Question

why do carbon mainly covalent compound

Answer 1

as outer most orbit of Carbon contain 4 electron so Carbon can't generate polar bond.. so mostly Carbon form other compounds by co-valent bondsonly… Carbon has 4 valence electrons so it is difficult to gain 4 more electrons or lose 4 electrons to attain octet in the outermost shell.

Answer 2

Because carbon has 4 electrons in its outermost shell (and because each covalent bond requires a donation of 1 electron, per atom, to the bond) carbon has exactly four bonds to make, and is only stable if all 4 of these bonds are used. This same rule applies to all of the atoms in the second row of the periodic table.