Most nuclear excited states are very unstable and "immediately" radiate away the extra energy after existing on the order of 10 seconds. As a result, the characterization "nuclear isomer" is usually applied only to configurations with half-lives of 10 seconds or longer. Quantum mechanics predicts that certain atomic species should possess isomers with unusually long lifetimes even by this stricter standard and have interesting properties. Some nuclear isomers are so long-lived that they are relatively stable and can be produced and observed in quantity.