Transitions made by the free valence electrons of the molecules are responsible for the production of scintillation light in organic crystals.

An example is The preferred CRU is an acyclic subunit of 4 carbon atoms with 4 free valences, one at each atom, as shown.

The structure of (MAO) consists of aluminum and oxygen atoms arranged alternately, as well as free valence electrons that are saturated by methyl substitutes.

To identify the preferred CRU, the chain is broken so that the senior ring is retained with the maximum number of heteroatoms and the minimum number of free valences.

The concept of nearly free valence electrons in metallic alloys explained the special stability of the Hume-Rothery phases if the Fermi sphere of the sp Valence electron, treated as free, would be scattered by the Brillouin zone boundaries of the atomic structure.