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= Actinide =
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Introduction
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The actinide () or actinoid () series encompasses at least the 14
metallic chemical elements in the 5f series, with atomic numbers from
89 to 102, actinium through nobelium. Number 103, lawrencium, is also
generally included despite being part of the 6d transition series. 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.
The 1985 IUPAC 'Red Book' recommends that 'actinoid' be used rather
than 'actinide', since the suffix '-ide' normally indicates a negative
ion. However, owing to widespread current use, 'actinide' is still
allowed.
Actinium through nobelium are f-block elements, while lawrencium is a
d-block element and a transition metal. The series mostly corresponds
to the filling of the 5f electron shell, although as isolated atoms in
the ground state many have anomalous configurations involving the
filling of the 6d shell due to interelectronic repulsion. 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
curium onwards) behave similarly to the lanthanides, the elements
thorium, protactinium, and uranium are much more similar to transition
metals in their chemistry, with neptunium, plutonium, and americium
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
have been used in nuclear reactors, and uranium and plutonium are
critical elements of 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.
Due to their long half-lives, only thorium and uranium are found on
Earth and astrophysically 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 the 1952 first test of a hydrogen
bomb showed the presence of americium, curium, berkelium, californium,
and the discovery of einsteinium and fermium.
In presentations of the periodic table, the f-block elements are
customarily shown as two additional rows below the main body of the
table. This convention is entirely a matter of aesthetics and
formatting practicality; a rarely used wide-formatted periodic table
inserts the 4f and 5f series in their proper places, as parts of the
table's sixth and seventh rows (periods).
Actinides
Discovery, isolation and synthesis
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Synthesis of transuranium elements
Element !Year !Method
Neptunium align=center| 1940 Bombarding 238U with neutrons
Plutonium align=center| 1941 Bombarding 238U with deuterons
Americium align=center| 1944 Bombarding 239Pu with neutrons
Curium align=center| 1944 Bombarding 239Pu with α-particles
Berkelium align=center| 1949 Bombarding 241Am with α-particles
Californium align=center| 1950 Bombarding 242Cm with α-particles
Einsteinium align=center| 1952 As a product of nuclear explosion
Fermium align=center| 1952 As a product of nuclear explosion
Mendelevium align=center| 1955 Bombarding 253Es with α-particles
Nobelium align=center| 1965 Bombarding 243Am with 15N or 238U with
22Ne
Lawrencium align=center| 1961 -1971 Bombarding 252Cf with 10B or 11B
and of 243Am with 18O
Like the lanthanides, the actinides form a family of elements with
similar properties. Within the actinides, there are two overlapping
groups: transuranium elements, which follow uranium in the periodic
table; and transplutonium elements, which follow plutonium. Compared
to the lanthanides, which (except for promethium) are found in nature
in appreciable quantities, most actinides are rare. Most do not occur
in nature, and of those that do, only thorium and uranium do so in
more than trace quantities. The most abundant or easily synthesized
actinides are uranium and thorium, followed by plutonium, americium,
actinium, protactinium, neptunium, and curium.
The existence of transuranium elements was suggested in 1934 by Enrico
Fermi, based on his experiments. However, even though four actinides
were known by that time, it was not yet understood that they formed a
family similar to lanthanides. The prevailing view that dominated
early research into transuranics was that they were regular elements
in the 7th period, with thorium, protactinium and uranium
corresponding to 6th-period hafnium, tantalum and tungsten,
respectively. Synthesis of transuranics gradually undermined this
point of view. By 1944, an observation that curium failed to exhibit
oxidation states above 4 (whereas its supposed 6th period homolog,
platinum, can reach oxidation state of 6) prompted Glenn Seaborg to
formulate an "actinide hypothesis". Studies of known actinides and
discoveries of further transuranic elements provided more data in
support of this position, but the phrase "actinide hypothesis" (the
implication being that a "hypothesis" is something that has not been
decisively proven) remained in active use by scientists through the
late 1950s.
At present, there are two major methods of producing isotopes of
transplutonium elements: (1) irradiation of the lighter elements with
neutrons; (2) irradiation with accelerated charged particles. The
first method is more important for applications, as only neutron
irradiation using nuclear reactors allows the production of sizeable
amounts of synthetic actinides; however, it is limited to relatively
light elements. The advantage of the second method is that elements
heavier than plutonium, as well as neutron-deficient isotopes, can be
obtained, which are not formed during neutron irradiation.
In 1962-1966, there were attempts in the United States to produce
transplutonium isotopes using a series of six underground nuclear
explosions. Small samples of rock were extracted from the blast area
immediately after the test to study the explosion products, but no
isotopes with mass number greater than 257 could be detected, despite
predictions that such isotopes would have relatively long half-lives
of α-decay. This non-observation was attributed to spontaneous fission
owing to the large speed of the products and to other decay channels,
such as neutron emission and nuclear fission.
From actinium to uranium
==========================
Uranium and thorium were the first actinides discovered. Uranium was
identified in 1789 by the German chemist Martin Heinrich Klaproth in
pitchblende ore. He named it after the planet Uranus, which had been
discovered eight years earlier. Klaproth was able to precipitate a
yellow compound (likely sodium diuranate) by dissolving pitchblende in
nitric acid and neutralizing the solution with sodium hydroxide. He
then reduced the obtained yellow powder with charcoal, and extracted a
black substance that he mistook for metal. Sixty years later, the
French scientist Eugène-Melchior Péligot identified it as uranium
oxide. He also isolated the first sample of uranium metal by heating
uranium tetrachloride with metallic potassium. The atomic mass of
uranium was then calculated as 120, but Dmitri Mendeleev in 1872
corrected it to 240 using his periodicity laws. This value was
confirmed experimentally in 1882 by K. Zimmerman.
Thorium oxide was discovered by Friedrich Wöhler in the mineral
thorianite, which was found in Norway (1827). Jöns Jacob Berzelius
characterized this material in more detail in 1828. By reduction of
thorium tetrachloride with potassium, he isolated the metal and named
it thorium after the Norse god of thunder and lightning Thor. The same
isolation method was later used by Péligot for uranium.
Actinium was discovered in 1899 by André-Louis Debierne, an assistant
of Marie Curie, in the pitchblende waste left after removal of radium
and polonium. He described the substance (in 1899) as similar to
titanium and (in 1900) as similar to thorium. The discovery of
actinium by Debierne was however questioned in 1971 and 2000, arguing
that Debierne's publications in 1904 contradicted his earlier work of
1899-1900. This view instead credits the 1902 work of Friedrich Oskar
Giesel, who discovered a radioactive element named 'emanium' that
behaved similarly to lanthanum. The name actinium comes from the ,
meaning beam or ray. This metal was discovered not by its own
radiation but by the radiation of the daughter products. Owing to the
close similarity of actinium and lanthanum and low abundance, pure
actinium could only be produced in 1950. The term actinide was
probably introduced by Victor Goldschmidt in 1937.
Protactinium was possibly isolated in 1900 by William Crookes. It was
first identified in 1913, when Kasimir Fajans and Oswald Helmuth
Göhring encountered the short-lived isotope 234mPa (half-life 1.17
minutes) during their studies of the 238U decay chain. They named the
new element 'brevium' (from Latin 'brevis' meaning brief); the name
was changed to 'protoactinium' (from Greek πρῶτος + ἀκτίς meaning
"first beam element") in 1918 when two groups of scientists, led by
the Austrian Lise Meitner and Otto Hahn of Germany and Frederick Soddy
and John Arnold Cranston of Great Britain, independently discovered
the much longer-lived 231Pa. The name was shortened to 'protactinium'
in 1949. This element was little characterized until 1960, when Alfred
Maddock and his co-workers in the U.K. isolated 130 grams of
protactinium from 60 tonnes of waste left after extraction of uranium
from its ore.
Neptunium and above
=====================
Neptunium (named for the planet Neptune, the next planet out from
Uranus, after which uranium was named) was discovered by Edwin
McMillan and Philip H. Abelson in 1940 in Berkeley, California. They
produced the 239Np isotope (half-life 2.4 days) by bombarding uranium
with slow neutrons. It was the first transuranium element produced
synthetically.
Transuranium elements do not occur in sizeable quantities in nature
and are commonly synthesized via nuclear reactions conducted with
nuclear reactors. For example, under irradiation with reactor
neutrons, uranium-238 partially converts to plutonium-239:
:
This synthesis reaction was used by Fermi and his collaborators in
their design of the reactors located at the Hanford Site, which
produced significant amounts of plutonium-239 for the nuclear weapons
of the Manhattan Project and the United States' post-war nuclear
arsenal.
