Last revised by Calum Worsley on 11 Jan 2023

In nuclear physics, isomers are atomic species that are identical in nuclear composition, sharing the same mass and atomic numbers, but differ in their relative energy states, and will therefore differ in their manner of radioactive decay.

Examples of isomers include:

  • metastable technetium-99, which decays into technetium-99 with a half-life of 6 hours by emission of gamma rays 4,5

  • metastable krypton-81, which decays into krypton-81 with a half-life of 13 seconds also by releasing gamma rays 5

This term should not be confused with the similarly sounding isotope, isobar and isotone.

The term should also not be confused with isomerism in organic chemistry, which describes molecules with an identical chemical formula but differ from one another with respect to spatial configuration.

History and etymology

Nuclear isomerism was discovered by Otto Hahn in 1921, the German scientist widely known as the father of nuclear chemistry 3.

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