Question

what is the reson that carbon atoms form only covalent compound

Answer 1

Carbon has atomic number 6. So it's electronic configuration is ( 2,4). That is it has two electrons in it's first (K) shell and four electrons in its second (L) shell. Like every atom carbon atom also tries to complete it's octate (fill it's outermost shell with 8 electrons). But here comes problem, carbon has to either gain four electrons or lose four electrons. When it will gain four electrons it will acquire four negative charges and become C-4 , and went it loses four electrons and become C+4. C-4 is very unstable as negatively charged carbon repels extra electrons . As these both forms are very unstable and requires much energy carbon completes it's octate by sharing electrons with other atoms (forming covalent bonds). As a result carbon has unique capacity of forming chains and branches.

Answer 2

4 electron in outer most shell