Question

Arrange the following AF_n species in order of increasing F-A-F bond angles:${\mathbf{BF}}_3$ ,role="math" localid="1656920312687" ${\mathbf{BeF}}_2$ , ${\mathbf{CF}}_4\mathbf{,}{\mathbf{NF}}_3\mathbf{,}{\mathbf{OF}}_2^{-}$.

Short answer

The bond order is ${\mathbf{OF}}_2^{-}$< role="math" localid="1656920489555" ${\mathbf{NF}}_3$ <role="math" localid="1656920512465" ${\mathbf{CF}}_4$ <role="math" localid="1656920536301" ${\mathbf{BeF}}_2$ < ${\mathbf{BF}}_3$

Step-by-step solution

Step 1:   BeF2 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.

Conclusion 

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