Isotopes
There are 32 known isotopes of astatine, with atomic masses (mass numbers) of 191 and 193–223. No stable or even long-lived astatine isotope is known, and no such isotope is expected to exist.
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Alpha decay characteristics for sample astatine isotopes Mass
numberMass
excessMass
excess of
daughterAverage
energy of
alpha
decayHalf-life Probability
of alpha
decayAlpha
half-life207 −13.243 MeV −19.116 MeV 5.873 MeV 1.80 h 8.6% 20.9 h 208 −12.491 MeV −18.243 MeV 5.752 MeV 1.63 h 0.55% 12.3 d 209 −12.880 MeV −18.638 MeV 5.758 MeV 5.41 h 4.1% 5.5 d 210 −11.972 MeV −17.604 MeV 5.632 MeV 8.1 h 0.175% 193 d 211 −11.647 MeV −17.630 MeV 5.983 MeV 7.21 h 41.8% 17.2 h 212 −8.621 MeV −16.436 MeV 7.825 MeV 0.31 s ≈100% 0.31 s 213 −6.579 MeV −15.834 MeV 9.255 MeV 125 ns 100% 125 ns 214 −3.380 MeV −12.366 MeV 8.986 MeV 558 ns 100% 558 ns 219 10.397 MeV 4.073 MeV 6.324 MeV 56 s 97% 58 s 220 14.350 MeV 8.298 MeV 6.052 MeV 3.71 min 8% 46.4 min 221 16.810 MeV 11.244 MeV 5.566 MeV 2.3 min ∞
Astatine has 23 nuclear isomers, which are nuclei with one or more nucleons (protons or neutrons) in an excited state. A nuclear isomer may also be called a "meta-state", meaning the system has more internal energy than the "ground state" (the state with the lowest possible internal energy), making the former likely to decay into the latter. There may be more than one isomer for each isotope. The most stable of these nuclear isomers is astatine-202m1, which has a half-life of about 3 minutes, longer than those of all the ground states except for those of isotopes 203–211 and 220. The least stable one is astatine-214m1; its half-life of 265 nanoseconds is shorter than those of all ground states except that of astatine-213.
Astatine's alpha decay energies follow the same trend as for other heavy elements. Lighter astatine isotopes have quite high energies of alpha decay, which become lower as the nuclei become heavier. Astatine-211, however, has a significantly higher energy than the previous isotope, because it has a nucleus with 126 neutrons, and 126 is a magic number corresponding to a filled neutron shell. Despite having a similar half-life to the previous isotope (8.1 hours for astatine-210 and 7.2 hours for astatine-211), the alpha decay probability is much higher for the latter: 41.81% against only 0.18%. The two following isotopes release even more energy, with astatine-213 releasing the highest amount of energy of all astatine isotopes. For this reason, it is the shortest-lived astatine isotope. Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists, due to the increasing role of beta decay (electron emission). This decay mode is especially important for astatine; as early as 1950 it was postulated that the element has no beta-stable isotopes (i.e., ones that do not beta decay at all). Beta decay modes have been found for all astatine isotopes except astatine-213, astatine-214, astatine-215, and astatine-216m. Astatine-210 and lighter isotopes exhibit beta plus decay (positron emission), astatine-216 and heavier isotopes exhibit beta (minus) decay, and astatine-212 decays via both modes, while astatine-211 undergoes electron capture instead.
The most stable isotope is astatine-210, which has a half-life of 8.1 hours. This isotope's primary decay mode is beta plus decay to the relatively long-lived (in comparison to astatine isotopes) alpha emitter polonium-210. In total, only five isotopes have half-lives exceeding one hour (those with mass numbers between 207 and 211). The least stable ground state isotope is astatine-213, with a half-life of 125 nanoseconds. It alpha decays to the extremely long-lived (in practice, stable) bismuth-209.
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