Why carbon froms covalent compounds whereas lead forms ionic compounds ?

Explanation

Carbon cannot lose all the four valence electrons to form `C^(4+)` ions because the sum of the first four ioniatization enthalpies of carbon is very large. It also cannot gain four electron to form `C^(4-)` ion as their anion would be highly unstable because it would be very difficult for a nucleus with only 6 protons to hold on family 10 electrons. Therefore, carbon forms only covalent compounds by sharing of electrons:
In contrast due to inert pair effect, +2 oxidation state of Pb is more stable thanits +4 oxidation state.
Further on moving down the group, the ionization enthalpy decreases. As a result , the sum of the first two ionization enthalpies of Pb is quite low and hence Pb can easily lose two electrons to form `Pb^(2+)` ions.
Consequently, Pb form ionic compounds.