Question
Account for the following:
  1. BF, molecule has a zero dipole moment although $\text{B}-\text{F}$ bonds are polar.
  2. The structure of $\mathrm{NH}_3$ molecule is pyramidal.

Answer

  1. Although $\text{B}-\text{F}$ bonds are polar but net dipole moment of $\mathrm{BF}_3$ molecule is zero because of the symmetry of the molecule, individual dipole moments cancel out as shown below.
  1. $\mathrm{NH}_3$ is pyramidal due to the presence of lone pair of electrons on nitrogen and 3 bonded pair of electrons.

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