Carbon has four valence electrons. To attain noble gas configuration of the nearest noble or inert gas (He = 2 or Ne = 2,8), there are following possibilities –
(1) It could gain 4 electrons and form C –4 anion.
By that way, it can achieve noble gas configuration (Ne = 2,8) having 8 electrons in the valence shell. Practically, it is difficult to get such an anion having nucleus with six protons to hold on to ten electrons i.e. four extra electrons.
(2) It could lose 4 electrons and form C +4 cation.
By that way, it can achieve configuration of helium (He = 2). This is not possible practically as it requires large amount of energy to remove four electrons leaving behind a carbon cation having nucleus with six protons to hold on to two electrons.
Ionic bond forms only when an element gains or loses its electrons, which are not possible in the case of carbon. Thus, carbon does not form ionic bond.
(3) Carbon can complete its octet by sharing its valence electrons with other atoms of carbon or with atoms of other elements.
Sharing of four pairs of electrons lead to both atoms attaining the inert gas configuration. Thus, carbon always form covalent bond.
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(a) Carbon dioxide molecule (CO2)
Each oxygen atom has 2 shared pairs of electrons and 2 lone pairs of electrons, complete its requirement of two more electrons to attain inert gas configuration. Similarly, carbon has 4 shared pairs of electrons, 2 with each oxygen, complete its octet.