Actinides with the highest mass numbers are synthesized by bombarding
uranium, plutonium, curium and californium with ions of nitrogen,
oxygen, carbon, neon or boron in a particle accelerator. Thus nobelium
was produced by bombarding uranium-238 with neon-22 as
: _{92}^{238}U + _{10}^{22}Ne -> _{102}^{256}No + 4_0^1n.
The first isotopes of transplutonium elements, americium-241 and
curium-242, were synthesized in 1944 by Glenn T. Seaborg, Ralph A.
James and Albert Ghiorso. Curium-242 was obtained by bombarding
plutonium-239 with 32-MeV α-particles:
: _{94}^{239}Pu + _2^4He -> _{96}^{242}Cm + _0^1n.
The americium-241 and curium-242 isotopes also were produced by
irradiating plutonium in a nuclear reactor. The latter element was
named after Marie Curie and her husband Pierre who are noted for
discovering radium and for their work in radioactivity.
Bombarding curium-242 with α-particles resulted in an isotope of
californium 245Cf in 1950, and a similar procedure yielded
berkelium-243 from americium-241 in 1949. The new elements were named
after Berkeley, California, by analogy with its lanthanide homologue
terbium, which was named after the village of Ytterby in Sweden.
In 1945, B. B. Cunningham obtained the first bulk chemical compound of
a transplutonium element, namely americium hydroxide. Over the few
years, milligram quantities of americium and microgram amounts of
curium were accumulated that allowed production of isotopes of
berkelium and californium. Sizeable amounts of these elements were
produced in 1958, and the first californium compound (0.3 μg of CfOCl)
was obtained in 1960 by B. B. Cunningham and J. C. Wallmann.
Einsteinium and fermium were identified in 1952-1953 in the fallout
from the "Ivy Mike" nuclear test (1 November 1952), the first
successful test of a hydrogen bomb. Instantaneous exposure of
uranium-238 to a large neutron flux resulting from the explosion
produced heavy isotopes of uranium, which underwent a series of beta
decays to nuclides such as einsteinium-253 and fermium-255. The
discovery of the new elements and the new data on neutron capture were
initially kept secret on the orders of the US military until 1955 due
to Cold War tensions. Nevertheless, the Berkeley team were able to
prepare einsteinium and fermium by civilian means, through the neutron
bombardment of plutonium-239, and published this work in 1954 with the
disclaimer that it was not the first studies that had been carried out
on those elements. The "Ivy Mike" studies were declassified and
published in 1955. The first significant (submicrogram) amounts of
einsteinium were produced in 1961 by Cunningham and colleagues, but
this has not been done for fermium yet.
The first isotope of mendelevium, 256Md (half-life 87 min), was
synthesized by Albert Ghiorso, Glenn T. Seaborg, Gregory Robert
Choppin, Bernard G. Harvey and Stanley Gerald Thompson when they
bombarded an 253Es target with alpha particles in the 60-inch
cyclotron of Berkeley Radiation Laboratory; this was the first isotope
of any element to be synthesized one atom at a time.
There were several attempts to obtain isotopes of nobelium by Swedish
(1957) and American (1958) groups, but the first reliable result was
the synthesis of 256No by the Russian group of Georgy Flyorov in 1965,
as acknowledged by the IUPAC in 1992. In their experiments, Flyorov et
al. bombarded uranium-238 with neon-22.
In 1961, Ghiorso et al. obtained the first isotope of lawrencium by
irradiating californium (mostly californium-252) with boron-10 and
boron-11 ions. The mass number of this isotope was not clearly
established (possibly 258 or 259) at the time. In 1965, 256Lr was
synthesized by Flyorov et al. from 243Am and 18O. Thus IUPAC
recognized the nuclear physics teams at Dubna and Berkeley as the
co-discoverers of lawrencium.
Isotopes
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!+ colspan=7 | Nuclear properties of isotopes of the most important
transplutonium isotopes
rowspan="2"| Isotope rowspan="2"| Half-life rowspan="2"| Probability
of spontaneous fission in % colspan="2"| Emission energy (MeV) (yield
in %) colspan="2"| Specific activity (Bq/kg) of
α γ α, β-particles fission
|241Am 432.2(7) y 4.3(18) 5.485 (84.8) 5.442 (13.1) 5.388
(1.66) 0.059 (35.9) 0.026 (2.27) 1.27 546.1
|243Am 7.37(4) y 3.7(2) 5.275 (87.1) 5.233 (11.2) 5.181 (1.36)
0.074 (67.2) 0.043 (5.9) 7.39 273.3
|242Cm 162.8(2) d 6.2(3) 6.069 (25.92) 6.112 (74.08) 0.044
(0.04) 0.102 (4) 1.23 7.6
|244Cm 18.10(2) y 1.37(3) 5.762 (23.6) 5.804 (76.4) 0.043
(0.02) 0.100 (1.5) 2.96 4.1
|245Cm 8.5(1) y 6.1(9) 5.529 (0.58) 5.488 (0.83) 5.361 (93.2)
0.175 (9.88) 0.133 (2.83) 6.35 3.9
|246Cm 4.76(4) y 0.02615(7) 5.343 (17.8) 5.386 (82.2) 0.045
(19) 1.13 2.95
|247Cm 1.56(5) y -- 5.267 (13.8) 5.212 (5.7) 5.147 (1.2)
0.402 (72) 0.278 (3.4) 3.43 --
|248Cm 3.48(6) y 8.39(16) 5.034 (16.52) 5.078 (75) -- 1.40
1.29
|249Bk 330(4) d 4.7(2) 5.406 (1) 5.378 (2.6) 0.32 (5.8)
5.88 2.76
|249Cf 351(2) y 5.0(4) 6.193 (2.46) 6.139 (1.33) 5.946 (3.33)
0.388 (66) 0.333 (14.6) 1.51 7.57
|250Cf 13.08(9) y 0.077(3) 5.988 (14.99) 6.030 (84.6) 0.043
4.04 3.11
|251Cf 900(40) y ? 6.078 (2.6) 5.567 (0.9) 5.569 (0.9) 0.177
(17.3) 0.227 (6.8) 5.86 --
|252Cf 2.645(8) y 3.092(8) 6.075 (15.2) 6.118 (81.6) 0.042
(1.4) 0.100 (1.3) 1.92 6.14
|254Cf 60.5(2) d ≈100 5.834 (0.26) 5.792 (5.3) -- 9.75 3.13
|253Es 20.47(3) d 8.7(3) 6.540 (0.85) 6.552 (0.71) 6.590 (6.6)
0.387 (0.05) 0.429 (8) 9.33 8.12
|254Es 275.7(5) d < 3 6.347 (0.75) 6.358 (2.6) 6.415 (1.8)
0.042 (100) 0.034 (30) 6.9 --
|255Es 39.8(12) d 0.0041(2) 6.267 (0.78) 6.401 (7) -- 4.38(β)
3.81(α) 1.95
|255Fm 20.07(7) h 2.4(10) 7.022 (93.4) 6.963 (5.04) 6.892
(0.62) 0.00057 (19.1) 0.081 (1) 2.27 5.44
|256Fm 157.6(13) min 91.9(3) 6.872 (1.2) 6.917 (6.9) -- 1.58
1.4
|257Fm 100.5(2) d 0.210(4) 6.752 (0.58) 6.695 (3.39) 6.622
(0.6) 0.241 (11) 0.179 (8.7) 1.87 3.93
|256Md 77(2) min -- 7.142 (1.84) 7.206 (5.9) -- 3.53 --
|257Md 5.52(5) h -- 7.074 (14) 0.371 (11.7) 0.325 (2.5) 8.17
--
|258Md 51.5(3) d -- 6.73 -- 3.64 --
|255No 3.1(2) min -- 8.312 (1.16) 8.266 (2.6) 8.121 (27.8)
0.187 (3.4) 8.78 --
|259No 58(5) min -- 7.455 (9.8) 7.500 (29.3) 7.533 (17.3) --
4.63 --
|256Lr 27(3) s < 0.03 8.319 (5.4) 8.390 (16) 8.430 (33) --
5.96 --
|257Lr 646(25) ms -- 8.796 (18) 8.861 (82) -- 1.54 --
Thirty-four isotopes of actinium and eight excited isomeric states of
some of its nuclides are known, ranging in mass number from 203 to
236. Three isotopes, 225Ac, 227Ac and 228Ac, were found in nature and
the others were produced in the laboratory; only the three natural
isotopes are used in applications. Actinium-225 is a member of the
radioactive neptunium series; it was first discovered in 1947 as a
decay product of uranium-233 and it is an α-emitter with a half-life
of 10 days. Actinium-225 is less available than actinium-228, but is
more promising in radiotracer applications. Actinium-227 (half-life
21.77 years) occurs in all uranium ores, but in small quantities. One
gram of uranium (in radioactive equilibrium) contains only 2 gram of
227Ac. Actinium-228 is a member of the radioactive thorium series
formed by the decay of 228Ra; it is a β− emitter with a half-life of
6.15 hours. In one tonne of thorium there is 5 gram of 228Ac. It was
discovered by Otto Hahn in 1906.
