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.
|Main isotopes of actinium (89Ac)|
Actinides and fission products by half-life
|Actinides by decay chain||Half-life
|Fission products of 235U by yield|
No fission products
|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|
|230Th№||231Pa№||32 k – 76 k|
|236Npƒ||233Uƒ||234U№||150 k – 250 k||‡||99Tc₡||126Sn|
|248Cm||242Pu||327 k – 375 k||79Se₡|
|237Npƒ||2.1 M – 6.5 M||135Cs₡||107Pd|
|236U||247Cmƒ||15 M – 24 M||129I₡|
... nor beyond 15.7 M years
|232Th№||238U№||235Uƒ№||0.7 G – 14.1 G|
Legend for superscript symbols
isotopic mass (u)
|range of natural|
|206m1Ac||80(50) keV||15(6) ms|
|206m2Ac||290(110)# keV||41(16) ms||(10−)|
|208mAc||506(26) keV||28(7) ms
|209Ac||89||120||209.00949(5)||92(11) ms||α (99%)||205Fr||(9/2−)|
|210Ac||89||121||210.00944(6)||350(40) ms||α (96%)||206Fr||7+#|
|211Ac||89||122||211.00773(8)||213(25) ms||α (99.8%)||207Fr||9/2−#|
|212Ac||89||123||212.00781(7)||920(50) ms||α (97%)||208Fr||6+#|
|214Ac||89||125||214.006902(24)||8.2(2) s||α (89%)||210Fr||(5+)#|
|215Ac||89||126||215.006454(23)||0.17(1) s||α (99.91%)||211Fr||9/2−|
|216mAc||44(7) keV||443(7) µs||(9−)|
|217Ac||89||128||217.009347(14)||69(4) ns||α (98%)||213Fr||9/2−|
|217mAc||2012(20) keV||740(40) ns||(29/2)+|
|218mAc||584(50)# keV||103(11) ns||(11+)|
|222Ac||89||133||222.017844(6)||5.0(5) s||α (99%)||218Fr||1−|
|222mAc||200(150)# keV||1.05(7) min||α (88.6%)||218Fr||high|
|223Ac||89||134||223.019137(8)||2.10(5) min||α (99%)||219Fr||(5/2−)|
|224Ac||89||135||224.021723(4)||2.78(17) h||β+ (90.9%)||224Ra||0−|
|225Ac[n 3]||89||136||225.023230(5)||10.0(1) d||α||221Fr||(3/2−)|
|226Ac||89||137||226.026098(4)||29.37(12) h||β− (83%)||226Th||(1)(−#)|
|227Ac||Actinium[n 4]||89||138||227.0277521(26)||21.772(3) y||β− (98.61%)||227Th||3/2−||Trace[n 5]|
|228Ac||Mesothorium 2||89||139||228.0310211(27)||6.13(2) h||β−||228Th||3+||Trace[n 6]|
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
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.