Question

Boron and aluminium belong to the same group but boron is unable to exceed its covalencyfrom four to six.why? explain.

Answer 1

because boron doesn't have d orbital to expand its covalency from 4 to 6

Answer 2

Boron belongs to 2nd period and doesn't have a d-orbital to extend it's valency from 2 to 4.
Aluminum belongs to 3rd period and has vacant d- orbitals