Topic 3 of 6
Bond polarity versus molecular polarity
Electronegativity produces bond dipoles; shape determines their vector sum.
A-Level 9476 (2026-2027)
A polar bond need not make a polar molecule
Add the bond-dipole directions using the actual three-dimensional shape.
Electronegativity is an atom's ability to attract the shared electrons of a covalent bond. Unequal attraction creates partial charges, δ+ and δ-. In H-Cl, chlorine attracts the pair more strongly, so H is δ+ and Cl is δ-. These are partial charges within a covalent bond, not a declaration that HCl is an ionic lattice.
| Molecule | Bond dipoles | Overall result |
|---|---|---|
| CO2 | Two C-O dipoles point in opposite directions along a straight line. | Cancel: non-polar molecule. |
| BF3 | Three identical B-F dipoles arranged symmetrically at 120 degrees. | Cancel: non-polar. |
| CH4 / CCl4 | Four equivalent bonds point to the corners of a tetrahedron. | Cancel: non-polar. |
| SF6 | Opposite S-F dipoles pair in an octahedron. | Cancel: non-polar. |
| H2O | O-H dipoles meet at a bent angle. | Do not cancel: polar. |
| NH3 | Three N-H bonds form a pyramid, not a flat triangle. | Do not cancel: polar. |
| CHCl3 | A tetrahedron with one H and three Cl substituents is not dipole-symmetric. | Polar. |
Worked example
Same general shape, different polarity
Both CO2 and OCS are linear. Why can OCS be polar?
- Linearity can make equal and opposite dipoles cancel.
- In CO2 the two C-O bonds are equivalent.
- OCS has different bonds at the two ends, so equal magnitudes cannot be assumed. Their vector sum need not be zero.
Answer
Shape alone does not decide polarity. Cancellation requires the appropriate symmetry and equivalent opposing bond dipoles.