Question

Which among the following molecules has a permanent dipole moment?

• A. $Si{F}_4$
• B. $B{F}_3$
• C. $P{F}_3$
• D. $P{F}_5$

Answer 1

The correct option is C $P{F}_3$

(a) $Si{F}_4$ has 4 $\sigma$ bond pairs. Therefore, $Si{F}_4$ is $s{p}^3$ hybridised and has a tetrahedral shape. All the dipole moment vectors cancel out each other. Hence, the dipole moment is zero.



(b) $B{F}_3$ has three $\sigma$ bonds. Therefore, $B{F}_3$ is $s{p}^2$ hybridised. The resultant of the two dipole moment vectors cancel out with the third dipole moment vector as the bond angle is ${120}^{\circ }$. Hence, it is also non polar in nature.

(c) In $P{F}_3$, $P$ has three $\sigma$ bonds and one lone pair. Hence, it is $s{p}^3$ hybridised and has trigonal pyramidal shape. It has two types of dipole moment vectors which cannot cancel out each other and hence it is polar in nature i.e. has a permanent dipole.

(d) In $P{F}_5$, $P$ has five $\sigma$ bond pairs. Hence, it is $s{p}^{3}d$ hybridised and has trigonal bipyramidal shape. The dipole moment vectors are at ${120}^{\circ }$ with each other. Hence, the resultant of the two dipole moment vectors cancel out with the third dipole moment vector. Similarly, above and below the triangular plane, dipole moment vectors cancel out each other. Therefore, $P{F}_5$ is non-polar in nature and does not have any permanent dipole.