Number of compounds from the following with zero dipole moment is _________.
HF, H₂, H₂ S, CO₂, NH₃, BF₃, CH₄, CHCl₃, SiF₄, H₂ O, BeF₂

Solution

Chemistry ma’am · 3 steps

Look at the shape first: if identical bond dipoles point symmetrically, they cancel to zero.

1. When it is zero. A molecule has zero dipole moment when it has no polar bonds, or when its identical polar bonds are arranged symmetrically and cancel. A lone pair on the central atom or a mix of different atoms breaks the symmetry.

   Molecule	Shape	Why	Net dipole
   H₂	linear	same two atoms, no polar bond	zero
   CO₂, BeF₂	linear	two equal bond dipoles at 180° cancel	zero
   BF₃	trigonal planar	three equal dipoles at 120° cancel	zero
   CH₄, SiF₄	tetrahedral	four identical bonds cancel	zero
   HF	linear	only one polar bond, nothing to cancel it	non-zero
   H₂O, H₂S, NH₃	bent or pyramidal	lone pairs make the shape unsymmetrical	non-zero
   CHCl₃	tetrahedral	one C–H and three C–Cl bonds are not equal	non-zero

2. Count them. The zero dipole set is H₂, CO₂, BeF₂, BF₃, CH₄ and SiF₄.
3. So. The number of compounds with zero dipole moment is 6.

The common slip: 7. Counting CHCl₃ as non-polar because it is tetrahedral gives 7. Its four bonds are not identical, so the dipoles do not cancel.

The idea to keep. Zero dipole needs a symmetric shape AND identical groups on the central atom. A tetrahedron with one different atom, like CHCl₃, is polar.

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