Tetravalency of Carbon:
The atomic number of carbon is 6 and its ground state electronic configuration is 1s22s22px1 2py1 2pz°. Since it has four electrons in its outermost shell, its valency is four. To achieve noble gas configuration, carbon atom cannot lose or gain four electrons to form C4+ and C4- ions because of the large amount of energy required for these processes (ionization energy and electron affinity). Thus, it has a very little tendency to form ionic compounds. Therefore, carbon achieves the noble gas configuration only by sharing electrons with other atoms and it forms covalent bonds.
Thus a carbon atom forms four covalent bonds in its compounds. For example, a molecule of methane (CH4) is formed when four electrons of carbon are shared with four hydrogen atoms as shown below:
Similarly, carbon can complete its octet by sharing its valence electrons with the electrons of other atoms as well. This characteristic of carbon atom by virtue of which it forms four covalent bonds is generally referred to as tetravalency of carbon.