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Table of Contents
(Subject Area: Inorganic Chemistry)
Article
Authors
Pages in the
Encyclopedia
Actinide Elements
Bioinorganic
Chemistry
Boron Hydrides
Coordination
Compounds
Dielectric Gases
Electron Transfer
Reactions
Halogen Chemistry
Inclusion (Clathrate)
Compounds
Inorganic Exotic
Molecules
Liquid Alkali Metals
Main Group
Elements
Mesoporous
Materials, Synthesis
Metal Cluster
Chemistry
Metal Hydrides
Siegfried Hübener
Brian T. Farrer and Vincent L.
Pecoraro
Herbert Beall and Donald F.
Gaines
R. D. Gillard
L. G. Christophorou and S. J.
Dale
Gilbert P. Haight, Jr.
Marianna Anderson Busch
Jerry L. Atwood
Joel F. Liebman, Kay Severin and
Thomas M. Klapötke
C. C. Addison
Russell L. Rasmussen, Joseph G.
Morse and Karen W. Morse
Robert Mokaya
D. F. Shriver
Holger Kohlmann
Pages 211-236
Pages 117-139
Pages 301-316
Pages 739-760
Pages 357-371
Pages 347-361
Pages 197-222
Pages 717-729
Pages 817-838
Pages 661-671
Pages 1-30
Pages 369-381
Pages 407-409
Pages 441-458
Pages 513-550
Pages 303-317
Pages 463-492
Pages 449-461
Pages 671-695
Pages 1-22
Allan W. Olsen and Kenneth J.
Metal Particles and
Cluster Compounds
Klabunde
Nano sized Inorganic
Leroy Cronin, Achim Müller and
Dieter Fenske
Clusters
Hubert Schmidbaur and John L.
Noble Metals
Cihonski
(Chemistry)
Noble-Gas Chemistry
Gary J. Schrobilgen
Periodic Table
N. D. Epiotis and D. K. Henze
(Chemistry)
Rare Earth Elements
Zhiping Zheng and John E.
Greedan
and Materials
Actinide Elements
Siegfried Hubener
¨
Forschungszentrum Rossendorf
I. Discovery, Occurrence, and
Synthesis of the Actinides
II. Radioactivity and Nuclear Reactions of Actinides
III. Applications of Actinides
IV. Actinide Metals
V. Actinide Ions
VI. Actinide Compounds and Complexes
GLOSSARY
Actinyl ion
Dioxo actinide cations MO
+
and MO
2+
.
2
2
Decay chain
A series of nuclides in which each member
transforms into the next through nuclear decay until a
stable nuclide has been formed.
Lanthanides
Fourteen elements with atomic numbers 58
(cerium) to 71 (lutetium) that are a result of filling the
4
f
orbitals with electrons.
Nuclear fission
The division of a nucleus into two or
more parts, usually accompanied by the emission of
neutrons and
γ
radiation.
Nuclide
A species of atom characterized by its mass num-
ber, atomic number, and nuclear energy state. A ra-
dionuclide is a radioactive nuclide.
Primordial radionuclides
Nuclides which were pro-
duced during element evolution and which have
partly survived since then due to their long half-
lives.
Radioactivity
The property of certain nuclides of show-
ing radioactive decay in which particles or
γ
radia-
tion are emitted or the nucleus undergoes spontaneous
fission.
Speciation
Characterization of physical and chemical
states of (actinide) species in a given (chemical)
environment.
Transactinide elements
Artificial elements beyond the
actinide elements, beginning with rutherfordium (Rf),
element 104. The heaviest elements, synthesized until
now, are the elements 114, 116, and 118. At present,
bohrium (Bh), element 107, is the heaviest element
which has been characterized chemically; chemical
studies of element 108, hassium (Hs), and element 112
are in preparation.
THE ACTINIDE ELEMENTS
(actinoids) comprise the
14 elements with atomic numbers 90–103, which fol-
low actinium in the periodic table: thorium (Th), pro-
tactinium (Pa), uranium (U), neptunium (Np), plutonium
(Pu), americium (Am), curium (Cm), berkelium (Bk), cal-
ifornium (Cf), einsteinium (Es), fermium (Fm), mendele-
vium (Md), nobelium (No), and lawrencium (Lr). The ac-
tinides constitute a unique series of elements which are
formed by the progressive filling of the 5
f
electron shell.
Although not formally an actinide element, actinium (Ac;
211
212
atomic number 89) is usually included in discussions about
the actinides.
