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 also exist 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 decay 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	7001999900000000000♠≈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	5000000000000000000♠experimentally alpha stable	9000000000000000000♠∞

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.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
	excitation energy
191At	85	106		1.7(+11−5) ms			(1/2+)
191mAt				2.1(+4−3) ms			(7/2−)
193At	85	108	192.99984(6)	28(+5−4) ms	α	189Bi	(1/2+)
193m1At	50 keV			21(5) ms			(7/2−)
193m2At	39 keV			27(+4−5) ms			(13/2+)
194At	85	109	193.99873(20)	~40 ms	α	190Bi	3+#
					β+ (rare)	194Po
194mAt	480(190) keV			~250 ms	α	190Bi	10−#
					IT (rare)	194At
195At	85	110	194.996268(10)	328(20) ms	α (75%)	191Bi	(1/2+)
					β+ (25%)	195Po
195mAt	34(7) keV			147(5) ms			9/2−#
196At	85	111	195.99579(6)	253(9) ms	α (96%)	192Bi	3+#
					β+ (4.0%)	196Po
196m1At	−30(80) keV			20# ms			10−#
196m2At	157.9(1) keV			11 µs			5+#
197At	85	112	196.99319(5)	0.390(16) s	α (96%)	193Bi	(9/2−)
					β+ (4.0%)	197Po
197mAt	52(10) keV			2.0(2) s			(1/2+)
198At	85	113	197.99284(5)	4.2(3) s	α (94%)	194Bi	(3+)
					β+ (6%)	198Po
198mAt	330(90)# keV			1.0(2) s			(10−)
199At	85	114	198.99053(5)	6.92(13) s	α (89%)	195Bi	(9/2−)
					β+ (11%)	199Po
200At	85	115	199.990351(26)	43.2(9) s	α (57%)	196Bi	(3+)
					β+ (43%)	200Po
200m1At	112.7(30) keV			47(1) s	α (43%)	196Bi	(7+)
					IT	200At
					β+	200Po
200m2At	344(3) keV			3.5(2) s			(10−)
201At	85	116	200.988417(9)	85(3) s	α (71%)	197Bi	(9/2−)
					β+ (29%)	201Po
202At	85	117	201.98863(3)	184(1) s	β+ (88%)	202Po	(2,3)+
					α (12%)	198Bi
202m1At	190(40) keV			182(2) s			(7+)
202m2At	580(40) keV			460(50) ms			(10−)
203At	85	118	202.986942(13)	7.37(13) min	β+ (69%)	203Po	9/2−
					α (31%)	199Bi
204At	85	119	203.987251(26)	9.2(2) min	β+ (96%)	204Po	7+
					α (3.8%)	200Bi
204mAt	587.30(20) keV			108(10) ms	IT	204At	(10−)
205At	85	120	204.986074(16)	26.2(5) min	β+ (90%)	205Po	9/2−
					α (10%)	201Bi
205mAt	2339.65(23) keV			7.76(14) µs			29/2+
206At	85	121	205.986667(22)	30.6(13) min	β+ (99.11%)	206Po	(5)+
					α (0.9%)	202Bi
206mAt	807(3) keV			410(80) ns			(10)−
207At	85	122	206.985784(23)	1.80(4) h	β+ (91%)	207Po	9/2−
					α (8.6%)	203Bi
208At	85	123	207.986590(28)	1.63(3) h	β+ (99.5%)	208Po	6+
					α (0.55%)	204Bi
209At	85	124	208.986173(8)	5.41(5) h	β+ (96%)	209Po	9/2−
					α (4.0%)	205Bi
210At	85	125	209.987148(8)	8.1(4) h	β+ (99.8%)	210Po	(5)+
					α (0.18%)	206Bi
210m1At	2549.6(2) keV			482(6) µs			(15)−
210m2At	4027.7(2) keV			5.66(7) µs			(19)+
211At	85	126	210.9874963(30)	7.214(7) h	EC (58.2%)	211Po	9/2−
					α (42%)	207Bi
212At	85	127	211.990745(8)	0.314(2) s	α (99.95%)	208Bi	(1−)
					β+ (0.05%)	212Po
					β− (2×10−6%)	212Rn
212m1At	223(7) keV			0.119(3) s	α (99%)	208Bi	(9−)
					IT (1%)	212At
212m2At	4771.6(11) keV			152(5) µs			(25−)
213At	85	128	212.992937(5)	125(6) ns	α	209Bi	9/2−
214At	85	129	213.996372(5)	558(10) ns	α	210Bi	1−
214m1At	59(9) keV			265(30) ns
214m2At	231(6) keV			760(15) ns			9−
215At	85	130	214.998653(7)	0.10(2) ms	α	211Bi	9/2−	Trace
216At	85	131	216.002423(4)	0.30(3) ms	α (99.99%)	212Bi	1−
					β− (.006%)	216Rn
					EC (3×10−7%)	216Po
216mAt	413(5) keV			100# µs			(9−)
217At	85	132	217.004719(5)	32.3(4) ms	α (99.98%)	213Bi	9/2−
					β− (.012%)	217Rn
218At	85	133	218.008694(12)	1.5(3) s	α (99.9%)	214Bi	1−#	Trace
					β− (0.10%)	218Rn
219At	85	134	219.011162(4)	56(3) s	α (97%)	215Bi	5/2−#	Trace
					β− (3.0%)	219Rn
220At	85	135	220.01541(6)	3.71(4) min	β− (92%)	220Rn	3(−#)
					α (8.0%)	216Bi
221At	85	136	221.01805(21)#	2.3(2) min	β−	221Rn	3/2−#
222At	85	137	222.02233(32)#	54(10) s	β−	222Rn
223At	85	138	223.02519(43)#	50(7) s			3/2−#

Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic masses from IUPAC, which use expanded uncertainties.

References

• Isotope masses from:
• Isotopic compositions and standard atomic masses from:
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

Bibliography

Isotopes of polonium	Isotopes of astatine	Isotopes of radon
Table of nuclides