Why boron halides do not exist as dimers while AlCl_(3) exist as Al_(2

1.	Why boron halides do not exist as dimers while AlCl_(3) exist as Al_(2)Cl_(6) ?
Answer» SOLUTION :Boronatom beingsmall in sizeis unableto accommodate FOUR largesizedhalogen ATOMS (exept F) aroundit. G HOWEVER, because of biggersize, Al can easily accommodatefour big sizedCl atomsaround it. Since in AlCl_(3), there areonly six electrons aroundAl atom, therefore, it completesits octet by ACCEPTING a lone pair of electronsfrom Cl atom ofanother `AlCl_(3)` molecule. As a result, `AlCl_(3)` existsas a chlorinebridgeddimericstructure.

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