Question

Which of the following is a polar molecule?
(a) ${\text{B}}{{\text{F}}_{\text{3}}}$
(b) ${\text{S}}{{\text{F}}_{\text{4}}}$
(C) ${\text{Si}}{{\text{F}}_{\text{4}}}$
(d) ${\text{Xe}}{{\text{F}}_{\text{4}}}$

Answer 1

If an even number of lone pair of electrons are present at the centre of the atom then they are considered as polar molecule and if even number of lone pair is present at the central atom they are considered as non-polar molecule.

Complete step by step answer:
A molecule will be considered as polar if there are different terminal atoms or lone pairs of electrons surrounding the central atoms otherwise it is non-polar.
Eg: ${\text{B}}{{\text{F}}_{\text{3}}}$
In the Lewis structure of ${\text{B}}{{\text{F}}_{\text{3}}}$
Boron is the central atom and Fluorine is the terminal atom, since all three of the terminal atoms are the same i.e., fluorine the first criteria is not fulfilled. Boron does not have any lone pair of electrons and all the electrons surrounding Boron are connected to Fluorine. Thus ${\text{B}}{{\text{F}}_{\text{3}}}$ is not a polar molecule.
From the Lewis structure of ${\text{S}}{{\text{F}}_{\text{4}}}$ there is a lone pair of electrons around the Sulphur atom and thus it is a polar molecule. From the Lewis structure of ${\text{Xe}}{{\text{F}}_{\text{4}}}$, there are single bonds to each of the Fluorine atoms and there are 2 lone pairs on Xenon atom that are not bonded to Fluorine. By looking at the symmetry of ${\text{Xe}}{{\text{F}}_{\text{4}}}$ the bond between Xenon and Fluorine has got a large difference in electronegativity. Then ${\text{Xe}}{{\text{F}}_{\text{4}}}$ is symmetrical in nature, no net dipole and thus ${\text{Xe}}{{\text{F}}_{\text{4}}}$ is non-polar in nature.
From the Lewis structure of ${\text{Si}}{{\text{F}}_{\text{4}}}$ it is known that it is a nonpolar molecule with no net dipoles.

So, the correct answer is Option B.

Additional information:
${\text{B}}{{\text{F}}_{\text{3}}}$ are used in the reactions such as alkylation of aromatic hydrocarbons, condensation and esterification reactions because of its catalytic properties. Sulphur tetrafluoride is used as an organic fluorinating agent. Silicon tetrafluoride has got a strong odour and it is used to seal water out of oil wells. Xenon tetrafluoride can be used to determine the trace metals that contaminate silicone rubber.

Note: ${\text{Si}}{{\text{F}}_{\text{4}}}$ has a tetrahedral geometry, it undergoes ${\text{s}}{{\text{p}}^{\text{3}}}$ hybridization and has a bond angle of ${\text{109}}{\text{.}}{{\text{5}}^{\text{o}}}$. Whereas ${\text{Xe}}{{\text{F}}_{\text{4}}}$ forms a square planar geometry where Xenon undergoes ${\text{s}}{{\text{p}}^{\text{3}}}{{\text{d}}^{\text{2}}}$ hybridization. ${\text{S}}{{\text{F}}_{\text{4}}}$ forms a distorted tetrahedral geometry with ${\text{s}}{{\text{p}}^{\text{3}}}{\text{d}}$hybridization and ${\text{B}}{{\text{F}}_{\text{3}}}$ shows trigonal planar geometry with ${\text{s}}{{\text{p}}^{\text{2}}}$ hybridization.