Assertion : Boron can only form $\left[\mathrm{BF}_{4}\right]^{-}$, whereas aluminium forms $\left[\

Question

Assertion : Boron can only form $\left[\mathrm{BF}_{4}ight]^{-}$, whereas aluminium forms $\left[\mathrm{AlF}_{6}ight]^{3-}$.\ Reason : The first member of a group of elements in the $s$ - and $p$-blocks shows anomalous behaviour.

Professor PC Thomas Classes · Answer

(b) : The maximum covalency of first member of each group of $s$ - and $p$-block elements is 4 , (due to non-availability of $d$-orbitals) whereas the other members can expand their valence shell to accommodate more than four pairs of electrons (due to availability of $d$-orbitals).