Question

Why aluminium exists as a dimer but boron doesn't?

Answer 1

BCl3 and AlCl3 both are the example of electron deficient compounds, as there are only six electrons in the valency shell of central atom after the formation of the molecule, thus, both the compounds have strong tendency to gain (two) electrons, so that their octet is completed. In both the cases central atom undergoes sp2 hybridisation and both acquire trigonal planar geometry.