There are 32 known isotopes of thorium ranging in mass number from 207
to 238. Of these, the longest-lived is 232Th, whose half-life of
means that it still exists in nature as a primordial nuclide. The next
longest-lived is 230Th, an intermediate decay product of 238U with a
half-life of 75,400 years. Several other thorium isotopes have
half-lives over a day; all of these are also transient in the decay
chains of 232Th, 235U, and 238U.
Twenty-nine isotopes of protactinium are known with mass numbers
211-239 as well as three excited isomeric states. Only 231Pa and 234Pa
have been found in nature. All the isotopes have short lifetimes,
except for protactinium-231 (half-life 32,760 years). The most
important isotopes are 231Pa and 233Pa, which is an intermediate
product in obtaining uranium-233 and is the most affordable among
artificial isotopes of protactinium. 233Pa has convenient half-life
and energy of γ-radiation, and thus was used in most studies of
protactinium chemistry. Protactinium-233 is a β-emitter with a
half-life of 26.97 days.
There are 27 known isotopes of uranium, having mass numbers 215-242
(except 220). Three of them, 234U, 235U and 238U, are present in
appreciable quantities in nature. Among others, the most important is
233U, which is a final product of transformation of 232Th irradiated
by slow neutrons. 233U has a much higher fission efficiency by
low-energy (thermal) neutrons, compared e.g. with 235U. Most uranium
chemistry studies were carried out on uranium-238 owing to its long
half-life of 4.4 years.
There are 25 isotopes of neptunium with mass numbers 219-244 (except
221); they are all highly radioactive. The most popular among
scientists are long-lived 237Np (t1/2 = 2.20 years) and short-lived
239Np, 238Np (t1/2 ~ 2 days).
There are 21 known isotopes of plutonium, having mass numbers 227-247.
The most stable isotope of plutonium is 244Pu with half-life of 8.13
years.
Eighteen isotopes of americium are known with mass numbers from 229 to
247 (with the exception of 231). The most important are 241Am and
243Am, which are alpha-emitters and also emit soft, but intense
γ-rays; both of them can be obtained in an isotopically pure form.
Chemical properties of americium were first studied with 241Am, but
later shifted to 243Am, which is almost 20 times less radioactive. The
disadvantage of 243Am is production of the short-lived daughter
isotope 239Np, which has to be considered in the data analysis.
Among 19 isotopes of curium, ranging in mass number from 233 to 251,
the most accessible are 242Cm and 244Cm; they are α-emitters, but with
much shorter lifetime than the americium isotopes. These isotopes emit
almost no γ-radiation, but undergo spontaneous fission with the
associated emission of neutrons. More long-lived isotopes of curium
(245-248Cm, all α-emitters) are formed as a mixture during neutron
irradiation of plutonium or americium. Upon short irradiation, this
mixture is dominated by 246Cm, and then 248Cm begins to accumulate.
Both of these isotopes, especially 248Cm, have a longer half-life
(3.48 years) and are much more convenient for carrying out chemical
research than 242Cm and 244Cm, but they also have a rather high rate
of spontaneous fission. 247Cm has the longest lifetime among isotopes
of curium (1.56 years), but is not formed in large quantities because
of the strong fission induced by thermal neutrons.
Seventeen isotopes of berkelium have been identified with mass numbers
233, 234, 236, 238, and 240-252. Only 249Bk is available in large
quantities; it has a relatively short half-life of 330 days and emits
mostly soft β-particles, which are inconvenient for detection. Its
alpha radiation is rather weak (1.45% with respect to β-radiation),
but is sometimes used to detect this isotope. 247Bk is an
alpha-emitter with a long half-life of 1,380 years, but it is hard to
obtain in appreciable quantities; it is not formed upon neutron
irradiation of plutonium because β-decay of curium isotopes with mass
number below 248 is not known. (247Cm would actually release energy by
β-decaying to 247Bk, but this has never been seen.)
The 20 isotopes of californium with mass numbers 237-256 are formed in
nuclear reactors; californium-253 is a β-emitter and the rest are
α-emitters. The isotopes with even mass numbers (250Cf, 252Cf and
254Cf) have a high rate of spontaneous fission, especially 254Cf of
which 99.7% decays by spontaneous fission. Californium-249 has a
relatively long half-life (352 years), weak spontaneous fission and
strong γ-emission that facilitates its identification. 249Cf is not
formed in large quantities in a nuclear reactor because of the slow
β-decay of the parent isotope 249Bk and a large cross section of
interaction with neutrons, but it can be accumulated in the
isotopically pure form as the β-decay product of (pre-selected) 249Bk.
Californium produced by reactor-irradiation of plutonium mostly
consists of 250Cf and 252Cf, the latter being predominant for large
neutron fluences, and its study is hindered by the strong neutron
radiation.
Properties of some transplutonium isotope pairs
Parent isotope t1/2 Daughter isotope t1/2 Time to establish
radioactive equilibrium
243Am 7370 years 239Np 2.35 days 47.3 days
245Cm 8265 years 241Pu 14 years 129 years
247Cm 1.64 years 243Pu 4.95 hours 7.2 days
254Es 270 days 250Bk 3.2 hours 35.2 hours
255Es 39.8 days 255Fm 22 hours 5 days
257Fm 79 days 253Cf 17.6 days 49 days
Among the 18 known isotopes of einsteinium with mass numbers from 240
to 257, the most affordable is 253Es. It is an α-emitter with a
half-life of 20.47 days, a relatively weak γ-emission and small
spontaneous fission rate as compared with the isotopes of californium.
Prolonged neutron irradiation also produces a long-lived isotope 254Es
(t1/2 = 275.5 days).
Twenty isotopes of fermium are known with mass numbers of 241-260.
254Fm, 255Fm and 256Fm are α-emitters with a short half-life (hours),
which can be isolated in significant amounts. 257Fm (t1/2 = 100 days)
can accumulate upon prolonged and strong irradiation. All these
isotopes are characterized by high rates of spontaneous fission.
Among the 17 known isotopes of mendelevium (mass numbers from 244 to
260), the most studied is 256Md, which mainly decays through electron
capture (α-radiation is ≈10%) with a half-life of 77 minutes. Another
alpha emitter, 258Md, has a half-life of 53 days. Both these isotopes
are produced from rare einsteinium (253Es and 255Es respectively),
that therefore limits their availability.
Long-lived isotopes of nobelium and isotopes of lawrencium (and of
heavier elements) have relatively short half-lives. For nobelium, 13
isotopes are known, with mass numbers 249-260 and 262. The chemical
properties of nobelium and lawrencium were studied with 255No (t1/2 =
3 min) and 256Lr (t1/2 = 35 s). The longest-lived nobelium isotope,
259No, has a half-life of approximately 1 hour. Lawrencium has 14
known isotopes with mass numbers 251-262, 264, and 266. The most
stable of them is 266Lr with a half-life of 11 hours.
Among all of these, the only isotopes that occur in sufficient
quantities in nature to be detected in anything more than traces and
have a measurable contribution to the atomic weights of the actinides
are the primordial 232Th, 235U, and 238U, and three long-lived decay
products of natural uranium, 230Th, 231Pa, and 234U. Natural thorium
consists of 0.02(2)% 230Th and 99.98(2)% 232Th; natural protactinium
consists of 100% 231Pa; and natural uranium consists of 0.0054(5)%
234U, 0.7204(6)% 235U, and 99.2742(10)% 238U.
Formation in nuclear reactors
======================================================================
The figure 'buildup of actinides' is a table of nuclides with the
number of neutrons on the horizontal axis (isotopes) and the number of
protons on the vertical axis (elements). The red dot divides the
nuclides in two groups, so the figure is more compact. Each nuclide is
represented by a square with the mass number of the element and its
half-life. Naturally existing actinide isotopes (Th, U) are marked
with a bold border, alpha emitters have a yellow colour, and beta
emitters have a blue colour. Pink indicates electron capture (236Np),
whereas white stands for a long-lasting metastable state (242Am).
The formation of actinide nuclides is primarily characterised by:
* Neutron capture reactions (n,γ), which are represented in the figure
by a short right arrow.
* The (n,2n) reactions and the less frequently occurring (γ,n)
reactions are also taken into account, both of which are marked by a
short left arrow.
* Even more rarely and only triggered by fast neutrons, the (n,3n)
reaction occurs, which is represented in the figure with one example,
marked by a long left arrow.
