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Oil and water don’t mix. Salt dissolves in water but not in petrol. A charged balloon held near a thin stream of tap water bends it sideways. All three observations come down to one property: polarity.
Deciding whether a molecule is polar takes two steps — first the bonds, then the shape. Most mistakes come from skipping the second step.
Step 1: are the bonds polar?
A covalent bond is a shared pair of electrons. If the two atoms pull on those electrons equally, the bond is nonpolar. If one atom pulls harder, the electrons spend more time near it, giving it a partial negative charge (δ−) and leaving the other atom partially positive (δ+). That’s a polar bond.
How hard an atom pulls is its electronegativity (see the electronegativity trend). The bigger the difference between the two atoms, the more polar the bond. A rough guide using Pauling values:
| Electronegativity difference | Bond type |
|---|---|
| less than about 0.4 | nonpolar covalent (C–H, C–C, H–H) |
| about 0.4 to 1.7 | polar covalent (O–H, N–H, C–O, C–Cl) |
| more than about 1.7 | largely ionic (Na–Cl) |
These boundaries are conventions, not sharp lines — bonding is a continuum. See ionic vs. covalent bonds.
Step 2: does the shape cancel the bond dipoles?
Each polar bond is a dipole — it has a direction, from δ+ to δ−. A molecule’s overall polarity is the combined effect of all its bond dipoles, and that depends on the molecule’s 3D shape (from VSEPR theory).
- If the bond dipoles point in directions that cancel, the molecule is nonpolar overall — even though its bonds are polar.
- If they don’t cancel, the molecule has a net dipole and is polar.
Examples
Carbon dioxide, CO₂ — nonpolar. Both C=O bonds are polar (oxygen is more electronegative). But CO₂ is linear, so the two dipoles point in exactly opposite directions and cancel, like two equal teams in a tug-of-war.
Water, H₂O — polar. The O–H bonds are polar, and the molecule is bent (because of oxygen’s two lone pairs). The two dipoles both point partly towards the oxygen, so they add up. The oxygen side is δ−, the hydrogen side δ+.
Methane, CH₄ — nonpolar. C–H bonds are barely polar anyway, and the perfect tetrahedron cancels them.
Carbon tetrachloride, CCl₄ — nonpolar. C–Cl bonds are quite polar, but four identical bonds in a symmetrical tetrahedron cancel exactly.
Chloromethane, CH₃Cl — polar. Replace one hydrogen in methane with chlorine and the symmetry breaks. The C–Cl dipole isn’t cancelled.
Ammonia, NH₃ — polar. The trigonal pyramidal shape, with a lone pair on top, means the N–H dipoles don’t cancel.
Boron trifluoride, BF₃ — nonpolar. Three polar B–F bonds in a flat, symmetrical triangle cancel.
A quick checklist
A molecule is usually nonpolar if:
- all its bonds are nonpolar (H₂, Cl₂, hydrocarbons), or
- it’s symmetrical, with identical atoms around a central atom that has no lone pairs (CO₂, BF₃, CH₄, CCl₄, SF₆).
A molecule is usually polar if:
- the central atom has lone pairs (H₂O, NH₃, SO₂), or
- the atoms around the central atom are not all identical (CH₃Cl, HCN, CHCl₃).
(A few symmetrical shapes with lone pairs, like linear XeF₂ and square planar XeF₄, are still nonpolar because the lone pairs themselves are arranged symmetrically.)
Why polarity matters
- Solubility: “like dissolves like”. Polar solvents like water dissolve polar and ionic substances (sugar, salt, alcohol). Nonpolar solvents like hexane dissolve nonpolar substances (oils, fats, waxes). That’s why you need soap — whose molecules have one polar end and one nonpolar end — to wash grease off your hands.
- Boiling points. Polar molecules attract each other through dipole-dipole forces, so they generally boil higher than nonpolar molecules of similar size. See intermolecular forces.
- Microwave ovens heat food mainly by making polar water molecules rotate back and forth in the oscillating electric field.
- The bending stream. A charged balloon attracts the stream of water because water molecules turn so that their oppositely charged end faces the balloon.
Quick answers
Can a molecule with polar bonds be nonpolar? Yes — whenever the shape makes the bond dipoles cancel, as in CO₂ and CCl₄.
Is H₂O polar or nonpolar? Polar, because its bent shape stops the O–H dipoles cancelling.
Is oil polar? No. Oils are mostly long hydrocarbon chains, which are nonpolar.
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