Molecules in polar molecular solids are held together by dipole-dipole interactions. Identify the polar molecules from the following:
A. Solid \(SO_2\)
B. \(NH_3\)
C. \(Cl_2\)
D. \(HCl\)
E. \(He\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A, B and D only
A molecule is polar when its bonds are polar and the bond dipoles do not cancel because of its shape. Such molecules form polar molecular solids, held together by dipole–dipole interactions.
(A) Solid SO2 — Polar. \( \ce{SO2} \) is a bent molecule, so the two S–O bond dipoles do not cancel; solid \( \ce{SO2} \) is a polar molecular solid.
(B) NH3 — Polar. \( \ce{NH3} \) is pyramidal with a lone pair on N, so it has a net dipole moment; solid \( \ce{NH3} \) is another polar molecular solid.
(C) Cl2 — Non-polar. Both atoms are the same, so the bond has no dipole; solid \( \ce{Cl2} \) is a non-polar molecular solid held by London forces.
(D) HCl — Polar. Cl is more electronegative than H, so the H–Cl bond is polar.
(E) He — Non-polar. Helium is a single atom with no dipole; it is held only by weak dispersion forces.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Polar Molecular Solids – “The molecules of substances like HCl, SO2 etc. are formed by polar covalent bonds. The molecules in such solids are held together by relatively stronger dipole-dipole interactions. These solids are soft and non-conductors of electricity. Their melting points are higher than those of non polar molecular solids yet most of these are gases or liquids under room temperature and pressure. Solid SO2 and solid NH3 are some examples of such solids.”