In addition to these neutron- or gamma-induced nuclear reactions, the
radioactive conversion of actinide nuclides also affects the nuclide
inventory in a reactor. These decay types are marked in the figure by
diagonal arrows. The beta-minus decay, marked with an arrow pointing
up-left, plays a major role for the balance of the particle densities
of the nuclides. Nuclides decaying by positron emission (beta-plus
decay) or electron capture (ϵ) do not occur in a nuclear reactor
except as products of knockout reactions; their decays are marked with
arrows pointing down-right. Due to the long half-lives of the given
nuclides, alpha decay plays almost no role in the formation and decay
of the actinides in a power reactor, as the residence time of the
nuclear fuel in the reactor core is rather short (a few years).
Exceptions are the two relatively short-lived nuclides 242Cm (T1/2 =
163 d) and 236Pu (T1/2 = 2.9 y). Only for these two cases, the α decay
is marked on the nuclide map by a long arrow pointing down-left. A few
long-lived actinide isotopes, such as 244Pu and 250Cm, cannot be
produced in reactors because neutron capture does not happen quickly
enough to bypass the short-lived beta-decaying nuclides 243Pu and
249Cm; they can however be generated in nuclear explosions, which have
much higher neutron fluxes.
Distribution in nature
======================================================================
Thorium and uranium are the most abundant actinides in nature with the
respective mass concentrations of 16 ppm and 4 ppm. Uranium mostly
occurs in the Earth's crust as a mixture of its oxides in the mineral
uraninite, which is also called pitchblende because of its black
color. There are several dozens of other uranium minerals such as
carnotite (KUO2VO4·3H2O) and autunite (Ca(UO2)2(PO4)2·nH2O). The
isotopic composition of natural uranium is 238U (relative abundance
99.2742%), 235U (0.7204%) and 234U (0.0054%); of these 238U has the
largest half-life of 4.51 years. The worldwide production of uranium
in 2009 amounted to 50,572 tonnes, of which 27.3% was mined in
Kazakhstan. Other important uranium mining countries are Canada
(20.1%), Australia (15.7%), Namibia (9.1%), Russia (7.0%), and Niger
(6.4%).
Content of plutonium in uranium and thorium ores
!Ore !Location Uranium content, % Mass ratio 239Pu/ore Ratio
239Pu/U ()
Uraninite Canada 13.5 9.1 7.1
Uraninite Congo 38 4.8 12
Uraninite Colorado, US 50 3.8 7.7
Monazite Brazil 0.24 2.1 8.3
Monazite North Carolina, US 1.64 5.9 3.6
Fergusonite - 0.25 <1 <4
Carnotite - 10 <4 <0.4
The most abundant thorium minerals are thorianite (), thorite () and
monazite, (). Most thorium minerals contain uranium and vice versa;
and they all have significant fraction of lanthanides. Rich deposits
of thorium minerals are located in the United States (440,000 tonnes),
Australia and India (~300,000 tonnes each) and Canada (~100,000
tonnes).
The abundance of actinium in the Earth's crust is only about 5%.
Actinium is mostly present in uranium-containing, but also in other
minerals, though in much smaller quantities. The content of actinium
in most natural objects corresponds to the isotopic equilibrium of
parent isotope 235U, and it is not affected by the weak Ac migration.
Protactinium is more abundant (10−12%) in the Earth's crust than
actinium. It was discovered in uranium ore in 1913 by Fajans and
Göhring. As actinium, the distribution of protactinium follows that of
235U.
The half-life of the longest-lived isotope of neptunium, 237Np, is
negligible compared to the age of the Earth. Thus neptunium is present
in nature in negligible amounts produced as intermediate decay
products of other isotopes. Traces of plutonium in uranium minerals
were first found in 1942, and the more systematic results on 239Pu are
summarized in the table (no other plutonium isotopes could be detected
in those samples). The upper limit of abundance of the longest-living
isotope of plutonium, 244Pu, is 3%. Plutonium could not be detected in
samples of lunar soil. Owing to its scarcity in nature, most plutonium
is produced synthetically.
Extraction
======================================================================
Owing to the low abundance of actinides, their extraction is a
complex, multistep process. Fluorides of actinides are usually used
because they are insoluble in water and can be easily separated with
redox reactions. Fluorides are reduced with calcium, magnesium or
barium:
:
Among the actinides, thorium and uranium are the easiest to isolate.
Thorium is extracted mostly from monazite: thorium pyrophosphate
(ThP2O7) is reacted with nitric acid, and the produced thorium nitrate
treated with tributyl phosphate. Rare-earth impurities are separated
by increasing the pH in sulfate solution.
In another extraction method, monazite is decomposed with a 45%
aqueous solution of sodium hydroxide at 140 °C. Mixed metal hydroxides
are extracted first, filtered at 80 °C, washed with water and
dissolved with concentrated hydrochloric acid. Next, the acidic
solution is neutralized with hydroxides to pH = 5.8 that results in
precipitation of thorium hydroxide (Th(OH)4) contaminated with ~3% of
rare-earth hydroxides; the rest of rare-earth hydroxides remains in
solution. Thorium hydroxide is dissolved in an inorganic acid and then
purified from the rare earth elements. An efficient method is the
dissolution of thorium hydroxide in nitric acid, because the resulting
solution can be purified by extraction with organic solvents:
:Th(OH)4 + 4 HNO3 → Th(NO3)4 + 4 H2O
Metallic thorium is separated from the anhydrous oxide, chloride or
fluoride by reacting it with calcium in an inert atmosphere:
:ThO2 + 2 Ca → 2 CaO + Th
Sometimes thorium is extracted by electrolysis of a fluoride in a
mixture of sodium and potassium chloride at 700-800 °C in a graphite
crucible. Highly pure thorium can be extracted from its iodide with
the crystal bar process.
Uranium is extracted from its ores in various ways. In one method, the
ore is burned and then reacted with nitric acid to convert uranium
into a dissolved state. Treating the solution with a solution of
tributyl phosphate (TBP) in kerosene transforms uranium into an
organic form UO2(NO3)2(TBP)2. The insoluble impurities are filtered
and the uranium is extracted by reaction with hydroxides as (NH4)2U2O7
or with hydrogen peroxide as UO4·2H2O.
When the uranium ore is rich in such minerals as dolomite, magnesite,
etc., those minerals consume much acid. In this case, the carbonate
method is used for uranium extraction. Its main component is an
aqueous solution of sodium carbonate, which converts uranium into a
complex [UO2(CO3)3]4−, which is stable in aqueous solutions at low
concentrations of hydroxide ions. The advantages of the sodium
carbonate method are that the chemicals have low corrosivity (compared
to nitrates) and that most non-uranium metals precipitate from the
solution. The disadvantage is that tetravalent uranium compounds
precipitate as well. Therefore, the uranium ore is treated with sodium
carbonate at elevated temperature and under oxygen pressure:
:2 UO2 + O2 + 6 → 2 [UO2(CO3)3]4−
This equation suggests that the best solvent for the uranyl carbonate
processing is a mixture of carbonate with bicarbonate. At high pH,
this results in precipitation of diuranate, which is treated with
hydrogen in the presence of nickel yielding an insoluble uranium
tetracarbonate.
Another separation method uses polymeric resins as a polyelectrolyte.
Ion exchange processes in the resins result in separation of uranium.
Uranium from resins is washed with a solution of ammonium nitrate or
nitric acid that yields uranyl nitrate, UO2(NO3)2·6H2O. When heated,
it turns into UO3, which is converted to UO2 with hydrogen:
: UO3 + H2 → UO2 + H2O
Reacting uranium dioxide with hydrofluoric acid changes it to uranium
tetrafluoride, which yields uranium metal upon reaction with magnesium
metal:
: 4 HF + UO2 → UF4 + 2 H2O
To extract plutonium, neutron-irradiated uranium is dissolved in
nitric acid, and a reducing agent (FeSO4, or H2O2) is added to the
resulting solution. This addition changes the oxidation state of
plutonium from +6 to +4, while uranium remains in the form of uranyl
nitrate (UO2(NO3)2). The solution is treated with a reducing agent and
neutralized with ammonium carbonate to pH = 8 that results in
precipitation of Pu4+ compounds.
In another method, Pu4+ and are first extracted with tributyl
phosphate, then reacted with hydrazine washing out the recovered
plutonium.
The major difficulty in separation of actinium is the similarity of
its properties with those of lanthanum. Thus actinium is either
synthesized in nuclear reactions from isotopes of radium or separated
using ion-exchange procedures.
