Which set has zero dipole moment?

Correct answer: D.

The question concerns the permanent dipole moment of each individual molecule in its usual molecular geometry. Add the dipole contributions as vectors; polar bonds do not necessarily make the whole molecule polar.

In D, BF₃ is trigonal planar: three equivalent B–F bond dipoles at 120° cancel. An isolated BeF₂ molecule is linear, so its two equal, opposite bond dipoles cancel. CO₂ is also linear with equal, opposite bond dipoles. In 1,4-dichlorobenzene (para), the two identical chlorine substituents occupy opposite positions; the molecular symmetry gives zero net dipole moment.

A is incorrect because NH₃ is trigonal pyramidal and polar, and H₂O is bent and polar. B is incorrect because HF is polar and the dipole contributions in 1,3-dichlorobenzene (meta) do not cancel. C contains polar NF₃, H₂O and 1,3-dichlorobenzene.

NF₃ is trigonal pyramidal, not trigonal planar like BF₃. In NCERT's description, its lone-pair contribution opposes the resultant of the N–F bond dipoles, leaving a small nonzero dipole moment; these contributions do not reinforce each other. A lone pair alone is not a universal test of molecular polarity.

Clarification: the original app explanation's BF₃/NF₃ shape comparison and “reinforcing” wording were misleading. The answer remains D; this clarification corrects that wording.

This separate clarification does not mean the original app explanation has been updated.

NCERT: pp.111–112 · NIST: 1,4 geometry · NIST: 1,3 geometry