Isotopes of actinium

Actinium (89Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 32 known isotopes, from 205Ac to 236Ac, and 7 isomers. Three isotopes are found in nature, 225Ac, 227Ac and 228Ac, as intermediate decay products of, respectively, 237Np, 235U, and 232Th. 228Ac and 225Ac are extremely rare, so almost all natural actinium is 227Ac.

The most stable isotopes are 227Ac with a half-life of 21.772 years, 225Ac with a half-life of 10.0 days, and 226Ac with a half-life of 29.37 hours. All other isotopes have half-lives under 10 hours, and most under a minute. The shortest-lived known isotope is 217Ac with a half-life of 69 ns.

Purified 227Ac comes into equilibrium with its decay products (227Th and 223Fr) after 185 days.[1]

Main isotopes of actinium (89Ac)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
225Ac trace 10 d α 221Fr
226Ac syn 29.37 h β 226Th
ε 226Ra
α 222Fr
227Ac trace 21.772 y β 227Th
α 223Fr

Actinides vs fission products

Actinides and fission products by half-life
Actinides[2] by decay chain Half-life
range (y)
Fission products of 235U by yield[3]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[4] 249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210 k years ...

241Amƒ 251Cfƒ[5] 430–900
226Ra 247Bk 1.3 k – 1.6 k
240Pu 229Th 246Cmƒ 243Amƒ 4.7 k – 7.4 k
245Cmƒ 250Cm 8.3 k – 8.5 k
239Puƒ 24.1 k
230Th 231Pa 32 k – 76 k
236Npƒ 233Uƒ 234U 150 k – 250 k 99Tc 126Sn
248Cm 242Pu 327 k – 375 k 79Se
1.53 M 93Zr
237Npƒ 2.1 M – 6.5 M 135Cs 107Pd
236U 247Cmƒ 15 M – 24 M 129I
244Pu 80 M

... nor beyond 15.7 M years[6]

232Th 238U 235Uƒ№ 0.7 G – 14.1 G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  primarily a naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 y: Medium-lived fission product
‡  over 200,000 y: Long-lived fission product

