Question

Select the incorrect statements amongst the following:

• A. The dipole moment of $C{H}_3-F$ is greater than $C{H}_3-Cl$
• B. The presence of polar bonds in a polyatomic molecule suggests that the molecule has non-zero dipole moment
• C. In benzene carbon uses only one s and two p-orbitals for hybridisation.
• D. $NM{e}_3$ and $N(SiM{e}_3{)}_3$ are isostructural

Answer 1

Answer: (A), (B), (D)

The correct options are
A The dipole moment of $C{H}_3-F$ is greater than $C{H}_3-Cl$
B The presence of polar bonds in a polyatomic molecule suggests that the molecule has non-zero dipole moment
D $NM{e}_3$ and $N(SiM{e}_3{)}_3$ are isostructural

(a) Here the internuclear distance decides dipole moment. Hence, $C-Cl$ bond has greater dipole moment than $C-F$ bond.
(b) If a molecule has polar bonds but is symmetric then net dipole moment comes out to be zero.
(c) Carbons in benzene are $s{p}^2$ hybridised, i.e. uses the s-orbital and two of its p-orbitals in hybridisation.
(d) $NM{e}_3$ is pyramidal while $N(SiM{e}_3{)}_3$ is planar. In the latter case, $p\pi -d\pi$ back bonding is possible between $N$ and $Si$ which makes the molecule planar.