(We have made it to the
actinides - that second row of the two at the bottom of your periodic table)
Actinium (ak-TIN-nee-əm) is a
radioactive chemical element with symbol Ac (not to be confused with the
abbreviation for an acetyl group) and atomic number 89, which was discovered in
1899. It was the first non-primordial radioactive element to be isolated.
Polonium, radium and radon were ...observed
before actinium, but they were not isolated until 1902. Actinium gave the name
to the actinide series, a group of 15 similar elements between actinium and
lawrencium in the periodic table.
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 actinides, actinium assumes oxidation state +3 in nearly all
its chemical compounds. Actinium is found only in traces in uranium ores as the
isotope 227Ac, which decays with a half-life of 21.772 years, predominantly
emitting beta particles. One tonne of uranium ore contains about 0.2 milligrams
of actinium. 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.
Name: actinium
Symbol: Ac
Atomic Number: 89
Pronunciation: ak-TIN-nee-əm
Element category: actinide
(sometimes considered a transition metal)
Period: 7
Block: f
Standard atomic weight: 227 amu
Electron configuration: [Rn] 6d1
7s2
First isolation: Friedrich Oskar
Giesel (1902)
Physical properties
Phase: solid
Density (near r.t.): 10 g•cm−3
Melting point: (circa) 1323 K,1050
°C,1922 °F
Boiling point: 3471 K,3198 °C,5788
°F
Heat of fusion: 14 kJ•mol−1
Heat of vaporization: 400 kJ•mol−1
Molar heat capacity: 27.2
J•mol−1•K−1
Atomic properties
Oxidation states: 3 (neutral
oxide)
Electronegativity: 1.1 (Pauling
scale)
Ionization energies: 1st: 499
kJ•mol−1; 2nd: 1170 kJ•mol−1
Covalent radius: 215 pm
History
André-Louis Debierne, a French
chemist, announced the discovery of a new element in 1899. He separated it from
pitchblende residues left by Marie and Pierre Curie after they had extracted
radium. In 1899, Debierne described the substance as similar to titanium and
(in 1900) as similar to thorium. Friedrich Oskar Giesel independently
discovered actinium in 1902 as a substance being similar to lanthanum and
called it "emanium" in 1904. After a comparison of the substances
half-lives determined by Debierne,[6] Hariett Brooks in 1904, and Otto Hahn and
Otto Sackur in 1905, Debierne's chosen name for the new element was retained
because it had seniority.
Articles published in the 1970s
and later suggest that Debierne's results published in 1904 conflict with those
reported in 1899 and 1900. This has led some authors to advocate that Giesel
alone should be credited with the discovery. A less confrontational vision of
scientific discovery is proposed by Adloff. He suggests that hindsight
criticism of the early publications should be mitigated by the nascent state of
radiochemistry: highlighting the prudence of Debierne's claims in the original
papers, he notes that nobody can contend that Debierne's substance did not
contain actinium. Debierne, who is now considered by the vast majority of
historians as the discoverer, lost interest in the element and left the topic.
Giesel, on the other hand, can rightfully be credited with the first
preparation of radiochemically pure actinium and with the identification of its
atomic number 89.
The name actinium originates from
the Ancient Greek aktis, aktinos, meaning beam or ray. Its symbol Ac is also
used in abbreviations of other compounds that have nothing to do with actinium,
such as acetyl, acetate and sometimes acetaldehyde.
Properties
Actinium is a soft, silvery-white,
radioactive, metallic element. Its estimated shear modulus is similar to that
of lead. Owing to its strong radioactivity, actinium glows in the dark with a
pale blue light, which originates from the surrounding air ionized by the
emitted energetic particles. Actinium has similar chemical properties as
lanthanum and other lanthanides, and therefore these elements are difficult to
separate when extracting from uranium ores. Solvent extraction and ion
chromatography are commonly used for the separation.
The first element of the
actinides, actinium gave the group its name, much as lanthanum had done for the
lanthanides. The group of elements is more diverse than the lanthanides and
therefore it was not until 1945 that Glenn T. Seaborg proposed the most
significant change to Dmitri Mendeleev's periodic table, by introducing the
actinides.
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 actinides, actinium
exists in the oxidation state +3, and the Ac3+ ions are colorless in solutions.
The oxidation state +3 originates from the 6d17s2 electronic configuration of
actinium that is it easily donates 3 electrons assuming a stable closed-shell
structure of the noble gas radon. The rare oxidation state +2 is only known for
actinium dihydride (AcH2).
