Hybridisation energy

The characteristic feature of the bands of cubic nitrides, which is different from those of the isostructural carbides (see Chapter 2), is the position of the N2s-bands, which, due to the lower energy of the 2s states of free N atoms is situated at a lower energy. Hybridised Mnd-bands have almost the same energy as their counterparts in the carbides. But their width in nitrides is essentially less than in the carbides, and this results in the large interval between the edges of the hybridised and metallic state bands. Moreover, in contrast to carbides, the Fermi level in iva and va metal nitrides is always located inside the Mnd-band. The lower covalency... 

Similar, but different, redeployment is envisaged when a carbon atom combines with three other atoms, e.g. in ethene (ethylene) (p. 8) three sp2 hybrid atomic orbitals disposed at 120° to each other in the same plane (plane trigonal hybridisation) are then employed. Finally, when carbon combines with two other atoms, e.g. in ethyne (acetylene) (p. 9) two sp1 hybrid atomic orbitals disposed at 180° to each other (idigonal hybridisation) are employed. In each case the s orbital is always involved as it is the one of lowest energy level. 

These are all valid ways of deploying one 2s and three 2p atomic orbitals—in the case of sp2 hybridisation there will be one unhybridised p orbital also available (p. 8), and in the case of sp1 hybridisation there will be two (p. 10). Other, equally valid, modes of hybridisation are also possible in which the hybrid orbitals are not necessarily identical with each other, e.g. those used in CH2C12 compared with the ones used in CC14 and CH4. Hybridisation takes place so that the atom concerned can form as strong bonds as possible, and so that the other atoms thus bonded (and the electron pairs constituting the bonds) are as far apart from each other as possible, i.e. so that the total intrinsic energy of the resultant compound is at a minimum. 

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