The total number of molecules with zero dipole moment among CH₄, BF₃, H₂ O, HF, NH₃, CO₂ and SO₂ is ________.

Solution

Chemistry ma’am · 3 steps

Polar bonds are not enough. Ask whether the shape lets the bond dipoles cancel each other out.

1. The rule. Each polar bond has a dipole, and the molecule's dipole moment is their vector sum. It is zero only when the shape is symmetric with identical bonds and no lone pair on the central atom pulling it out of balance.

   Molecule	Shape	Lone pairs on centre	Dipole moment
   CH₄	tetrahedral	0	zero
   BF₃	trigonal planar	0	zero
   CO₂	linear	0	zero
   H₂O	bent	2	non-zero
   NH₃	trigonal pyramidal	1	non-zero
   SO₂	bent	1	non-zero
   HF	linear, only one bond	not applicable	non-zero

2. Why they cancel. In CO₂ the two C=O dipoles point in opposite directions at 180°. In BF₃ three equal dipoles at 120° add to zero, and in CH₄ four equal dipoles at the tetrahedral angle also sum to zero.
3. So. Only CH₄, BF₃ and CO₂ have zero dipole moment, so the count is 3.

The common slip: 4. Treating SO₂ as linear like CO₂ gives 4. The lone pair on sulphur bends SO₂, so its two dipoles do not cancel.

The idea to keep. Draw the shape first. Identical bonds arranged symmetrically around a central atom with no lone pairs cancel to zero, while a lone pair or a bent shape leaves a net dipole.

Answer from the NTA final answer key. Solution by PrepWiser.