Properties
======================================================================
Actinides have similar properties to lanthanides. Just as the 4f
electron shells are filled in the lanthanides, the 5f electron shells
are filled in the actinides. Because the 5f, 6d, 7s, and 7p shells are
close in energy, many irregular configurations arise; thus, in
gas-phase atoms, just as the first 4f electron only appears in cerium,
so the first 5f electron appears even later, in protactinium. However,
just as lanthanum is the first element to use the 4f shell in
compounds, so actinium is the first element to use the 5f shell in
compounds. The f-shells complete their filling together, at ytterbium
and nobelium. The first experimental evidence for the filling of the
5f shell in actinides was obtained by McMillan and Abelson in 1940. As
in lanthanides (see lanthanide contraction), the ionic radius of
actinides monotonically decreases with atomic number (see also
actinoid contraction).
The shift of electron configurations in the gas phase does not always
match the chemical behaviour. For example, the
early-transition-metal-like prominence of the highest oxidation state,
corresponding to removal of all valence electrons, extends up to
uranium even though the 5f shells begin filling before that. On the
other hand, electron configurations resembling the lanthanide
congeners already begin at plutonium, even though lanthanide-like
behaviour does not become dominant until the second half of the series
begins at curium. The elements between uranium and curium form a
transition between these two kinds of behaviour, where higher
oxidation states continue to exist, but lose stability with respect to
the +3 state. The +2 state becomes more important near the end of the
series, and is the most stable oxidation state for nobelium, the last
5f element. Oxidation states rise again only after nobelium, showing
that a new series of 6d transition metals has begun: lawrencium shows
only the +3 oxidation state, and rutherfordium only the +4 state,
making them respectively congeners of lutetium and hafnium in the 5d
row.
Properties of actinides (the mass of the most long-lived isotope is
in square brackets)
!Element Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
!Core charge 89 90 91 92 93 94 95 96 97 98 99 100 101
102 103
!Atomic mass [227] 232.0377(4) 231.03588(2) 238.02891(3) [237]
[244] [243] [247] [247] [251] [252] [257] [258] [259] [266]
!Number of natural isotopes 3 8 3 8 3 4 0 0 0 0 0 0 0
0 0
!Natural isotopes 225, 227, 228 227-234 231, 233, 234 233-240
237, 239, 240 238-240, 244 -- -- -- -- -- -- -- --
--
!Natural quantity isotopes -- 230, 232 231 234, 235, 238 --
-- -- -- -- -- -- -- -- -- --
!Longest-lived isotope 227 232 231 238 237 244 243 247 247
251 252 257 258 259 266
!Half-life of the longest-lived isotope 14 byr 4.47 byr
2.14 myr 80.8 myr 15.6 myr
!Most common isotope 227 232 231 238 237 239 241 244
249 252 253 255 256 255 260
!Half-life of the most common isotope | 14 byr 4.47 byr 2.14 myr
!Electronic configuration in the ground state (gas phase) 6d17s2
6d27s2 5f26d17s2 5f36d17s2 5f46d17s2 5f67s2 5f77s2 5f76d17s2
5f97s2 5f107s2 5f117s2 5f127s2 5f137s2 5f147s2 5f147s27p1
| 2, **3**|| 2, 3, **4**|| 2, 3, 4, **5**|| 2, 3, 4, 5, **6**|| 3, 4,
**5**, 6, 7|| 3, **4**, 5, 6, 7|| 2, **3**, 4, 5, 6, 7|| 2, **3**, 4,
6|| 2, **3**, 4|| 2, **3**, 4|| 2, **3**, 4|| 2, **3**|| 2, **3**||
**2**, 3|| **3**
!Oxidation states
!Metallic radius (nm) 0.203 0.180 0.162 0.153 0.150 0.162 0.173
0.174 0.170 0.186 0.186 ? 0.198 ? 0.194 ? 0.197 ? 0.171
! An4+ An3+ | -- 0.126 0.114 -- 0.104 0.118 0.103 0.118
0.101 0.116 0.100 0.115 0.099 0.114 0.099 0.112 0.097
0.110 0.096 0.109 0.085 0.098 0.084 0.091 0.084 0.090
0.084 0.095 0.083 0.088 !Temperature (°C): melting boiling 1050
3198 1842 4788 1568 ? 4027 1132.2 4131 639 ? 4174 639.4 3228
1176 ? 2607 1340 3110 986 2627 900 ? 1470 860 ? 996 1530 --
830 -- 830 -- 1630 --
!Density, g/cm3 |10.07 11.78 15.37 19.06 20.45 19.84 11.7 13.51
14.78 15.1 8.84 ? 9.7 ? 10.3 ? 9.9 ? 14.4 !Standard electrode
potential (V): 'E'° (An4+/An0) 'E'° (An3+/An0) -- −2.13 −1.83 --
−1.47 -- −1.38 −1.66 −1.30 −1.79 −1.25 −2.00 −0.90 −2.07
−0.75 −2.06 −0.55 −1.96 −0.59 −1.97 −0.36 −1.98 −0.29
−1.96 -- −1.74 -- −1.20 -- −2.10 Color: [M(H2O)n]4+
[M(H2O)n]3+ | -- Colorless Colorless Blue Yellow Dark blue
Green Purple Purple Brown Violet Red Rose Yellow
Colorless Beige Green Green -- Pink -- -- -- -- --
-- -- --
colspan=16 | Approximate colors of actinide ions in aqueous solution
Colors for the actinides 100-103 are unknown as sufficient quantities
have not yet been synthesized. The colour of was likewise not
recorded.
Actinide ('Z') 89 90 91 92 93 94 95 96 97 98
99 100 101 102 103
Oxidation state
+2 |
| **Ac3+**
| **Am3+**
| **Cm3+**
| **Bk3+**
| **Cf3+**
| **Es3+**
+3
| || **Th4+**
| **Pa4+**
| **U4+**
| **Np4+**
| **Pu4+**
| **Am4+**
| **Cm4+**
| **Bk4+**
| **Cf4+**
+4
| ****
| ****
| ****
|PuO|2|+}}**
| ****
+5
| ****
| ****
| ****
| ****
|CmO|2|2+}}**
+6
| ****
| ****
| ****
+7
Physical properties
=====================
400px 400px
|Major crystal structures of some actinides vs. temperature |Metallic
and ionic radii of actinides
Actinides are typical metals. All of them are soft and have a silvery
color (but tarnish in air), relatively high density and plasticity.
Some of them can be cut with a knife. Their electrical resistivity
varies between 15 and 150 μΩ·cm. The hardness of thorium is similar to
that of soft steel, so heated pure thorium can be rolled in sheets and
pulled into wire. Thorium is nearly half as dense as uranium and
plutonium, but is harder than either of them. All actinides are
radioactive, paramagnetic, and, with the exception of actinium, have
several crystalline phases: plutonium has seven, and uranium,
neptunium and californium three. The crystal structures of
protactinium, uranium, neptunium and plutonium do not have clear
analogs among the lanthanides and are more similar to those of the
3'd'-transition metals.
All actinides are pyrophoric, especially when finely divided, that is,
they spontaneously ignite upon reaction with air at room temperature.
The melting point of actinides does not have a clear dependence on the
number of 'f'-electrons. The unusually low melting point of neptunium
and plutonium (~640 °C) is explained by hybridization of 5'f' and 6'd'
orbitals and the formation of directional bonds in these metals.
Comparison of ionic radii of lanthanides and actinides
Lanthanides Ln3+, Å Actinides An3+, Å An4+, Å
Lanthanum 1.061 Actinium 1.11 -
Cerium 1.034 Thorium 1.08 0.99
Praseodymium 1.013 Protactinium 1.05 0.93
Neodymium 0.995 Uranium 1.03 0.93
Promethium 0.979 Neptunium 1.01 0.92
Samarium 0.964 Plutonium 1.00 0.90
Europium 0.950 Americium 0.99 0.89
Gadolinium 0.938 Curium 0.98 0.88
Terbium 0.923 Berkelium - -
Dysprosium 0.908 Californium - -
Holmium 0.894 Einsteinium - -
Erbium 0.881 Fermium - -
Thulium 0.869 Mendelevium - -
Ytterbium 0.858 Nobelium - -
Lutetium 0.848 Lawrencium - -
Chemical properties
=====================
Like the lanthanides, all actinides are highly reactive with halogens
and chalcogens; however, the actinides react more easily. Actinides,
especially those with a small number of 5'f'-electrons, are prone to
hybridization. This is explained by the similarity of the electron
energies at the 5'f', 7's' and 6'd' shells. Most actinides exhibit a
larger variety of valence states, and the most stable are +6 for
uranium, +5 for protactinium and neptunium, +4 for thorium and
plutonium and +3 for actinium and other actinides.
