Partial charges

Section 2 of 4

When one atom pulls the shared pair closer, the electrons' negative charge is spread unevenly along the bond. One end of the bond becomes a little negative, and the other end a little positive.

The electrons are still shared, so neither atom gains a whole electron. Each end carries only part of a charge, called a partial charge. Chemists mark it with the Greek letter delta, δ, which stands for "a little": δ− on the atom that pulls harder, and δ+ on the atom that pulls less.

Partial charges on an O–H bond and a C=O bond Two ball-and-stick bonds. Left, an O–H bond: the oxygen is marked delta minus and the hydrogen delta plus, and an arrow under the bond points to the oxygen. Right, a C=O double bond: the carbon is marked delta plus and the oxygen delta minus, and an arrow under the bond points to the oxygen. Each arrow shows which way the shared electrons are pulled. δ− δ+ O–⁠H in an -⁠OH group δ+ δ− C=O in propanone
Oxygen pulls harder than hydrogen and harder than carbon, so it carries δ− in both bonds. Each arrow points the way the shared electrons are pulled.

A bond with a δ+ end and a δ− end is a polar bond: it has two ends, like the poles of a magnet. A bond that shares its pair evenly, or almost evenly, is a non-polar bond. C–⁠C bonds are non-polar, and chemists count C–⁠H bonds as non-polar too.

Groups with charged ends

Now take the functional groups you met earlier. Every one of them holds an oxygen or a nitrogen atom, so they all bring polar bonds into a molecule:

  • the -⁠OH group of an alcohol, with δ− on the oxygen and δ+ on the hydrogen
  • the C=O of an acid, ketone, aldehyde or ester, with δ+ on the carbon and δ− on the oxygen
  • the -⁠NH₂ group of an amine, with δ− on the nitrogen and δ+ on each hydrogen

The carbon chain around these groups is all C–⁠C and C–⁠H bonds, so it is non-polar. An ethanol molecule has charged ends at its -⁠OH group and none along its two-carbon chain.

Quick check

Where do the partial charges sit on the C=O bond in propanone?