Isotopes of Astatine

Isotopes Of Astatine

Astatine (At) has 37 known isotopes, all of which are radioactive; the range of their mass numbers is from 191 to 229. There exist also 23 metastable excited states. The longest-lived isotope is 210At, which has a half-life of 8.1 hours; the longest-lived isotope existing in naturally occurring decay chains is 219At with a half-life of 56 seconds.

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.

Alpha decay characteristics for sample astatine isotopes

Mass number	Mass excess	Mass excess of daughter	Average energy of alpha decay	Half-life	Probability of alpha decay	Alpha half-life
207	−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 (nuclei with one or more nucleons – protons or neutrons – in an excited state). A nuclear isomer may also be called a "meta-state"; this means 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 them is astatine-202m1, which has a half-life of about 3 minutes; this is longer than those of all ground states except those of isotopes 203–211 and 220. The least stable one is astatine-214m1; its half-life of 265 ns is shorter than those of all ground states except that of astatine-213.

Alpha decay energy follows 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. However, astatine-211 has a significantly higher energy than the previous isotope; 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 time as 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: 41.8 percent versus just 0.18 percent. 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; this happens due to the increasing role of beta decay. 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 undergo beta decay at all). A beta decay mode has been found for all astatine isotopes except for astatine-213, astatine-214, astatine-215, and astatine-216m. Among other isotopes: astatine-210 and the lighter isotopes decay by positron emission); astatine-216 and the heavier isotopes undergo beta decay; astatine-212 decays via either ways; and astatine-211 decays by electron capture instead.

The most stable isotope of astatine is At-210, which has a half-life of about 8.1 hours. This isotope's primary decay mode is positron emission to the relatively long-lived alpha emitter, polonium-210. In total, only five isotopes of astatine have half-lives exceeding one hour: those between 207 and 211. The least stable ground state isotope is astatine-213, with a half-life of about 125 nanoseconds. It undergoes alpha decay to the extremely long-lived isotope (in practice, a stable one) bismuth-209.