Covalency

Covalency

This type of bonding involves a sharing of electrons in pairs, each atom contributing one electron to form a shared pair, each pair of electrons having their spins antiparallel. This method of completing an octet (or any of the other possible values) gives rise to the covalent bond.

Hydrogen is usually unicovalent: occasionally it is unielectrovalent, e.g., in sodium hydride, hydrogen exists as the hydride anion, formed by accepting an electron from the sodium:

Na.   +   H.    →   Na⁺H:^-

Carbon almost invariably forms covalent compounds. The electron configuration of carbon is (1s)2(2s)2(2p)2. Since the two 2s electrons are paired, it would appear that carbon is bivalent, only the two single 2p electrons being involved in compounds formation. As pointed out previously, carbon is almost always quadrivalent; thus the 2s and 2p electrons must be involved.

In methane the four hydrogen atoms each contribute one electron and the carbon atom four electrons towards the formation of fur shared pairs:

Each hydrogen atom has its duplet (as in helium), and the carbon atom has an octet.

Each pair of shared electrons is equivalent to the ordinary ‘valency-bond’, and so electronic formulae are readily transformed into the usual structural formulae, each bond representing a shared pair, e.g.,

From these examples it can be seen that there is a very important difference between an electronic formula and its equivalent structural formula. In the former, all valency electrons as shown whether they are used to form covalent bonds or not; in the latter, only those electrons which are actually used to form covalent bonds are indicated. This is a limitation of the usual structural formula. A widely used scheme is to represent structures with ordinary valency bonds and to indicate lone pairs by pairs of dots.