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Q.
$\left[ AlF _{6}\right]^{3-}$ exists, while $\left[ BF _{6}\right]^{3-}$ does not. Choose the most appropriate reason.
Classification of Elements and Periodicity in Properties
Solution:
The most appropriate reason is that, boron cannot expand its valence shell and its maximum valency is limited to $4$ , while $Al$ can expand its valence shell and hence can form $\left[ AlF _{6}\right]^{3-}$.