According to the International Union of Pure and Ap-
plied Chemistry (IUPAC), the name actinoid is prefer-
able to actinide because the ending “-ide” normally indi-
cates a negative ion. However, owing to wide current use,
“actinide” is still allowed.
Actinide Elements
I. DISCOVERY, OCCURRENCE, AND
SYNTHESIS OF THE ACTINIDES
A. Naturally Occurring Actinides
All of the isotopes of the actinide elements are radioac-
tive, and only four of the primordial isotopes,
232
Th,
235
U,
238
U, and
244
Pu, have a sufficient long half-life for there to
be any of these isotopes left in nature. Only three actinide
elements and actinium were known as late as 1940. In ad-
dition to thorium and uranium, protactinium and actinium
have been found to exist in uranium and thorium ores due
to the
238
U [Eq. (1)] and
235
U [Eq. (2)] decay series:
−β
−
−β
−
−α
238
U
−→
234
Th
−→
234
Pa
−→
234
U,
92
90
91
92
−β
−
−α
−α
235
U
−→
231
Th
−→
231
Pa
−→
227
Ac.
92
90
91
89
(1)
(2)
It was not until 1971 that the existence of primordial
244
Pu
in nature in trace amounts was shown by D. C. Hoffman
and co-workers.
Uranium was the first actinide element to be discov-
ered. M. H. Klaproth showed in 1789 that pitchblende con-
tained a new element and named it uranium after the then
newly discovered planet Uranus. Uranium is now known
to comprise 2.1 ppm of the Earth’s crust, which makes
it about as abundant as arsenic or europium. It is widely
distributed, with the principal sources being in Australia,
Canada, South Africa, and the United States. The two
most important oxide minerals of uranium are uraninite
(U
3
O
8
; 50–90% uranium), a variety of which is called
pitchblende, and carnotite (K
2
(UO
2
)(VO
4
)
2
·
3H
2
O; 54%
uranium). A very common uranium mineral is autu-
nite (Ca(UO
2
)
2
(PO
4
)
2
·
nH
2
O,
n
=
8–12). Natural ura-
nium consists of 99.3%
238
U and 0.72% of the fissionable
isotope
235
U. A third important isotope,
233
U, does not
occur in nature but can be produced by thermal-neutron
irradiation of
232
Th [Eq. (3)]:
232
90
Th
tained by reduction of its tetrachloride with potassium he
named thorium. (Later, in 1841, B. Peligot used the same
method to prepare uranium metal for the first time.) Tho-
rium constitutes 8.1 ppm of the Earth’s crust and is thus
as abundant as boron. Converted by neutron irradiation
to
233
U, it could yield an amount of neutron-fissile ma-
terial several hundred times the amount of the naturally
occurring fissile uranium isotope
235
U. The principal tho-
rium ore is monazite, a mixture of rare-earth and thorium
phosphates containing up to 30% ThO
2
. Monazite sands
are widely distributed throughout the world. In Canada
thorium is recovered from uranothorite (a mixed thorium-
uranium silicate accompanied by pitchblende) as a co-
product of uranium. Rarer minerals thorianite (90% ThO
2
)
and thorite (ThSiO
4
; 62% thorium) have been found in the
western United States and New zealand. Natural thorium
is 100%
232
Th.
In 1913 protactinium was discovered by K. Fajans and
O. G¨ hring, who identified
234m
Pa as an unstable member
o
of the
238
U decay series. They named the new element bre-
vium because of its short half-life of 1.15 min. In 1918 the
longer-lived isotope
231
Pa, with a half-life of 32,800 years,
was identified independently by two groups, O. Hahn and
L. Meitner, and F. Soddy and J. A. Cranston, as a prod-
uct of
235
U decay. Since the name brevium was obviously
inappropriate for such a long-lived radioelement, it was
changed to protactinium, thus naming element 91 as the
parent of actinium. Protactinium is one of the rarest of
the naturally occurring elements. Although not worth ex-
tracting from uranium ores, protactinium becomes con-
centrated in residues from uranium processing plants.
Actinium was discovered by A. Debierne in 1899. Its
name is derived from the Greek word for beam or ray,
referring to its radioactivity. The natural occurrence of
the longest lived actinium isotope
227
Ac, with a half-life
of 21.77 years, is entirely dependent on that of its pri-
mordial ancestor,
235
U. The natural abundance of
227
Ac
is estimated to be 5.7
·
10
−10
ppm. The most concentrated
actinium sample ever prepared from a natural raw material
consisted of about 7
µg
of
227
Ac in less than 0.1 mg of
La
2
O
3
.
