There are 38 known isotopes and 17 nuclear isomers of tellurium (Te), with atomic masses that range from 105 to 142. These are listed in the table below.
Naturally-occurring tellurium on Earth consists of eight isotopes. Two of these have been found to be radioactive: 128Te and 130Te undergo double beta decay with half-lives of, respectively, 2.2×1024 (2.2 septillion) years (the longest half-life of all nuclides proven to be radioactive) and 7.9×1020 (790 quintillion) years. The longest-lived artificial radioisotope of Tellerium is 121Te with a half-life of nearly 19 days. Several nuclear isomers have longer half-lives, the longest being 121mTe with a half-life of 154 days.
The very-long-lived radioisotopes 128Te and 130Te are the two most common isotopes of tellurium. Of elements with at least one stable isotope, only indium and rhenium likewise have a radioisotope in greater abundance than a stable one.
It has been claimed that electron capture of 123Te was observed, but the recent measurements of the same team have disproved this. The half-life of 123Te is longer than 9.2 × 1016 years, and probably much longer.
124Te can be used as a starting material in the production of radionuclides by a cyclotron or other particle accelerators. Some common radionuclides that can be produced from tellurium-124 are iodine-123 and iodine-124.
The short-lived isotope 135Te (half-life 19 seconds) is produced as a fission product in nuclear reactors. It decays, via two beta decays, to 135Xe, the most powerful known neutron absorber, and the cause of the iodine pit phenomenon.
With the exception of beryllium, tellurium is the lightest element observed to commonly undergo alpha decay, with isotopes 106Te to 110Te being seen to undergo this mode of decay. Some lighter elements have isotopes with alpha decay as a rare branch.
Tellurium's relative atomic mass is 127.60(3).