Question

the correct sequence of polarity of the following molecule
a) Benzene
b) Inorganic benzene
c) $PC{l}_3{F}_2$
d) $PC{l}_2{F}_3$
(Where P= polar, NP = non polar)
A. 1P, 2NP,3NP, 4P
B. 1NP, 2NP, 3NP, 4P
C. 1NP, 2P, 3NP, 4P
D. 1NP, 2P, 3P, 4NP

Answer 1

Hint: Polar molecules occur when there is electronegativity difference between the bonded atom. Non polar molecules occur when electrons are shared equally between the atoms, or when polar bonds in a larger molecule cancel out each other.

Complete Step by step answer: polar bonds form when there is electronegativity difference between the atoms. These bonds are partially ionic. While for non polar molecules, the electrons are shared equally between the two atoms and they are purely covalent in nature. for example the diatomic molecule like $O_2,N_2$ . or when all the polar bonds cancel out each other.
In case of benzene, it is a non polar molecule because it contains only C-H and C-C bonds. Since carbon is slightly more electronegative than H , a C-H bond is very slightly polar and has a very small dipole moment. But the dipole moment is a vector quantity. So that bond moment of one C-H bond got canceled out by the C-H bond which is opposite to it. Hence overall it has no dipole moment, hence benzene is nonpolar.
In case of inorganic benzene$B_3{N}_3{H}_6$, the bond arrangement in this is such that there are alternate $C=C$, $B=N$ bond just like $C=C$ in benzene with H attached to every B and N. and hence bond moment of each bond get cancelled out. Hence inorganic benzene is also non polar.
In the case of $PC{l}_3{F}_2$ has trigonal bipyramidal geometry. The 2 fluorine lies in axial position and all 3 chlorine lie in the equatorial position. The molecule is symmetrical and hence the bond moments get cancelled out and hence resultant dipole moment will be zero.
But in the case of $PC{l}_2{F}_3$, it also has a trigonal bipyramidal structure. But the molecule is not symmetrical as 2 fluorines are present in the axial position and 1 fluorine is present at equatorial. 2 chlorines are also there in equatorial position. The bond moment of axial fluorine gets canceled out but bond moment at equatorial will not. Hence, $PC{l}_2{F}_3$ is a polar compound.

Hence the correct option will be B.

Note: in trigonal bipyramidal structure, there are two positions, one is axial and the other is equatorial position. The more electronegative atom stays at the axial position to decrease the repulsion between them. Therefore fluorine stays at axial position only.