Question

Between boron and aluminium boron cannot have covalency more than 4 but al can have give reason

Answer 1

Boron does not have d susheel in n=2 level/it has only 4 valence orbitals/Al has d susheel in n=3 level or has more valence orbitals have coordination number-6

Answer 2

Explanation:

Covalency is the ability of an atom to share electron pairs with other atoms.

Boron has atomic number 4 and its electronic configuration is $1s^{2}2s^{2}$. So, there are only four orbitals in a boron atom, that is, 1s and 2s.

Whereas atomic number of aluminium is 13 and its electronic configuration is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{1}$, that is, in case of aluminium there is availability of vacant 3d orbitals.

Hence, due to the presence of vacant 3d orbitals aluminium can expand its octet whereas boron cannot expand its octet due to the non-availability of vacant d-orbital.