Tetra covalency and catenation property of carbon


A carbon atom has a total of six electrons occupying the first two shells, i.e., the K-shell has two electrons and the L-shell has four electrons. This distribution indicates that in the outermost shell there are one completely filled 's' orbital and two half-filled 'p' orbitals, showing carbon to be a divalent atom. But in actuality, carbon displays tetravalency in the combined state. Therefore, a carbon atom has four valence electrons. It could gain four electrons to form C4- anion or lose four electrons to form C4+ cation. Both these conditions would take carbon far away from achieving stability by the octect rule. To overcome this problem carbon undergoes bonding by sharing its valence electrons. This allows it to be covalently bonded to one, two, three or four carbon atoms or atoms of other elements or groups of atoms. Let us see how carbon forms the single, double and triple bonds in the following examples.

Methane Molecule

Carbon atom has four electrons in its outermost shell. Thus, it requires four more electrons to acquire a stable noble gas configuration. Each of the hydrogen atoms has only one electron in its outermost shell and requires one more electron to complete its outermost shell (to acquire He configuration). This is done as follows.

molecules of methane

Carbon Dioxide Molecule

The electronic configurations of carbon and oxygen are:


Thus, each carbon atom requires four, and each oxygen atom requires two more electrons to acquire noble gas configurations. To achieve this, two oxygen atoms form a double covalent bond with carbon as follows.

molecule of carbon di oxide

Acetylene Molecule

Carbon atom has four electrons in its outermost shell and hydrogen atoms have only one electron in its outermost shell. Carbon share one of its electrons with hydrogen to form a single bond each. Each carbon then requires three more electrons to acquire a stable configuration of the nearest noble gas (neon). This is done by mutually sharing three pairs of electrons between the two carbon atoms to form a triple bond as shown below.

molecules arrangement in acetylene

The property of self-linking with atoms of the same element is termed Catenation. Carbon has a unique property of linking itself to other carbon atoms to give open chain or/and cyclic structures. Catenation is favored by atoms where atom to atom covalent bond is quite strong.

In carbon, C-C bond energy is very high (347.3 kJ mol-1) causing catenation. Further, the carbon atom due to its tetravalency, can be bonded to two, three or four carbon atoms by forming single and multiple bonds. Therefore, chains of carbon atoms may be linear, branched or cyclic. For example,

linear carbon chain
branched carbon chain

cyclic carbon chain
cyclic carbon chain

Catenation is responsible for the existence of a large number of organic compounds.

Strength of Bonds with Other Elements

The C-C and C-H bonds in organic compounds are very strong. During chemical reactions, these bonds generally do not break easily. However other atoms or groups such as Cl, OH, etc., attached to the carbon atom may get replaced easily. For example, aqueous KOH reacts with haloalkanes to give the corresponding alcohol

formation of ethanol from chloroethane

All these features make carbon to form a mole variety of compounds