Answer to Question #54600 in General Chemistry for ramesh kumar

Question

At normal temperature, the trihalides of boron are all monomeric compounds whereas those of aluminium, except AlF3, are dimeric solids, explain

Answer 1

Aluminium halides exist as dimers. In the dimeric structure each aluminium atom accepts a lone pair of electrons from the chloride atom bonded to the other aluminium atom. In doing so each aluminium completes its octet.
Boron halides do not form dimmers because the size of boron is so small that it is unable to co-ordinate four large-sized halide ions.