Term Symbols

Term symbols are used by chemists to denote the electron configuration of an atom or molecule, slightly different systems are used for both so we shall only look at molecules as these are the only excited species we shall meet in this context. Of these, there is only one part which is commonly mentioned, the multiplicity.

The multiplicity is often referred to as being a triplet or singlet state which are directly related to the total spin of the molecule, S (the multiplicity is 2S+1). Where the multiplicity is 1, the total spin, S is 0 and there is no net spin on the molecule. This effectively means that all electrons are coupled in orbitals. So He, electron configuration 1s² has no net spin and a multiplicity of 1 (in fact all closed shells have multiplicity of 1).

A multiplicity of 3 indicates a total spin of 1 – here there are two unpaired orbitals (each electron can be spin up or down, the lowest energy will be where the electrons have spin in the same direction, each contributes a ½ making a total of 1.

Looking at the multiplicity of a molecule tells you something about its energy. A term symbol for a particular species with a higher multiplicity always has a larger energy than one with a lower multiplicity.

Go back to the Introduction ....

More information on term symbols can be found here. You will need the Adobe Acrobat Reader to view this file.