The chemical element having the greatest binding energy per nucleon. The ion Fe⁺² displays a diagnostic band at 1m (while Cu⁺² also has a feature near 1 m = 1000 nm, its is generally not present in appreciable amounts). In the Fe⁺³ ion, all the bands are spin-forbidden, so one might expect weak absorption. Instead, Fe⁺³has a sharp band at 440 nm.