Amongst BeF₂, BF₃, H₂O, NH₃, CCl₄ and HCl, the number of molecules with non-zero net dipole moment is ____________.

Solution

Chemistry ma’am · 2 steps

Picture each shape first: if the bond dipoles are arranged symmetrically, they cancel to zero.

1. The test. Every polar bond has a dipole, but the molecule's net dipole is their vector sum. A symmetric shape with identical bonds cancels them; a lone pair or an unbalanced shape leaves a net dipole.

   Molecule	Shape	Bond dipoles cancel?	Net dipole
   BeF₂	linear	yes, two equal and opposite	zero
   BF₃	trigonal planar	yes, three at 120°	zero
   H₂O	bent (2 lone pairs on O)	no	non-zero
   NH₃	pyramidal (1 lone pair on N)	no	non-zero
   CCl₄	tetrahedral	yes, four identical bonds	zero
   HCl	linear, one bond	nothing to cancel with	non-zero

2. So. H₂O, NH₃ and HCl have a net dipole moment, so the answer is 3.

The common slip: 6. Counting every molecule with polar bonds as polar gives 6, but the bond dipoles in BeF₂, BF₃ and CCl₄ cancel by symmetry.

The idea to keep. Polar bonds do not guarantee a polar molecule. Identical bonds in a symmetric shape (linear, trigonal planar, tetrahedral) cancel; lone pairs on the centre usually break that symmetry.

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