Polar and non-polar molecules

Section 3 of 4

A molecule with polar bonds has several pulls at once. The directions its bonds point in decide whether those pulls add up or cancel out.

Think of each polar bond as a pull in one direction, towards its δ− end. When all the pulls in a molecule add up to an overall pull one way, the molecule has a negative end and a positive end. It is a polar molecule. When the pulls cancel out, so the molecule has no overall negative end and positive end, it is a non-polar molecule.

Carbon dioxide and water

Carbon dioxide, as you have seen, is straight, with its two oxygens 180° apart. Each C=O bond is polar, and each oxygen pulls just as hard as the other, in exactly the opposite direction. The two pulls cancel, so carbon dioxide is non-polar.

Water is bent, with its bonds 104.5° apart. Both O–⁠H bonds pull towards the oxygen, so both pulls lean the same way and add up. The oxygen side of the molecule is δ− and the hydrogen side is δ+. Water is a polar molecule.

Carbon dioxide is non-polar and water is polar Left, a straight carbon dioxide molecule, O=C=O, with delta minus on each oxygen and delta plus on the carbon. Two arrows under it point away from the carbon to the two oxygens, in exactly opposite directions, so they cancel. Right, a bent water molecule with delta minus on the oxygen at the top and delta plus on each hydrogen below. An arrow beside each bond points from the hydrogen to the oxygen, and a larger arrow between the hydrogens points straight up to the oxygen: the two pulls add up, giving water a negative end and a positive end. δ− δ+ δ− carbon dioxide the pulls cancel non-polar δ− δ+ δ+ water the pulls add up polar
Carbon dioxide's two pulls point in opposite directions and cancel. Water's two pulls both lean towards the oxygen and add up to one overall pull.

Ammonia works the same way as water: its pyramid has the nitrogen on top and three δ+ hydrogens below, so ammonia is polar. Methane, hexane and every other hydrocarbon have only carbon-carbon and C–⁠H bonds, so hydrocarbons are non-polar, or very nearly so.

Same bonds, different result

Carbon tetrachloride, CCl₄, has four polar C–⁠Cl bonds pointing to the corners of a tetrahedron. All four pull equally hard in evenly spread directions, so they cancel and the molecule is non-polar. Chloroform, CHCl₃, swaps one chlorine for a hydrogen. Its three C–⁠Cl pulls all lean away from the hydrogen and add up, so the chlorine side is δ− and chloroform is polar.

Carbon tetrachloride (tetrachloromethane), CCl₄One carbon atom joined by single bonds to four chlorine atoms, which point to the corners of a tetrahedron.ClClClClC
Carbon tetrachloride (tetrachloromethane), CCl₄: four C–⁠Cl pulls spread evenly, so it is non-polar.
Chloroform (trichloromethane), CHCl₃One carbon atom joined by single bonds to three chlorine atoms and one hydrogen atom, arranged as a tetrahedron.ClClClCH
Chloroform (trichloromethane), CHCl₃: three C–⁠Cl pulls on one side, so it is polar.

Quick check

Carbon dioxide has two polar C=O bonds. Why is the molecule non-polar?