11th Class Chemistry The p-Block Elements-I

  • question_answer 22) Suggest reasons why the BF bond lengths in \[B{{F}_{3}}\](130 pm) and \[BF_{4}^{-}\] (143 pm) do differ?

    Answer:

    \[B{{F}_{3}}\]has a planar structure as boron is in \[sp\] -hybridized state. It has one empty p-orbital. F atom has lone pairs. On account of small size, a lone pair of F atom is transferred to B atom and thus forms a\[p\pi -p\pi \] back bonding, i.e., double bond character comes and due to resonance in the molecule, the bond length is shorter. In \[BF_{4}^{-}\] ion, B atom is \[s{{p}^{3}}\] -hybridized and it has no p-orbital, i.e., back bonding or \[p\pi -p\pi \] bonding is not possible. Thus, the bond length, B-F is larger as the bond is purely a single bond.    


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