Editing Stable nuclide (section)

=== Nuclear isomers, including a "stable" one ===
The 251 known stable nuclides include tantalum-180m, since even though its decay is automatically implied by its being "metastable", this has not been observed. All "stable" isotopes (stable by observation, not theory) are the ground states of nuclei, except for tantalum-180m, which is a [[nuclear isomer]] or excited state. The ground state, tantalum-180, is radioactive with half-life 8 hours; in contrast, the decay of the nuclear isomer is extremely strongly forbidden by spin-parity selection rules. It has been reported by direct observation that the half-life of {{sup|180m}}Ta to gamma decay must be >10{{sup|15}} years. Other possible modes of {{sup|180m}}Ta decay (beta decay, electron capture, and alpha decay) have also never been observed.

[[File:Binding energy curve - common isotopes.svg|thumb|upright=1.2|Binding energy per nucleon of common isotopes.]]