Bonding in Carbon Compounds
BONDING IN CARBON COMPOUNDS
Bond formation between two atoms is then envisaged as the progres
sive overlapping of the atomic orbitals of the two participating
The 2s orbital takes up its full complement of two electrons before
the 2p orbitals begin to be occupied, however, as it is at a slightly
lower ene^y level. This, however, represents the ground state of the
carbon atom in which only two unpaired electrons (in the 2px and
2py orbitals) are available for the formation of bonds with other
atoms, i.e. at first sight carbon might appear to be only divalent.
It is however energetically worthwhile for the carbon atom to assume
an excited state by uncoupling the 2s^2 electrons and promoting one of
them to the vacant 2pz orbital for, by doing so, it now has four unpaired
electrons and is thus able to form four, rather than only two, bonds
with other atoms or groups ;14he large amount of energy produced by
forming these two extra bonds considerably outweighs that required
- (« 97 kcal/mole) for the initial 2«^2 uncoupling and 25->2JP promotion.
Carbon in order to exhibit its normal and characteristic quadrivalency
thus assumes the electron distribution, Is^2 2sl 2p\ 2p\ 2p.
0.
^ HYBRIDATION
Carbon does not, however, exert its quadrivalency by the direct use of
these four orbitals to form thrie bonds of one type with the three 2p
orbitals and one of a different nature with the 2s orbital. Calculation
shows that by blending these four orbitals so as to form four new,
identical and symmetrically disposed orbitals inclined to each other
at 109° 28' (the normal tetrahedral angle), it is possible to form four
stronger, more stable bonis. The observed behaviour of a carbon atom
dm thus again be justified on energetic grounds. These four new
orbitals are designated as sp^3 hybrids and the process by which they are
obtained as hybridisation:
