Why boron halides do not exidising as dimers while `AlCl_(3)` exists as `Al_(2)Cl_(6)` ?

Answer

Boron atom being small in size is unable to accommodate four large sized halogen atoms (except `F`) around it.However, because of bigger size, `Al` can easily accommodate four big size `Cl` atoms around it. Since in `AlCl_(3)`, there are only six electrons around `Al` atom, therefore, it completes its octet by accepting a lone pair of electrons from `Cl` atom of another `AlCl_(3)` molecule.As a result, `AlCl_(3)` exists as a chlorine bridged dimeric structure.