Actinium is chemically similar to lanthanum, which is explained by
their similar ionic radii and electronic structures. Like lanthanum,
actinium almost always has an oxidation state of +3 in compounds, but
it is less reactive and has more pronounced basic properties. Among
other trivalent actinides Ac3+ is least acidic, i.e. has the weakest
tendency to hydrolyze in aqueous solutions.
Thorium is rather active chemically. Owing to lack of electrons on
6'd' and 5'f' orbitals, tetravalent thorium compounds are colorless.
At pH < 3, solutions of thorium salts are dominated by the cations
[Th(H2O)8]4+. The Th4+ ion is relatively large, and depending on the
coordination number can have a radius between 0.95 and 1.14 Å. As a
result, thorium salts have a weak tendency to hydrolyse. The
distinctive ability of thorium salts is their high solubility both in
water and polar organic solvents.
Protactinium exhibits two valence states; the +5 is stable, and the +4
state easily oxidizes to protactinium(V). Thus tetravalent
protactinium in solutions is obtained by the action of strong reducing
agents in a hydrogen atmosphere. Tetravalent protactinium is
chemically similar to uranium(IV) and thorium(IV). Fluorides,
phosphates, hypophosphates, iodates and phenylarsonates of
protactinium(IV) are insoluble in water and dilute acids. Protactinium
forms soluble carbonates. The hydrolytic properties of pentavalent
protactinium are close to those of tantalum(V) and niobium(V). The
complex chemical behavior of protactinium is a consequence of the
start of the filling of the 5'f' shell in this element.
Uranium has a valence from 3 to 6, the last being most stable. In the
hexavalent state, uranium is very similar to the group 6 elements.
Many compounds of uranium(IV) and uranium(VI) are non-stoichiometric,
i.e. have variable composition. For example, the actual chemical
formula of uranium dioxide is UO2+x, where 'x' varies between −0.4 and
0.32. Uranium(VI) compounds are weak oxidants. Most of them contain
the linear "uranyl" group, . Between 4 and 6 ligands can be
accommodated in an equatorial plane perpendicular to the uranyl group.
The uranyl group acts as a hard acid and forms stronger complexes with
oxygen-donor ligands than with nitrogen-donor ligands. and are also
the common form of Np and Pu in the +6 oxidation state. Uranium(IV)
compounds exhibit reducing properties, e.g., they are easily oxidized
by atmospheric oxygen. Uranium(III) is a very strong reducing agent.
Owing to the presence of d-shell, uranium (as well as many other
actinides) forms organometallic compounds, such as UIII(C5H5)3 and
UIV(C5H5)4.
Neptunium has valence states from 3 to 7, which can be simultaneously
observed in solutions. The most stable state in solution is +5, but
the valence +4 is preferred in solid neptunium compounds. Neptunium
metal is very reactive. Ions of neptunium are prone to hydrolysis and
formation of coordination compounds.
Plutonium also exhibits valence states between 3 and 7 inclusive, and
thus is chemically similar to neptunium and uranium. It is highly
reactive, and quickly forms an oxide film in air. Plutonium reacts
with hydrogen even at temperatures as low as 25-50 °C; it also easily
forms halides and intermetallic compounds. Hydrolysis reactions of
plutonium ions of different oxidation states are quite diverse.
Plutonium(V) can enter polymerization reactions.
The largest chemical diversity among actinides is observed in
americium, which can have valence between 2 and 6. Divalent americium
is obtained only in dry compounds and non-aqueous solutions
(acetonitrile). Oxidation states +3, +5 and +6 are typical for aqueous
solutions, but also in the solid state. Tetravalent americium forms
stable solid compounds (dioxide, fluoride and hydroxide) as well as
complexes in aqueous solutions. It was reported that in alkaline
solution americium can be oxidized to the heptavalent state, but these
data proved erroneous. The most stable valence of americium is 3 in
aqueous solution and 3 or 4 in solid compounds.
Valence 3 is dominant in all subsequent elements up to lawrencium
(with the exception of nobelium). Curium can be tetravalent in solids
(fluoride, dioxide). Berkelium, along with a valence of +3, also shows
the valence of +4, more stable than that of curium; the valence 4 is
observed in solid fluoride and dioxide. The stability of Bk4+ in
aqueous solution is close to that of Ce4+. Only valence 3 was observed
for californium, einsteinium and fermium. The divalent state is proven
for mendelevium and nobelium, and in nobelium it is more stable than
the trivalent state. Lawrencium shows valence 3 both in solutions and
solids.
The redox potential \mathit E_\frac{M^4+}{AnO2^2+} increases from
−0.32 V in uranium, through 0.34 V (Np) and 1.04 V (Pu) to 1.34 V in
americium revealing the increasing reduction ability of the An4+ ion
from americium to uranium. All actinides form AnH3 hydrides of black
color with salt-like properties. Actinides also produce carbides with
the general formula of AnC or AnC2 (U2C3 for uranium) as well as
sulfides An2S3 and AnS2.
File:Uranylnitrate_crystals.jpg|Uranyl nitrate (UO2(NO3)2)
File:U Oxstufen.jpg|Aqueous solutions of uranium III, IV, V, VI salts
File:Np ox st .jpg|Aqueous solutions of neptunium III, IV, V, VI, VII
salts
File:Plutonium in solution.jpg|Aqueous solutions of plutonium III, IV,
V, VI, VII salts
File:UCl4.jpg|Uranium tetrachloride
File:Uranium hexafluoride crystals sealed in an ampoule.jpg|Uranium
hexafluoride
File:Yellowcake.jpg|U3O8 (yellowcake)
Oxides and hydroxides
=======================
Oxides of actinides
rowspan="2"|Compound rowspan="2"|Color rowspan="2"|Crystal symmetry,
type colspan="3"|Lattice constants, Å rowspan="2"|Density, g/cm3
rowspan="2"|Temperature, °C
!'a' !'b' !'c'
Ac2O3 White Hexagonal, La2O3 4.07 - 6.29 9.19 -
PaO2 - Cubic, CaF2 5.505 - - - -
Pa2O5 White cubic, CaF2 Cubic Tetragonal Hexagonal Rhombohedral
Orthorhombic 5.446 10.891 5.429 3.817 5.425 6.92 - - - - -
4.02 - 10.992 5.503 13.22 - 4. 18 - 700 700-1100 1000
1000-1200 1240-1400 -
ThO2 Colorless Cubic 5.59 - - 9.87 -
UO2 Black-brown Cubic 5.47 - - 10.9 -
NpO2 Greenish-brown Cubic, CaF2 5.424 - - 11.1 -
PuO Black Cubic, NaCl 4.96 - - 13.9 -
PuO2 Olive green Cubic 5.39 - - 11.44 -
Am2O3 Red-brown Red-brown Cubic, Mn2O3 Hexagonal, La2O3 11.03
3.817 - - 5.971 10.57 11.7 -
AmO2 Black Cubic, CaF2 5.376 - - - -
Cm2O3 White - - Cubic, Mn2O2 Hexagonal, LaCl3 Monoclinic, Sm2O3
11.01 3.80 14.28 - - 3.65 - 6 8.9 11.7 -
CmO2 Black Cubic, CaF2 5.37 - - - -
Bk2O3 Light brown Cubic, Mn2O3 10.886 - - - -
BkO2 Red-brown Cubic, CaF2 5.33 - - - -
Cf2O3 Colorless Yellowish - Cubic, Mn2O3 Monoclinic, Sm2O3
Hexagonal, La2O3 10.79 14.12 3.72 - 3.59 - - 8.80 5.96 - -
CfO2 Black Cubic 5.31 - - - -
Es2O3 - Cubic, Mn2O3 Monoclinic Hexagonal, La2O3 10.07 14.1
3.7 - 3.59 - - 8.80 6 - -
Approximate colors of actinide oxides (most stable are bolded)
Oxidation state 89 90 91 92 93 94 95 96 97 98 99
| +3||**Ac2O3**|| || ||
| **Cm2O3**
| **Cf2O3**
| **Es2O3**
|bgcolor=black Am2O3 Bk2O3
| +4|| || **ThO2**
| **NpO2**
| **PuO2**
|bgcolor=brown| **BkO2**
|bgcolor=black |bgcolor=black |bgcolor=black |bgcolor=black
| **Pa2O5**
+5 |bgcolor=black
+5,+6
+6 UO3
Dioxides of some actinides
|Chemical formula ThO2 PaO2 UO2 NpO2 PuO2 AmO2 CmO2 BkO2 CfO2
|CAS Number 1314-20-1 12036-03-2 1344-57-6 12035-79-9 12059-95-9
12005-67-3 12016-67-0 12010-84-3 12015-10-0
|Molar mass 264.04 263.035 270.03 269.047 276.063 275.06
270-284** 279.069 283.078
|Melting point 3390 °C 2865 °C 2547 °C 2400 °C 2175 °C
|Crystal structure Colspan = "9"|[[Space group Colspan = "9"|Fmm
| Colspan = "9" |**An**[8], O[4]
|Coordination number
: An - actinide **Depending on the isotopes
Some actinides can exist in several oxide forms such as An2O3, AnO2,
An2O5 and AnO3. For all actinides, oxides AnO3 are amphoteric and
An2O3, AnO2 and An2O5 are basic, they easily react with water, forming
bases:
: An2O3 + 3 H2O → 2 An(OH)3.
