question_answer

Why boron halides do not exist as dimers while \[AlC{{l}_{3}}\] exists as \[A{{l}_{2}}C{{l}_{6}}\]?

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 sized Cl atoms around it. Since in\[AlC{{l}_{3}},\] 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 \[AlC{{l}_{3}}\] molecule. As a result, \[AlC{{l}_{3}}\] exists as a chlorine bridged dimeric structure. Refer to Fig. 11.2, page 11/10.