Explain why carbon from covalent bond? …

Carbon has 4 electrons in its outermost shell and requires 4 more electrons to attain a noble gas electronic configuration. It cannot form an  ion, as a huge amount of energy is required for the removal of 4 valence electrons. The  ion thus formed will have 6 protons and 2 electrons, which will make it highly unstable. Carbon cannot form an  ion, as its nucleus with 6 protons cannot hold 10 electrons due to interelectronic repulsion. So, carbon achieves a noble gas electronic configuration only by sharing its 4 valence electrons with other elements. Thus, it forms compounds mainly by covalent bonds.
 The two main reasons for carbon forming a large number of compounds are as follows:
(a) Catenation: It is the ability of carbon to form bonds with other carbon atoms; this results in compounds having long branched chains and rings. 
(b) Tetravalency: Carbon has 4 valence electrons, so it is capable of forming covalent bonds with 4 other atoms.
The small atomic size of carbon enables it’s nucleus to strongly hold on to the shared pair of electrons. Hence, carbon forms strong bonds with most other elements.