|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
|180Ta||179.947 46(2)||0.000 1201(32)|
|181Ta||180.948 00(2)||0.999 8799(32)|
In 1969, the Commission recommended Ar(Ta) = 180.9479(3) based on new mass-spectrometric measurements, and recognized tantalum as the element whose atomic weight could be stated very precisely because of single predominant isotope, 181Ta (99.99 %). Standard atomic weight of tantalum was last revised in 2005 to Ar(Ta) = 180.947 88(2). Mass discrimination in the isotopic-abundance measurements have a very small effect to the atomic weight of tantalum.
The minor isotope, 180Ta, is radioactive with a very long half-life of more than 1013 a. It is the last quasi-stable isotope to be discovered. Its interest is heightened by the discovery that it is not the ground-state isomer.
© IUPAC 2003