BCl₃ exists as monomer whereas AlCl₃ is dimerised through halogen bridging. Give reason. Explain the structure of the dimer of AlCl₃ also.

BCl₃ is in trivalent state and has 6 electrons in its valence shell and need two more electrons to complete the octet. BCl₃ act as a lewis acid and NH₃ can donate its electron. BCl₃readily accepts the lone pair of an electron from NH₃ and form BCl₃.NH₃.

BCl₃ + NH₃ → BCl₃.NH₃.

AlCl₃ forms a stable molecule by making a Dimer. Aluminium has 6 electrons in the valence shell and need 2 more electrons to complete the octet whereas chlorine has 3 lone pair of electron. Aluminium takes up 1 lone pair and forms Dimer with chlorine.