1. Electron duplet is complete in each in a hydrogen molecule.
2. Sodium chloride is an ionic compound while hydrogen chloride is a covalent compound.
Sodium chloride is an ionic compound because the number of electrons in the valence shell of Na is 1. The number of electrons in the valence shell of Cl is 7. So, Sodium donates its valance electron to chlorine and forms an ionic bond.
3. Chlorine molecule is formed by sharing of electrons between two chlorine atoms. When two chlorine atoms share one electron, it results in formation of covalent bond.