These bases are poorly soluble in water and by their activity are
close to the hydroxides of rare-earth metals.
Np(OH)3 has not yet been synthesized, Pu(OH)3 has a blue color while
Am(OH)3 is pink and Cm(OH)3 is colorless. Bk(OH)3 and Cf(OH)3 are also
known, as are tetravalent hydroxides for Np, Pu and Am and pentavalent
for Np and Am.
The strongest base is of actinium. All compounds of actinium are
colorless, except for black actinium sulfide (Ac2S3). Dioxides of
tetravalent actinides crystallize in the cubic system, same as in
calcium fluoride.
Thorium reacting with oxygen exclusively forms the dioxide:
: Th{} + O2 ->[\ce{1000^\circ C}]
\overbrace{ThO2}^{Thorium~dioxide}
Thorium dioxide is a refractory material with the highest melting
point among any known oxide (3390 °C). Adding 0.8-1% ThO2 to tungsten
stabilizes its structure, so the doped filaments have better
mechanical stability to vibrations. To dissolve ThO2 in acids, it is
heated to 500-600 °C; heating above 600 °C produces a very resistant
to acids and other reagents form of ThO2. Small addition of fluoride
ions catalyses dissolution of thorium dioxide in acids.
Two protactinium oxides have been obtained: PaO2 (black) and Pa2O5
(white); the former is isomorphic with ThO2 and the latter is easier
to obtain. Both oxides are basic, and Pa(OH)5 is a weak, poorly
soluble base.
Decomposition of certain salts of uranium, for example UO2(NO3)·6H2O
in air at 400 °C, yields orange or yellow UO3. This oxide is
amphoteric and forms several hydroxides, the most stable being uranyl
hydroxide UO2(OH)2. Reaction of uranium(VI) oxide with hydrogen
results in uranium dioxide, which is similar in its properties with
ThO2. This oxide is also basic and corresponds to the uranium
hydroxide U(OH)4.
Plutonium, neptunium and americium form two basic oxides: An2O3 and
AnO2. Neptunium trioxide is unstable; thus, only Np3O8 could be
obtained so far. However, the oxides of plutonium and neptunium with
the chemical formula AnO2 and An2O3 are well characterized.
Salts
=======
Trichlorides of some actinides
Chemical formula AcCl3 UCl3 NpCl3 PuCl3 AmCl3 CmCl3 BkCl3
CfCl3
CAS-number 22986-54-5 10025-93-1 20737-06-8 13569-62-5 13464-46-5
13537-20-7 13536-46-4 13536-90-8
Molar mass 333.386 344.387 343.406 350.32 349.42 344-358**
353.428 357.438
Melting point | 837 °C 800 °C 767 °C 715 °C 695 °C 603 °C 545 °C
Boiling point | 1657 °C 1767 °C 850 °C
Crystal structure Colspan = "8"|250px
[[Space group Colspan = "8"|P63/m
| Colspan = "8" |**An***[9], Cl [3]
Coordination number
Lattice constants 'a' = 762 pm 'c' = 455 pm 'a' = 745.2 pm 'c' =
432.8 pm 'a' = 739.4 pm 'c' = 424.3 pm 'a' = 738.2 pm 'c' =
421.4 pm 'a' = 726 pm 'c' = 414 pm 'a' = 738.2 pm 'c' = 412.7 pm
'a' = 738 pm 'c' = 409 pm
: *An - actinide **Depending on the isotopes
Actinide fluorides
Rowspan = "2"|Compound rowspan="2"|Color rowspan="2"|Crystal
symmetry, type colspan="3"|Lattice constants, Å rowspan="2"|Density,
g/cm3
!'a' !'b' !'c'
AcF3 White Hexagonal, LaF3 4.27 - 7.53 7.88
PaF4 Dark brown Monoclinic 12.7 10.7 8.42 -
PaF5 Black Tetragonal, β-UF5 11.53 - 5.19 -
ThF4 Colorless Monoclinic 13 10.99 8.58 5.71
UF3 Reddish-purple Hexagonal 7.18 - 7.34 8.54
UF4 Green Monoclinic 11.27 10.75 8.40 6.72
α-UF5 Bluish Tetragonal 6.52 - 4.47 5.81
β-UF5 Bluish Tetragonal 11.47 - 5.20 6.45
UF6 Yellowish Orthorhombic 9.92 8.95 5.19 5.06
NpF3 Black or purple Hexagonal 7.129 - 7.288 9.12
NpF4 Light green Monoclinic 12.67 10.62 8.41 6.8
NpF6 Orange Orthorhombic 9.91 8.97 5.21 5
PuF3 Violet-blue Trigonal 7.09 - 7.25 9.32
PuF4 Pale brown Monoclinic 12.59 10.57 8.28 6.96
PuF6 Red-brown Orthorhombic 9.95 9.02 3.26 4.86
AmF3 Pink or light beige hexagonal, LaF3 7.04 - 7.255 9.53
AmF4 Orange-red Monoclinic 12.53 10.51 8.20 -
CmF3 From brown to white Hexagonal 4.041 - 7.179 9.7
CmF4 Yellow Monoclinic, UF4 12.51 10.51 8.20 -
BkF3 Yellow-green Trigonal, LaF3 Orthorhombic, YF3 6.97 6.7 -
7.09 7.14 4.41 10.15 9.7
BkF4 - Monoclinic, UF4 12.47 10.58 8.17 -
CfF3 - - Trigonal, LaF3 Orthorhombic, YF3 6. 94 6.65 - 7.04
7.10 4.39 -
CfF4 - - Monoclinic, UF4 Monoclinic, UF4 1.242 1.233 1.047
1.040 8.126 8.113 -
Actinides easily react with halogens forming salts with the formulas
MX3 and MX4 (X = halogen). So the first berkelium compound, BkCl3, was
synthesized in 1962 with an amount of 3 nanograms. Like the halogens
of rare earth elements, actinide chlorides, bromides, and iodides are
water-soluble, and fluorides are insoluble. Uranium easily yields a
colorless hexafluoride, which sublimates at a temperature of 56.5 °C;
because of its volatility, it is used in the separation of uranium
isotopes with gas centrifuge or gaseous diffusion. Actinide
hexafluorides have properties close to anhydrides. They are very
sensitive to moisture and hydrolyze forming AnO2F2. The pentachloride
and black hexachloride of uranium were synthesized, but they are both
unstable.
Action of acids on actinides yields salts, and if the acids are
non-oxidizing then the actinide in the salt is in low-valence state:
: U + 2 H2SO4 → U(SO4)2 + 2 H2
: 2 Pu + 6 HCl → 2 PuCl3 + 3 H2
However, in these reactions the regenerating hydrogen can react with
the metal, forming the corresponding hydride. Uranium reacts with
acids and water much more easily than thorium.
Actinide salts can also be obtained by dissolving the corresponding
hydroxides in acids. Nitrates, chlorides, sulfates and perchlorates of
actinides are water-soluble. When crystallizing from aqueous
solutions, these salts form hydrates, such as Th(NO3)4·6H2O,
Th(SO4)2·9H2O and Pu2(SO4)3·7H2O. Salts of high-valence actinides
easily hydrolyze. So, colorless sulfate, chloride, perchlorate and
nitrate of thorium transform into basic salts with formulas Th(OH)2SO4
and Th(OH)3NO3. The solubility and insolubility of trivalent and
tetravalent actinides is like that of lanthanide salts. So phosphates,
fluorides, oxalates, iodates and carbonates of actinides are weakly
soluble in water; they precipitate as hydrates, such as ThF4·3H2O and
Th(CrO4)2·3H2O.
Actinides with oxidation state +6, except for the AnO22+-type cations,
form [AnO4]2−, [An2O7]2− and other complex anions. For example,
uranium, neptunium and plutonium form salts of the Na2UO4 (uranate)
and (NH4)2U2O7 (diuranate) types. In comparison with lanthanides,
actinides more easily form coordination compounds, and this ability
increases with the actinide valence. Trivalent actinides do not form
fluoride coordination compounds, whereas tetravalent thorium forms
K2ThF6, KThF5, and even K5ThF9 complexes. Thorium also forms the
corresponding sulfates (for example Na2SO4·Th(SO4)2·5H2O), nitrates
and thiocyanates. Salts with the general formula An2Th(NO3)6·'n'H2O
are of coordination nature, with the coordination number of thorium
equal to 12. Even easier is to produce complex salts of pentavalent
and hexavalent actinides. The most stable coordination compounds of
actinides - tetravalent thorium and uranium - are obtained in
reactions with diketones, e.g. acetylacetone.
