Because chlorine is MORE electronegative than hydrogen, chlorine in the HCl molecule polarizes electron density towards itself. We could represent this as:
+δH−Clδ−.
The H−Cl molecule is thus a polar covalent molecule, in which the electronegative chlorine atom strongly polarizes electron density. In water, the polarization is so pronounced that the H−Cl bond completely ionizes:
+δH−Clδ−+H2O→H3O++Cl−.
And thus solutions of HCl(aq) are stoichiometric in H3O+, hydronium ion, and Cl−, the chloride ion. In the gas phase, however, we have the H−Cl molecule, which has a normal boiling point of −85.0 ∘C. Sometimes, you will see gas cylinders of HCl, the which requires special regulators and special precautions and apparatus to handle. Why so......??
So the moral? As a gas, HCl is molecular; as a solution in water, HCl ionizes. Gaseous ammonia is the same sort of molecule, however, ammonia acts as a weak base in water.
