 effect of the electron gas. In an ionic crystal, like NaCl, there is no 

 electron gas and the coulomb energy must be considered in the manner 

 described above. 



Energy Bands for Other Crystals 



There are chemical compounds which lie between the homopolar 



and ionic types. For example in the sequence of compounds NaF, 



MgO, AIN, SiC the compounds are progressively less and less definitely 



ionic — the least ionic, SiC or carborundum, being homopolar. Simi- 



