how aluminium has covalency of 6 however boron has 4
 

Dear student,

Covalency of element is its total no. of electrons involved in the covalent bonding or no. of unpaired electrons available for covalent bonding.

Electronic configuration of Al is Ne3s2,3p1.
There are vacant 3d orbitals available so it can exhibit a covalency up to 6.
example: AlF63-.
Electronic configuration of B is 1s22s22p1.
Here are one s and 3p orbitals are available for bonding, so covalency of B is only restricted to 4 as boron does not contain d orbitals in its valence shell.
So Boron has a maximum covalency of 4.

Regards

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