Promotion, Hybridization, and the Tetrahedral Carbon Atom

In very nearly all of its covalent compounds, carbon forms four bonds. The carbon atom (configuration is Is2 2s2 2p2), however, has only two unpaired electrons in its ground state and by a process of reasoning similar to that in the preceding paragraph, carbon would be expected to form only two covalent bonds. Confronted with this inconsistency, one is tempted to look for ways in which the carbon configuration can be modified to give four unpaired electrons. Suppose, for instance that the 

Thus the reshuffling of electrons of the lone carbon atom will allow formation of four bonds but the directions of the four bonds do not correspond to the directions of the four original electrons. The quantum-mechanical treatment is said to hybridize these electrons (that is, reorient the shape of the electron cloud picture without increasing the number of bonds) the bonds formed by carbon are said to be sp3 hybrid bonds, the bonding made possible by vacancies in the and p tvpe subshells. 

Such hybridization is possible with other sets of orbitals indeed most covalent bonds form from hybridized orbitals. Thus, for an atom forming six covalent bonds (for example, sulfur in SF6), we speak of spzd2 hybridization—that is, six bonds derived from a combination of one s, three p, and two d orbitals. Similarly, for an atom forming five covalent bonds (for example, phosphorus in PCls vapor), we may speak of spzd hybridization. A number of workers describe complexes of the transition metals in an analogous manner but here the description is not a good one, for it glosses over some important complicating features associated with such complexes (see Chap. 22). 

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