Hydrogen atom has one electron in the outermost shell, which can form one covalent bond with another atom to attain stability. For example, in the case of water (H2O), each hydrogen atom forms one bond with oxygen.

Carbon

A single atom of carbon can form a maximum of four covalent bonds. This versatility is due to carbon's four valence electrons, which allow it to share electron pairs with up to four other atoms. For example, in methane (CH4), carbon shares its four valence electrons with four hydrogen atoms, forming four covalent C-H bonds.

Nitrogen

Nitrogen has five valence electrons and can typically form three covalent bonds. By sharing three electron pairs with other atoms, nitrogen achieves a stable configuration. For example, in ammonia (NH3), each nitrogen forms three bonds with three hydrogen atoms. However, nitrogen can occasionally form four or even five covalent bonds under specific conditions.