Nuclear Isomers:

Isotones are those nuclides that have the same number of neutrons (N) but different number of protons (Z).  For example ¹⁴ ₆C₈, ¹⁵₇N₈, and ¹⁶₈O₈ , all have 8 neutrons.

Nuclear Isomers are the nuclides that have similar Z and A but different nuclear excitation states.  Therefore these have different half-lives and are designated through using m (meaning metastable). For example ^80m Br and ⁸⁰Br. this denotes that one isomer is in a nuclear excited state along with a different half life.