Number of compounds / species from the following with non-zero dipole moment is _________.
BeCl₂, BCl₃, NF₃, XeF₄, CCl₄, H₂ O, H₂ S, HBr, CO₂, H₂, HCl

Solution

Chemistry ma’am · 3 steps

Look at the shape first: when identical bonds sit symmetrically, their dipoles cancel to zero.

1. The rule. Each polar bond has a dipole, a vector pointing along the bond. The molecule's dipole moment is the vector sum of these. It is zero only when identical bonds are arranged so that they cancel exactly.
2. When dipoles survive. Lone pairs on the central atom bend or pucker the shape, so the bond dipoles no longer cancel. A diatomic made of two different atoms has one polar bond with nothing to cancel it.

   Shape	Species	Dipole moment
   Linear, no lone pair on centre	BeCl₂, CO₂	zero, two equal dipoles point in opposite directions
   Trigonal planar	BCl₃	zero, three dipoles at 120° cancel
   Square planar (2 lone pairs, one above and one below the plane)	XeF₄	zero, opposite Xe–F dipoles cancel
   Tetrahedral, four identical bonds	CCl₄	zero
   Homonuclear diatomic	H₂	zero, the bond is not polar
   Bent or pyramidal (lone pairs on centre)	H₂O, H₂S, NF₃	non-zero, 3 species
   Heteronuclear diatomic	HBr, HCl	non-zero, 2 species

3. So. The species with a non-zero dipole moment are NF₃, H₂O, H₂S, HBr and HCl. That makes 5.

The common slip: 4. Counting NF₃ as zero because its dipole is very small gives 4. In NF₃ the lone pair partly opposes the N–F bond dipoles, but the cancellation is incomplete, so the dipole moment is small but not zero.

The idea to keep. Polar bonds do not guarantee a polar molecule. Find the shape: symmetric shapes with identical atoms around the centre give zero, while lone pairs that bend or pucker the shape leave a net dipole.

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