Naturaltantalum (₇₃Ta) consists of two stableisotopes:¹⁸¹Ta (99.988%) and^180mTa (0.012%).

There are also 35 known artificialradioisotopes, the longest-lived of which are¹⁷⁹Ta with a half-life of 1.82 years,¹⁸²Ta with a half-life of 114.74 days,¹⁸³Ta with a half-life of 5.1 days, and¹⁷⁷Ta with a half-life of 56.46 hours. All other isotopes have half-lives under a day, most under an hour. There are also numerous isomers, the most stable of which (other than^180mTa) is^182m2Ta with a half-life of 15.8 minutes. All isotopes andnuclear isomers of tantalum are either radioactive orobservationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.

Tantalum has been proposed as a "salting" material fornuclear weapons (cobalt is another, better-known salting material). A jacket of tantalum, irradiated by the intense high-energy neutron flux of the weapon, would be transmuted into the radioactive isotope¹⁸²
Ta, producing about 1.12 MeV ofgamma radiation per decay and significantly increasing the radioactivity of the weapon'sfallout for months. Such a weapon is not known to have ever been built, tested, or used.^[4]

The nuclide^180m
Ta (m denotes ametastable state) is one of a very fewnuclear isomers which are more stable than their ground states. Although it is not unique in this regard (this property is shared bybismuth-210m (^210mBi) andamericium-242m (^242mAm), among other nuclides), it is exceptional in that it isobservationally stable: no decay hasever been observed. In contrast, the ground state nuclide¹⁸⁰
Ta has a half-life of only 8 hours.

^180m
Ta has sufficient energy to decay in three ways:isomeric transition to theground state of¹⁸⁰
Ta,beta decay to¹⁸⁰
W, orelectron capture to¹⁸⁰
Hf. However, no radioactivity from any of these theoretically possible decay modes has ever been observed. As of 2023, the half-life of^180mTa is calculated from experimental observation to be at least2.9×10¹⁷ (290 quadrillion) years.^[6][8][9] The very slow decay of^180m
Ta is attributed to its high spin (9 units) and the low spin of lower-lying states. Gamma or beta decay would require many units of angular momentum to be removed in a single step, so that the process would be very slow.^[10] Similar suppression of gamma or beta decay occurs for^210mBi, a rather short-lived alpha emitter.^[11]

Because of this stability,^180m
Ta is aprimordial nuclide, the only naturally occurringnuclear isomer (excluding short-lived radiogenic and cosmogenic nuclides). It is also the rarest primordial nuclide in the Universe observed for any element which has any stable isotopes. In ans-process stellar environment with a thermal energyk_BT =26 keV (i.e. a temperature of 300 million kelvin), the nuclear isomers are expected to be fully thermalized, meaning that¹⁸⁰Ta is equilibrated between spin states and its overall half-life is predicted to be 11 hours.^[12]

It is one of onlyfive stable nuclides to have both an odd number of protons and an odd number of neutrons, the other four stableodd-odd nuclides being²H,⁶Li,¹⁰B and¹⁴N.^[13]

Daughter products other than tantalum

• Isotopes of tungsten
• Isotopes of hafnium
• Isotopes of lutetium

• Isotopes of tantalum
• Tantalum
• Lists of isotopes by element

• Articles with short description
• Short description with empty Wikidata description