Q10.46P - Chemistry: Molecular Nature Of Matter And Change

Question

Arrange the following AF_n species in order of increasing F-A-F bond angles:  ${\mathbf{BF}}_3$ , ${\mathbf{BeF}}_2$  ,  ${\mathbf{CF}}_4\mathbf{,}{\mathbf{NF}}_3\mathbf{,}{\mathbf{OF}}_2^{-}$. 

Step-by-Step Solution

Answer

The bond order is  ${\mathbf{OF}}_2^{-}$  <  ${\mathbf{NF}}_3$  <  ${\mathbf{CF}}_4$  <  ${\mathbf{BeF}}_2$  <  ${\mathbf{BF}}_3$

Step 1: BeF 2 is a linear molecule

${\mathbf{BeF}}_2$  is a linear molecule with a maximum angle of  $\mathbf{180}\mathbf{°}$ .

${\mathbf{BF}}_3$  is a planar molecule with trigonal geometry and bond angles of  $\mathbf{120}\mathbf{°}$ 

_{${\mathbf{CF}}_4\mathbf{,}{\mathbf{NF}}_3\mathbf{,}{\mathbf{OF}}_2^{-}$}   have tetrahedral geometry. The expected bond angle is $\mathbf{109}\mathbf{.}\mathbf{5}\mathbf{°}$ . 

However, nitrogen has one lone pair of electrons. This decreases the bond angle. Oxygen has two lone pairs of electrons and the angle decreases further.  

Step 2: Conclusion

Therefore, the correct order is  ${\mathbf{OF}}_2^{-}\mathbf{<}{\mathbf{NF}}_3\mathbf{<}{\mathbf{CF}}_4\mathbf{<}{\mathbf{BeF}}_2\mathbf{<}{\mathbf{BF}}_3$