Question:

The decreasing order of bond angles in $BF_{3} , NH_{3} , PF_{3}$ and $I_{3}{^{-}}$ is

Updated On: Aug 12, 2024
  • $I_3{^{-}} > NH_3 > PF_3 > BF_3$
  • $I_3{^{-}} > BF_3 > NH_3 > PF_3$
  • $BF_3 > I_3{^{-}} > PF_3 > NH_3$
  • $BF_3 > NH_3 > PF_3 > I_3{^{-}}$
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The Correct Option is B

Approach Solution - 1

The shape of the given molecules is as follows:

$BF_3$ : Planar triangle with bond angles of $120^{\circ}$.



$NH _{3}$ : Trigonal pyramidal with bond angles of $107^{\circ} .$

$I _{3}^{-}$ : Linear with bond angles $180^{\circ}$.



$PF _{3}$ : Trigonal pyramidal with bond angles of $97.8^{\circ} .$

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Approach Solution -2

The repulsion is in the order: lp-lp > lp-bp > bp-bp. 
In PF3, there is a lp-lp repulsion, the angle of PF3 will be less than BF3
The geometry of BF3 is trigonal planar while NH3 is tetrahedral 
The sequence will therefore be, BF3> NH3>PF3 
I3- is a linear molecule therefore, the angle is 180 degree.

The overall order will be I3- > BF3> NH3>PF

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Concepts Used:

Bond Parameters

Covalent bonds can be characterized on the basis of several bond parameters such as bond length, bond angle, bond order, and bond energy (also known as bond enthalpy). These bond parameters offer insight into the stability of a chemical compound and the strength of the chemical bonds holding its atoms together.

Bond parameters 

  • Bond length: it is an equilibrium distance between the centres of the nuclei of the two bonded atoms. It is expressed in terms of A.
  • Bond Angle: It is characterized as the point between the orbitals consisting of holding electron sets around the focal iota in an atom/complex particle. Bond points are communicated in degrees which are tentatively controlled by spectroscopic strategies.
  • Bond Enthalpy: the measure of energy needed to break one mole of obligations of a specific sort between two molecules in a vaporous state. The unit of bond enthalpy is kJ mol–1. The more the bond enthalpy, the more grounded the bond is.

For example, The H—H bond enthalpy in hydrogen is 435.8 kJ mol-1. \

  • Bond Order: According to Lewis, Bond Order is given by the number of connections between the two atoms in a particle.

Bond order of H2 (H —H) =1
Bond order of 02 (O = O) =2
Bond order of N2 (N = N) =3

  • Resonance Structures: It is observed that there are many molecules whose behaviour cannot be explained by a single Lewis structure.

Read More: Chemical Bonding and Molecular Structure