List of isotopes

Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[7][n 1]
isotope(s)[n 2]
(mole fraction)
range of natural
(mole fraction)
excitation energy
205Ac[8] 89 116 20(+97−9) ms α 201Fr
206Ac 89 117 206.01450(8) 25(7) ms (3+)
206m1Ac 80(50) keV 15(6) ms
206m2Ac 290(110)# keV 41(16) ms (10−)
207Ac 89 118 207.01195(6) 31(8) ms
[27(+11−6) ms]
α 203Fr 9/2−#
208Ac 89 119 208.01155(6) 97(16) ms
[95(+24−16) ms]
α (99%) 204Fr (3+)
β+ (1%) 208Ra
208mAc 506(26) keV 28(7) ms
[25(+9−5) ms]
α (89%) 204Fr (10−)
IT (10%) 208Ac
β+ (1%) 208Ra
209Ac 89 120 209.00949(5) 92(11) ms α (99%) 205Fr (9/2−)
β+ (1%) 209Ra
210Ac 89 121 210.00944(6) 350(40) ms α (96%) 206Fr 7+#
β+ (4%) 210Ra
211Ac 89 122 211.00773(8) 213(25) ms α (99.8%) 207Fr 9/2−#
β+ (.2%) 211Ra
212Ac 89 123 212.00781(7) 920(50) ms α (97%) 208Fr 6+#
β+ (3%) 212Ra
213Ac 89 124 213.00661(6) 731(17) ms α 209Fr (9/2−)#
β+ (rare) 213Ra
214Ac 89 125 214.006902(24) 8.2(2) s α (89%) 210Fr (5+)#
β+ (11%) 214Ra
215Ac 89 126 215.006454(23) 0.17(1) s α (99.91%) 211Fr 9/2−
β+ (.09%) 215Ra
216Ac 89 127 216.008720(29) 0.440(16) ms α 212Fr (1−)
β+ (7×10−5%) 216Ra
216mAc 44(7) keV 443(7) µs (9−)
217Ac 89 128 217.009347(14) 69(4) ns α (98%) 213Fr 9/2−
β+ (6.9×10−9%) 217Ra
217mAc 2012(20) keV 740(40) ns (29/2)+
218Ac 89 129 218.01164(5) 1.08(9) µs α 214Fr (1−)#
218mAc 584(50)# keV 103(11) ns (11+)
219Ac 89 130 219.01242(5) 11.8(15) µs α 215Fr 9/2−
β+ (10−6%) 219Ra
220Ac 89 131 220.014763(16) 26.36(19) ms α 216Fr (3−)
β+ (5×10−4%) 220Ra
221Ac 89 132 221.01559(5) 52(2) ms α 217Fr 9/2−#
222Ac 89 133 222.017844(6) 5.0(5) s α (99%) 218Fr 1−
β+ (1%) 222Ra
222mAc 200(150)# keV 1.05(7) min α (88.6%) 218Fr high
IT (10%) 222Ac
β+ (1.4%) 222Ra
223Ac 89 134 223.019137(8) 2.10(5) min α (99%) 219Fr (5/2−)
EC (1%) 223Ra
CD (3.2×10−9%) 209Bi
224Ac 89 135 224.021723(4) 2.78(17) h β+ (90.9%) 224Ra 0−
α (9.1%) 220Fr
β (1.6%) 224Th
225Ac[n 3] 89 136 225.023230(5) 10.0(1) d α 221Fr (3/2−)
CD (6×10−10%) 211Bi
226Ac 89 137 226.026098(4) 29.37(12) h β (83%) 226Th (1)(−#)
EC (17%) 226Ra
α (.006%) 222Fr
227Ac Actinium[n 4] 89 138 227.0277521(26) 21.772(3) y β (98.61%) 227Th 3/2− Trace[n 5]
α (1.38%) 223Fr
228Ac Mesothorium 2 89 139 228.0310211(27) 6.13(2) h β 228Th 3+ Trace[n 6]
α (5.5×10−6%) 224Fr
229Ac 89 140 229.03302(4) 62.7(5) min β 229Th (3/2+)
230Ac 89 141 230.03629(32) 122(3) s β 230Th (1+)
231Ac 89 142 231.03856(11) 7.5(1) min β 231Th (1/2+)
232Ac 89 143 232.04203(11) 119(5) s β 232Th (1+)
233Ac 89 144 233.04455(32)# 145(10) s β 233Th (1/2+)
234Ac 89 145 234.04842(43)# 44(7) s β 234Th
235Ac 89 146 235.05123(38)# 60(4) s β 235Th 1/2+#
236Ac[9] 89 147 236.05530(54)# 72(+345-33) s β 236Th
  1. ^ Abbreviations:
    CD: Cluster decay
    EC: Electron capture
    IT: Isomeric transition
  2. ^ Bold italics for nearly-stable isotopes (half-life longer than the age of the universe)
  3. ^ Has medical uses
  4. ^ Source of element's name
  5. ^ Intermediate decay product of 235U
  6. ^ Intermediate decay product of 232Th


  • 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 mass from IUPAC which use expanded uncertainties.