Chemical compounds
Only a limited number of actinium
compounds are known including AcF3, AcCl3, AcBr3, AcOF, AcOCl, AcOBr, Ac2S3,
Ac2O3 and AcPO4. Except for AcPO4, they are all similar to the corresponding
lanthanum compounds and contain actinium in the oxidation state +3. In
particular, the lattice constants of the analogous lanthanum and actinium
compounds differ by only a few percent.
Oxides
Actinium oxide (Ac2O3) can be
obtained by heating the hydroxide at 500 °C or the oxalate at 1100 °C, in
vacuum. It crystal lattice is isotypic with the oxides of most trivalent
rare-earth metals.
Halides
Actinium trifluoride can be
produced either in solution or in solid reaction. The former reaction is
carried out at room temperature, by adding hydrofluoric acid to a solution
containing actinium ions. In the latter method, actinium metal is treated with
hydrogen fluoride vapors at 700 °C in an all-platinum setup. Treating actinium
trifluoride with ammonium hydroxide at 900–1000 °C yields oxyfluoride AcOF.
Whereas lanthanum oxyfluoride can be easily obtained by burning lanthanum
trifluoride in air at 800 °C for an hour, similar treatment of actinium
trifluoride yields no AcOF and only results in melting of the initial product.
AcF3 + 2 NH3 + H2O → AcOF + 2 NH4F
Actinium trichloride is obtained
by reacting actinium hydroxide or oxalate with carbon tetrachloride vapors at
temperatures above 960 °C. Similar to oxyfluoride, actinium oxychloride can be
prepared by hydrolyzing actinium trichloride with ammonium hydroxide at 1000
°C. However, in contrast to the oxyfluoride, the oxychloride could well be
synthesized by igniting a solution of actinium trichloride in hydrochloric acid
with ammonia.
Reaction of aluminium bromide and
actinium oxide yields actinium tribromide:
Ac2O3 + 2 AlBr3 → 2 AcBr3 + Al2O3
Treating it with ammonium
hydroxide at 500 °C results in the oxybromide (AcOBr).
Other compounds
Actinium hydride was obtained by
reduction of actinium trichloride with potassium at 300 °C, and its structure
was deduced by analogy with the corresponding LaH2 hydride. The source of
hydrogen in the reaction was uncertain.
Mixing monosodium phosphate
(NaH2PO4) with a solution of actinium in hydrochloric acid yields white-colored
actinium phosphate hemihydrate (AcPO4•0.5H2O), and heating actinium oxalate
with hydrogen sulfide vapors at 1400 °C for a few minutes results in a black
actinium sulfide Ac2S3. It may possibly be produced by acting with a mixture of
hydrogen sulfide and carbon disulfide on actinium oxide at 1000 °C.
Isotopes
Naturally occurring actinium is
composed of one radioactive isotope; 227Ac.
Thirty-six radioisotopes have been
identified, the most stable being 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
remaining radioactive isotopes have half-lives that are less than 10 hours and
the majority of them have half-lives shorter than one minute. The
shortest-lived known isotope of actinium is 217
Ac (half-life of 69 nanoseconds)
which decays through alpha decay and electron capture. Actinium also has two
meta states.
Ac comes into equilibrium with its
decay products at the end of 185 days. It decays according to its 21.772-year
half-life emitting mostly beta (98.8%) and some alpha particles (1.2%);[21] the
successive decay products are part of the actinium series. Owing to the low
available amounts, low energy of its beta particles (46 keV) and low intensity
of alpha radiation, 227
Ac is difficult to detect directly
by its emission and it is therefore traced via its decay products.[21] The
isotopes of actinium range in atomic weight from 206 u (206
Ac) to 236 u (236Ac).
Occurrence and synthesis
Actinium is found only in traces
in uranium ores as 227Ac – one tonne of ore contains about 0.2 milligrams of
actinium.[32][33] The actinium isotope 227Ac is a transient member of the
actinium series decay chain, which begins with the parent isotope 235U (or
239Pu) and ends with the stable lead isotope 207Pb. Another actinium isotope
(225Ac) is transiently present in the neptunium series decay chain, beginning
with 237Np (or 233U) and ending with thallium (205Tl) and near-stable bismuth
(209Bi).
The low natural concentration and
the close similarity of physical and chemical properties to those of lanthanum
and other lanthanides, which are always abundant in actinium-bearing ores, render
separation of actinium from the ore impractical, and complete separation was
never achieved. Instead, actinium is prepared, in milligram amounts, by the
neutron irradiation of 226Ra in a nuclear reactor.
