Natural tantalum (Ta) consists of two stable isotopes: 181Ta (99.988%) and 180m
Ta
(0.012%).
The latter nuclide 180m
Ta
(m denotes a metastable state) has sufficient energy to decay in three ways: isomeric transition to the ground state of 180
Ta
, beta decay to 180
W
, and electron capture to 180
Hf
. However, no radioactivity from any decay mode of this nuclear isomer has ever been observed. Only a lower limit on its half-life of over 1015 years has been set, by observation. 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.
The very unusual nature of 180mTa is that the ground state of this isotope is less stable than the isomer. The same property is exhibited in americium-242m (242mAm). 180
Ta
has a half-life of only 8 hours. 180m
Ta
is the only naturally occurring nuclear isomer (excluding radiogenic and cosmogenic short-living nuclides). It is also the rarest primordial nuclide in the Universe observed for any element that has any stable isotopes.
As of October 3, 2016 the half life of 180mTa is calculated to be least 45x1015 (45 million billion) years.
There are also 35 known artificial radioisotopes, the longest-lived of which are 179Ta with a half-life of 1.82 years, 182Ta with a half-life of 114.43 days, 183Ta with a half-life of 5.1 days, and 177Ta with a half-life of 56.56 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 178m1Ta with a half-life of 2.36 hours.
Tantalum has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of 181Ta, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 182
Ta
with a half-life of 114.43 days and produce approximately 1.12 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several months. Such a weapon is not known to have ever been built, tested, or used.
Tantalum has a relative atomic mass of 180.94788(2).