See also


  1. ^ G. D. Considine, ed. (2005). "Chemical Elements". Van Nostrand's Encyclopedia of Chemistry. Wiley-Interscience. p. 332. ISBN 978-0-471-61525-5.
  2. ^ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  3. ^ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  4. ^ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
    "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 y. No growth of Cf248 was detected, and a lower limit for the β half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
  5. ^ This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
  6. ^ Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
  7. ^ "Universal Nuclide Chart". nucleonica.
  8. ^ Zhang, Z. Y.; Gan, Z. G.; Ma, L.; Yu, L.; Yang, H. B.; Huang, T. H.; Li, G. S.; Tian, Y. L.; Wang, Y. S.; Xu, X. X.; Huang, M. H.; Luo, C.; Ren, Z. Z.; Zhou, S.G.; Zhou, X. H.; Xu, H. S.; Xiao, G. Q. (January 2014). "α decay of the new neutron-deficient isotope 205Ac". Physical Review C. 89 (1): 014308. Bibcode:2014PhRvC..89a4308Z. doi:10.1103/PhysRevC.89.014308.
  9. ^ Chen, L.; et al. (2010). "Discovery and Investigation of Heavy Neutron-Rich Isotopes with Time-Resolved Schottky Spectrometry in the Element Range from Thallium to Actinium" (PDF). Physics Letters B. 691 (5): 234–237. doi:10.1016/j.physletb.2010.05.078.

The actinide or actinoid (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.Strictly speaking, both actinium and lawrencium have been labeled as group 3 elements, but both elements are often included in any general discussion of the chemistry of the actinide elements. Actinium is the more often omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "actinide" means "like actinium", it has been argued that actinium cannot logically be an actinide, but IUPAC acknowledges its inclusion based on common usage.The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium. The series mostly corresponds to the filling of the 5f electron shell, although actinium and thorium lack any 5f electrons, and curium and lawrencium have the same number as the preceding element. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position.

All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors.

Of the actinides, primordial thorium and uranium occur naturally in substantial quantities. The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).


Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated by French chemist André-Louis Debierne in 1899. Friedrich Oskar Giesel later independently isolated it in 1902 and, unaware that it was already known, gave it the name emanium. Actinium gave the name to the actinide series, a group of 15 similar elements between actinium and lawrencium in the periodic table. It is also sometimes considered the first of the 7th-period transition metals, although lawrencium is less commonly given that position. Together with polonium, radium, and radon, actinium was one of the first non-primordial radioactive elements to be isolated.

A soft, silvery-white radioactive metal, actinium reacts rapidly with oxygen and moisture in air forming a white coating of actinium oxide that prevents further oxidation. As with most lanthanides and many actinides, actinium assumes oxidation state +3 in nearly all its chemical compounds. Actinium is found only in traces in uranium and thorium ores as the isotope 227Ac, which decays with a half-life of 21.772 years, predominantly emitting beta and sometimes alpha particles, and 228Ac, which is beta active with a half-life of 6.15 hours. One tonne of natural uranium in ore contains about 0.2 milligrams of actinium-227, and one tonne of thorium contains about 5 nanograms of actinium-228. The close similarity of physical and chemical properties of actinium and lanthanum makes separation of actinium from the ore impractical. Instead, the element is prepared, in milligram amounts, by the neutron irradiation of 226Ra in a nuclear reactor. Owing to its scarcity, high price and radioactivity, actinium has no significant industrial use. Its current applications include a neutron source and an agent for radiation therapy targeting cancer cells in the body and killing them.

Isotopes of protactinium

Protactinium (91Pa) has no stable isotopes. The three naturally occurring isotopes allow a standard mass to be given.

Twenty-nine radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32,760 years, 233Pa with a half-life of 26.967 days, and 230Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives less than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. This element also has five meta states, 217mPa (t1/2 1.15 milliseconds), 220m1Pa (t1/2 308 nanoseconds), 220m2Pa (t1/2 69 nanoseconds), 229mPa (t1/2 420 nanoseconds), and 234mPa (t1/2 1.17 minutes).

The only naturally occurring isotopes are 231Pa, which occurs as an intermediate decay product of 235U, 234Pa and 234mPa, both of which occur as intermediate decay products of 238U. 231Pa makes up nearly all natural protactinium.

The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope 231Pa is alpha decay, except for 228Pa to 230Pa, which primarily decay by electron capture to isotopes of thorium. The primary mode for the heavier isotopes is beta minus (β−) decay. The primary decay products of 231Pa and isotopes of protactinium lighter than and including 227Pa are isotopes of actinium and the primary decay products for the heavier isotopes of protactinium are isotopes of uranium.

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