Question
Boron chloride exists a monomer while in the same group anhydrous, AlCl3 exists as a dimer?
Solution
Boron trichloride is a planar molecule and three covalent bond results due to sp2- hybridisation. 
BCl3 does not form a dimer. On the other hand, aluminium trichloride exists in the dimeric state (Al2Cl6).
In the dimeric state, each aluminium atom accepts a pair of electrons from the chlorine atom of another aluminium chloride molecule and thereby acquires an octet of electrons. In other words AlCl3 achieves stability by forming a dimer.
