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CHEMISTRY QUESTIONS AND ANSWERS

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EXAM OF IIT JAM

Q. The correct order of F-M-F bond angles (where M is the central atom) in NSF3, SiF4 and POF3 is:

(A) NSF3 > SiF4 > POF3

(B) SiF4 > NSF3 > POF3 

(C) NSF3 > POF3 > SiF4

(D) SiF4 > POF3 > NSF3

Answer: (D)

Concept:

First of all write the Lewis dot structures for the molecules and find the number and types of electrons pairs; magnitude of repulsions between them to arrive at the relative bond angles.

According to the VSEPR theory, the repulsion increases with increase in the volume occupied by electron pair(s). Since the electron density in triple bond occupies more space, it exerts more repulsion than that of double bond and which turn exerts more repulsion than that of single bond. 

Explanation:

In POF3, there is a double bond between Phosphorus and Oxygen atom, which also causes more repulsion than single bond, but less than the triple bond. Hence the bond angle will be medium in POCl3.(Bond angle almost 101o )

The bond angle is least affected in case of SiF4, since all the Si-F bonds are single bonds, which exert less repulsion on other bond pairs. Therefore the bond angle is same as the tetrahedral molecule i.e. 109o28'

In NSF3, there is a triple bond between N and S. Hence the repulsions will be more on S-F bond pairs. This will reduce the bond angle more than in other cases(94O).

 
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