Question

Why boron halides do not exist as dimer while $AlCl_{3}$ exists as $Al_{2}Cl_{6}$?

Answer 1

    Boron B which has the “three electrons in the outermost shell” can be shared with the other atoms. Therefore, the octet of the Boron is not fully filled so its octet is incomplete. Since Boron has only six electrons around it in the Boron compounds.

Hence, electro-deficit compounds were formed by the Boron.

     And also its atoms are small sized, so it cannot accommodate four halogen atoms which are large sized. Hence Boron Halides cannot form dimer. Thus, $AlCl_{3}$ exists as a dimer $Al_{2}Cl_{6}$ whereas Boron Halides cannot exist.  

Answer 2

Boron B which has the “three electrons in the outermost shell” can be shared with the other atoms. Therefore, the octet of the Boron is not fully filled so its octet is incomplete. Since Boron has only six electrons around it in the Boron compounds.

Hence, electro-deficit compounds were formed by the Boron.

And also its atoms are small sized, so it cannot accommodate four halogen atoms which are large sized. Hence Boron Halides cannot form dimer. Thus, AlCl_{3}AlCl

3

exists as a dimer Al_{2}Cl_{6}Al

2

Cl

6

whereas Boron Halides cannot exist.