Carbon, electronic configuration promotion energy

Other electron configurations are also possible in addition to the above-mentioned ground state, for example, one (5S) in which the four electrons are all unpaired and distributed over the 2s and the three 2p levels. Thus carbon is indeed quadrivalent and it will also so react if the energy necessary to promote the atom from the divalent ground state to this quadrivalent state finds sufficient compensation in the extra bond energy of the two new bonds. These four bonds would, however, not all be equivalent, in fact three would be p bonds, the fourth a probably weaker s bond. 

The hybridisation theory states that atomic orbitals can be mixed, i.e. hybridised, to form new atomic orbitals that are degenerate, i.e. equal in energy and orientation. Thus, for example, one of the 2s electrons in the ground state of carbon is promoted to give the electron configuration of 2s1, 2pr ,... 

FIGURE 1.20 (a) Electron configuration of carbon in its most stable state, (b) An electron is "promoted" from the 2s orbital to the vacant 2p orbital, (c) The 2s orbital and the three 2p orbitals are combined to give a set of four equal-energy sp -hybridized orbitals, each of which contains one electron. 

The simplest hydrocarbon observed imder normal laboratory conditions is methane, CH4. This is a stable, unreactive molecule with a molecular formula consistent with the octet rule of the Lewis theory. To obtain this molecular formula by the valence bond method, we need an orbital diagram for carbon in which there are four unpaired electrons so that orbital overlap leads to four C—H bonds. To get such a diagram, imagine that one of the 2s electrons in a ground-state C atom absorbs energy and is promoted to the empty 2p orbital. The resulting electron configuration is that of an excited state. 

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