Because chlorine is MORE electronegative than hydrogen, chlorine in the #HCl# molecule polarizes electron density towards itself. We could represent this as:
#""^(+delta)H-Cl^(delta-)#.
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:
#""^(+delta)H-Cl^(delta-) + H_2O rarr H_3O^+ + Cl^-#.
And thus solutions of #HCl(aq)# are stoichiometric in #H_3O^+#, #"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.
