Draft/Chemical polarity

In chemistry, polarity can be used to talk about both chemical bonds and molecules. For both, it talks about an unequal sharing of electrical charge between atoms.

In bonds

In a covalent bond, two atoms share their electrons. Different chemical elements pull harder on electrons than others, which is called their electronegativity. The bigger the difference in this pull, the more polar their bond will be.

As an example, ammonia and phosphine both have the same structure: a pnictogen atom (nitrogen or phosphorus) bonded to three hydrogen atoms. Nitrogen pulls on electrons much harder than hydrogen does, so most of the shared electrons are pulled in close to the nitrogen atom: the N–H bond is very polar. But phosphorus's pull is only a little weaker than hydrogen's, so the shared electrons are balanced between the atoms: the P–H bond is almost totally non-polar.

In molecules

A molecule is said to be polar if its electrons are pulled to or away from a particular part of the molecule. A polar molecule must have polar bonds, but not every polar bond needs to be in a polar molecule. If polar bonds pull in opposite directions, they will cancel each other out to give a non-polar molecule.

An example of this is chloroform (CH
3Cl) and carbon tetrachloride (CCl
4. Chloroform has one very polar bond between carbon and chlorine, which pulls all the electrons to the chlorine. Carbon tetrachloride has four very polar bonds, but all pulling out from one center, so the electrons do not shift in any specific direction and the molecule is not polar.

Lone pairs also impact the polarity of a molecule.