The reaction yield is about 2% of
the radium weight. 227Ac can further capture neutrons resulting in small
amounts of 228Ac. After the synthesis, actinium is separated from radium and
from the products of decay and nuclear fusion, such as thorium, polonium, lead
and bismuth. The extraction can be performed with thenoyltrifluoroacetone-benzene
solution from an aqueous solution of the radiation products, and the
selectivity to a certain element is achieved by adjusting the pH (to about 6.0
for actinium). An alternative procedure is anion exchange with an appropriate
resin in nitric acid, which can result in a separation factor of 1,000,000 for
radium and actinium vs. thorium in a two-stage process. Actinium can then be
separated from radium, with a ratio of about 100, using a low cross-linking
cation exchange resin and nitric acid as eluant.
225Ac was first produced
artificially at the Institute for Transuranium Elements (ITU) in Germany using
a cyclotron and at St George Hospital in Sydney using a linac in 2000. This
rare isotope has potential applications in radiation therapy and is most
efficiently produced by bombarding a radium-226 target with 20–30 MeV deuterium
ions. This reaction also yields 226Ac which however decays with a half-life of
29 hours and thus does not contaminate 225Ac.
Actinium metal has been prepared
by the reduction of actinium fluoride with lithium vapor in vacuum at a
temperature between 1100 and 1300 °C. Higher temperatures resulted in
evaporation of the product and lower ones lead to an incomplete transformation.
Lithium was chosen among other alkali metals because its fluoride is most
volatile.
Applications
Owing to its scarcity, high price
and radioactivity, actinium currently has no significant industrial use.
227Ac is highly radioactive and
was therefore studied for use as an active element of radioisotope
thermoelectric generators, for example in spacecraft. The oxide of 227Ac
pressed with beryllium is also an efficient neutron source with the activity
exceeding that of the standard americium-beryllium and radium-beryllium pairs.
In all those applications, 227Ac (a beta source) is merely a progenitor which
generates alpha-emitting isotopes upon its decay. Beryllium captures alpha
particles and emits neutrons owing to its large cross-section for the (α,n)
nuclear reaction:
The 227AcBe neutron sources can be
applied in a neutron probe – a standard device for measuring the quantity of
water present in soil, as well as moisture/density for quality control in
highway construction. Such probes are also used in well logging applications,
in neutron radiography, tomography and other radiochemical investigations.
225Ac is applied in medicine to
produce 213Bi in a reusable generator or can be used alone as an agent for
radiation therapy, in particular targeted alpha therapy (TAT). This isotope has
a half-life of 10 days that makes it much more suitable for radiation therapy
than 213Bi (half-life 46 minutes). Not only 225Ac itself, but also its decay
products emit alpha particles which kill cancer cells in the body. The major difficulty
with application of 225Ac was that intravenous injection of simple actinium
complexes resulted in their accumulation in the bones and liver for a period of
tens of years. As a result, after the cancer cells were quickly killed by alpha
particles from 225Ac, the radiation from the actinium and its decay products
might induce new mutations. To solve this problem, 225Ac was bound to a
chelating agent, such as citrate, ethylenediaminetetraacetic acid (EDTA) or
diethylene triamine pentaacetic acid (DTPA). This reduced actinium accumulation
in the bones, but the excretion from the body remained slow. Much better
results were obtained with such chelating agents as
HEHA(1,4,7,10,13,16-hexaazacyclohexadecane-N,N`,N``,N```,N````,N`````-hexaacetic
acid) or DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)
coupled to trastuzumab, a monoclonal antibody that interferes with the HER2/neu
receptor. The latter delivery combination was tested on mice and proved to be
effective against leukemia, lymphoma, breast, ovarian, neuroblastoma and
prostate cancers.
The medium half-life of 227Ac
(21.77 years) makes it very convenient radioactive isotope in modeling the slow
vertical mixing of oceanic waters. The associated processes cannot be studied
with the required accuracy by direct measurements of current velocities (of the
order 50 meters per year). However, evaluation of the concentration
depth-profiles for different isotopes allows estimating the mixing rates. The
physics behind this method is as follows: oceanic waters contain homogeneously
dispersed 235U. Its decay product, 231Pa, gradually precipitates to the bottom,
so that its concentration first increases with depth and then stays nearly
constant. 231Pa decays to 227Ac; however, the concentration of the latter
isotope does not follow the 231Pa depth profile, but instead increases toward
the sea bottom. This occurs because of the mixing processes which raise some
additional 227Ac from the sea bottom. Thus analysis of both 231Pa and 227Ac
depth profiles allows to model the mixing behavior.
Precautions [edit]
227Ac is highly radioactive and
experiments with it are carried out in a specially designed laboratory equipped
with a glove box. When actinium trichloride is administered intravenously to
rats, about 33% of actinium is deposited into the bones and 50% into the liver.
Its toxicity is comparable to, but slightly lower than that of americium and
plutonium
