Lesson 3 of 6

Bond polarity versus molecular polarity

Electronegativity produces bond dipoles; shape determines their vector sum.

A-Level 9476 (2026-2027)

I can explain bond polarity using electronegativity.Syllabus
Syllabus 9476, 2(f). Explain bond polarity using electronegativity.
I can deduce molecular polarity.Syllabus
Syllabus 9476, 2(g). Deduce molecular polarity.

A polar bond need not make a polar molecule

Add the bond-dipole directions using the actual three-dimensional shape.

Make a guess. You are not marked.

Each C=O bond in carbon dioxide is polar, because oxygen is more electronegative than carbon. Is the CO₂ molecule polar overall?

Yes, because each of its bonds is polar
No; it is linear, so the bond dipoles cancel
No; carbon and oxygen have equal electronegativity

Show the answer

No; it is linear, so the bond dipoles cancel

CO₂ is linear, so its two equal bond dipoles point in opposite directions and cancel.

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.

Use bond polarity and shape together
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 exampleSame general shape, different polarity

Both CO₂ and OCS are linear. Why can OCS be polar?

Try it first. Then show the worked stepsHide the worked steps

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.