The number of non-polar molecules from the following is _________. HF, H₂ O, SO₂, H₂, CO₂, CH₄, NH₃, HCl, CHCl₃, BF₃

Solution

Chemistry ma’am · 2 steps

A molecule is non-polar only when its bond dipoles cancel, so check the shape before the bonds.

1. The test. Each polar bond has a dipole pointing along it. The molecule is non-polar only if all these dipoles add to zero, which needs a symmetric shape with identical outer atoms and no lone pair on the central atom.

   Molecule	Shape	Do the dipoles cancel?	Result
   H₂	linear, same two atoms	no bond dipole at all	non-polar
   CO₂	linear, 180°	two equal C=O dipoles point opposite ways	non-polar
   CH₄	tetrahedral, four identical H	four equal dipoles cancel	non-polar
   BF₃	trigonal planar, 120°	three equal B-F dipoles cancel	non-polar
   HF, HCl	diatomic, two different atoms	only one dipole, nothing to cancel it	polar
   H₂O, SO₂	bent (lone pairs on the central atom)	dipoles point partly the same way	polar
   NH₃	trigonal pyramidal (one lone pair on N)	dipoles add up towards N	polar
   CHCl₃	tetrahedral, but outer atoms differ	one C-H and three C-Cl dipoles are unequal	polar

2. So. The non-polar molecules are H₂, CO₂, CH₄ and BF₃, so the answer is 4.

The common slip: 5. Treating CHCl₃ as non-polar because it is tetrahedral like CH₄ gives 5. Its C-H and C-Cl dipoles are unequal, so they cannot cancel.

The idea to keep. Zero dipole moment needs a symmetric shape (linear, trigonal planar, tetrahedral), identical outer atoms, and no lone pair on the central atom. Break any one of these and the molecule is polar.

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