Question

why carbon cannot gain or lose electrons

Answer 1

• Carbon has four electrons in its outermost shell and find easier to share electrons than to gain or lose four electrons.

• If carbon gains 4 electrons and becomes $C^{4-}$, it would be difficult for the nucleus which has six protons to hold the ten electrons.
• If carbon loses 4 electrons and forms $C^{4+}$ , large amount of energy would be required.

In order to avoid this, carbon shares its valence electrons with other atoms.  The bond formed by sharing of electrons between the two atoms is called covalent bond.