The type of chemical bond present in a hydrogen chloride (HCl) molecule is:

A covalent bond is formed when two atoms share a pair of electrons. Covalent bonds usually form between non-metal atoms, which have similar tendencies to gain electrons, so neither atom gives up its electron completely.

In a hydrogen chloride molecule, the hydrogen atom has one electron, and the chlorine atom has seven electrons in its outer shell. Each atom contributes one electron to form a shared pair. This gives hydrogen a stable configuration like helium (two electrons) and chlorine a stable octet like argon (eight electrons). So HCl has a single covalent bond, written H–Cl.

However, chlorine is much more electronegative than hydrogen; it pulls the shared electrons closer to itself. As a result, the chlorine end of the molecule gets a slight negative charge and the hydrogen end a slight positive charge. Such a bond is called a polar covalent bond, and HCl is a polar molecule.

When HCl gas dissolves in water, it ionises almost completely into hydrogen ions (H+, which combine with water to form hydronium ions, H3O+) and chloride ions (Cl–). This solution is hydrochloric acid, a strong acid. Hydrochloric acid is produced in our stomach to help digest food and to kill germs. Pure, dry HCl gas does not show acidic properties, because it does not ionise without water.

Other examples of covalent molecules include hydrogen (H2), chlorine (Cl2), water (H2O), methane (CH4), ammonia (NH3) and carbon dioxide (CO2). In H2 and Cl2, the electrons are shared equally, so the bond is non-polar.

Why not the other options?

• ✗An ionic bond is formed by the transfer of electrons, as in NaCl.
• ✗A metallic bond holds atoms together in metals through a "sea" of free electrons.
• ✗A hydrogen bond is a weak attraction between molecules, as between water molecules; it is not the bond within HCl.