Applications
======================================================================
While actinides have some established daily-life applications, such as
in smoke detectors (americium) and gas mantles (thorium), they are
mostly used in nuclear weapons and as fuel in nuclear reactors. The
last two areas exploit the property of actinides to release enormous
energy in nuclear reactions, which under certain conditions may become
self-sustaining chain reactions.
The most important isotope for nuclear power applications is
uranium-235. It is used in the thermal reactor, and its concentration
in natural uranium does not exceed 0.72%. This isotope strongly
absorbs thermal neutrons releasing much energy. One fission act of 1
gram of 235U converts into about 1 MW·day. Of importance, is that
emits more neutrons than it absorbs; upon reaching the critical mass,
enters into a self-sustaining chain reaction. Typically, uranium
nucleus is divided into two fragments with the release of 2-3
neutrons, for example:
: + ⟶ + + 3
Other promising actinide isotopes for nuclear power are thorium-232
and its product from the thorium fuel cycle, uranium-233.
Nuclear reactor
The core of most Generation II nuclear reactors contains a set of
hollow metal rods, usually made of zirconium alloys, filled with solid
nuclear fuel pellets - mostly oxide, carbide, nitride or monosulfide
of uranium, plutonium or thorium, or their mixture (the so-called MOX
fuel). The most common fuel is oxide of uranium-235. Nuclear reactor
scheme Fast neutrons are slowed by moderators, which contain water,
carbon, deuterium, or beryllium, as thermal neutrons to increase the
efficiency of their interaction with uranium-235. The rate of nuclear
reaction is controlled by introducing additional rods made of boron or
cadmium or a liquid absorbent, usually boric acid. Reactors for
plutonium production are called breeder reactor or breeders; they have
a different design and use fast neutrons.
Emission of neutrons during the fission of uranium is important not
only for maintaining the nuclear chain reaction, but also for the
synthesis of the heavier actinides. Uranium-239 converts via β-decay
into plutonium-239, which, like uranium-235, is capable of spontaneous
fission. The world's first nuclear reactors were built not for energy,
but for producing plutonium-239 for nuclear weapons.
About half of produced thorium is used as the light-emitting material
of gas mantles. Thorium is also added into multicomponent alloys of
magnesium and zinc. Mg-Th alloys are light and strong, but also have
high melting point and ductility and thus are widely used in the
aviation industry and in the production of missiles. Thorium also has
good electron emission properties, with long lifetime and low
potential barrier for the emission. The relative content of thorium
and uranium isotopes is widely used to estimate the age of various
objects, including stars (see radiometric dating).
The major application of plutonium has been in nuclear weapons, where
the isotope plutonium-239 was a key component due to its ease of
fission and availability. Plutonium-based designs allow reducing the
critical mass to about a third of that for uranium-235. The "Fat
Man"-type plutonium bombs produced during the Manhattan Project used
explosive compression of plutonium to obtain significantly higher
densities than normal, combined with a central neutron source to begin
the reaction and increase efficiency. Thus only 6.2 kg of plutonium
was needed for an explosive yield equivalent to 20 kilotons of TNT.
(See also Nuclear weapon design.) Hypothetically, as little as 4 kg of
plutonium--and maybe even less--could be used to make a single atomic
bomb using very sophisticated assembly designs.
Plutonium-238 is potentially more efficient isotope for nuclear
reactors, since it has smaller critical mass than uranium-235, but it
continues to release much thermal energy (0.56 W/g) by decay even when
the fission chain reaction is stopped by control rods. Its application
is limited by its high price (about US$1000/g). This isotope has been
used in thermopiles and water distillation systems of some space
satellites and stations. The Galileo and Apollo spacecraft (e.g.
Apollo 14) had heaters powered by kilogram quantities of plutonium-238
oxide; this heat is also transformed into electricity with
thermopiles. The decay of plutonium-238 produces relatively harmless
alpha particles and is not accompanied by gamma rays. Therefore, this
isotope (~160 mg) is used as the energy source in heart pacemakers
where it lasts about 5 times longer than conventional batteries.
Actinium-227 is used as a neutron source. Its high specific energy
(14.5 W/g) and the possibility of obtaining significant quantities of
thermally stable compounds are attractive for use in long-lasting
thermoelectric generators for remote use. 228Ac is used as an
indicator of radioactivity in chemical research, as it emits
high-energy electrons (2.18 MeV) that can be easily detected.
228Ac-228Ra mixtures are widely used as an intense gamma-source in
industry and medicine.
Development of self-glowing actinide-doped materials with durable
crystalline matrices is a new area of actinide utilization as the
addition of alpha-emitting radionuclides to some glasses and crystals
may confer luminescence.
Toxicity
======================================================================
Radioactive substances can harm human health via (i) local skin
contamination, (ii) internal exposure due to ingestion of radioactive
isotopes, and (iii) external overexposure by β-activity and
γ-radiation. Together with radium and transuranium elements, actinium
is one of the most dangerous radioactive poisons with high specific
α-activity. The most important feature of actinium is its ability to
accumulate and remain in the surface layer of skeletons. At the
initial stage of poisoning, actinium accumulates in the liver. Another
danger of actinium is that it undergoes radioactive decay faster than
being excreted. Adsorption from the digestive tract is much smaller
(~0.05%) for actinium than radium.
Protactinium in the body tends to accumulate in the kidneys and bones.
The maximum safe dose of protactinium in the human body is 0.03 μCi
that corresponds to 0.5 micrograms of 231Pa. This isotope, which might
be present in the air as aerosol, is 2.5 times more toxic than
hydrocyanic acid.
Plutonium, when entering the body through air, food or blood (e.g. a
wound), mostly settles in the lungs, liver and bones with only about
10% going to other organs, and remains there for decades. The long
residence time of plutonium in the body is partly explained by its
poor solubility in water. Some isotopes of plutonium emit ionizing
α-radiation, which damages the surrounding cells. The median lethal
dose (LD50) for 30 days in dogs after intravenous injection of
plutonium is 0.32 milligram per kg of body mass, and thus the lethal
dose for humans is approximately 22 mg for a person weighing 70 kg;
the amount for respiratory exposure should be approximately four times
greater. Another estimate assumes that plutonium is 50 times less
toxic than radium, and thus permissible content of plutonium in the
body should be 5 μg or 0.3 μCi. Such amount is nearly invisible under
microscope. After trials on animals, this maximum permissible dose was
reduced to 0.65 μg or 0.04 μCi. Studies on animals also revealed that
the most dangerous plutonium exposure route is through inhalation,
after which 5-25% of inhaled substances is retained in the body.
Depending on the particle size and solubility of the plutonium
compounds, plutonium is localized either in the lungs or in the
lymphatic system, or is absorbed in the blood and then transported to
the liver and bones. Contamination via food is the least likely way.
In this case, only about 0.05% of soluble and 0.01% of insoluble
compounds of plutonium absorbs into blood, and the rest is excreted.
Exposure of damaged skin to plutonium would retain nearly 100% of it.
Using actinides in nuclear fuel, sealed radioactive sources or
advanced materials such as self-glowing crystals has many potential
benefits. However, a serious concern is the extremely high
radiotoxicity of actinides and their migration in the environment. Use
of chemically unstable forms of actinides in MOX and sealed
radioactive sources is not appropriate by modern safety standards.
There is a challenge to develop stable and durable actinide-bearing
materials, which provide safe storage, use and final disposal. A key
need is application of actinide solid solutions in durable crystalline
host phases.
See also
======================================================================
* Actinides in the environment
* Lanthanides
* Major actinides
* Minor actinides
* Transuranics
External links
======================================================================
*
[
https://web.archive.org/web/20120220054516/http://imglib.lbl.gov/ImgLib/COLLECTIONS/BERKELEY-LAB/SEABORG-ARCHIVE/index/96B05654.html
Lawrence Berkeley Laboratory image of historic periodic table by
Seaborg showing actinide series for the first time]
*
[
https://web.archive.org/web/20120813024043/https://www.llnl.gov/str/pdfs/06_00.2.pdf#search=%22actinide%20series%22
Lawrence Livermore National Laboratory, 'Uncovering the Secrets of the
Actinides']
* [
https://www.lanl.gov/media/publications/actinide-research-quarterly
Los Alamos National Laboratory, 'Actinide Research Quarterly']
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Actinide