B. Synthetic Actinides
Stimulated by the discovery of the neutron in 1932 by
J. Chadwick and the first synthesis of artificial radioactive
nuclei using
α
particle-induced nuclear reactions in 1934
by F. Joliot and I. Curie, many attempts were made to
produce transuranium elements by neutron irradiation of
uranium. In 1934, E. Fermi and later O. Hahn, L. Meitner,
and F. Strassmann reported that they had created transura-
nium elements. But in 1938, O. Hahn and F. Strassmann
showed that the radioactive species produced by neutron
+
1
n
→
233
Th
−→
233
Pa
−→
233
U.
0
90
91
92
−β
−
−β
−
(3)
This process converts thorium to fissionable fuel in a
breeder reactor.
Thorium was discovered by J. J. Berzelius in 1828 when
he isolated a new oxide from a Norwegian ore then known
as thorite. He named the oxide thoria, and the metal he ob-
Actinide Elements
213
cium and curium failed, believing that they would have
chemical properties similar to uranium, neptunium, and
plutonium. Once it was recognized that these elements,
according to G. T. Seaborg’s actinide concept, might have
properties similar to europium and gadolinium, the use of
proper chemical procedures led to success. By analogy to
europium (named after Europe) and gadolinium (named
after Johan Gadolin, a Finnish rare-earth chemist), for el-
ements 95 and 96 the names americium after the continent
of America and curium to honor Pierre and Marie Curie
were proposed. The elements with the atomic numbers
97 and 98 at first could not be produced by irradiation
with neutrons, because
β
−
decaying isotopes of curium
were not known. By 1949 sufficient amounts of
241
Am
and
242
Cm had been accumulated to make it possible to
produce elements 97 and 98 in helium-ion bombardments.
The
α
particle-emitting species produced in the bombard-
ments could be identified as isotopes of elements 97 and
98, which were named berkelium and californium after
the city and state of discovery.
Elements 99 and 100, named einsteinium and fermium
to honor Albert Einstein and Enrico Fermi, were unex-
pectedly synthesized in the first U. S. thermonuclear ex-
plosion in 1952. The successive capture of numerous neu-
trons by
238
U and subsequent
β
−
decay chains ended in
the
β
stable nuclides
253
Es and
255
Fm. From tons of coral
collected at the explosion area, hundreds of atoms of the
new elements could be separated and positively identi-
fied. Further attempts to produce still heavier elements
in underground nuclear tests or in high-flux nuclear re-
actors failed.
257
Fm is the heaviest nuclide which can be
produced using neutron-capture reactions, owing to the
very short half-lives of the heavier fermium isotopes and
their spontaneous fission instead of
β
−
decay. To pro-
duce element 101, mendelevium, only about 10
9
atoms of
253
Es were made available for a bombardment with he-
lium ions in the Berkeley 60-in. cyclotron. For the first
time an element was discovered in “one-atom-at-a-time”
experiments on the basis of only 17 produced atoms re-
coiling from the einsteinium target. The discoverers of
element 101, A. Ghiorso, B. G. Harvey, G. R. Choppin,
S. G. Thompson, and G. T. Seaborg, suggested the name
mendelevium in honor of the Russian chemist Dmitri I.
Mendeleev, who was the first to use a periodic system of
the elements to predict the chemical properties of undis-
covered elements.
The synthesis of element 102 was even more compli-
cated, because a fermium target to apply the bombardment
with helium ions was not available. In order to make use of
lighter target elements, heavier ions had to be accelerated.
The discovery of element 102 was first reported in 1957
by an international group working at the Nobel Institute
of Physics in Stockholm. The name nobelium in honor of
irradiation of uranium were in fact fission fragments re-
sulting from the nuclear fission of uranium! Thus, the early
search for transuranium elements led to one of the greatest
discoveries of the 20th century.
The first transuranium element, neptunium, was discov-
ered in 1940 by E. M. McMillan and P. H. Abelson. They
were able to chemically separate and identify element 93
formed in the following reaction sequences [Eq. (4)]:
238
92
U
+
1
0
n
→
−β
−
−β
−
239
239
−
− →
239
Pu.
92
U
− →
93
Np
− −
94
23 min
2.3 days
(4)
They showed that element 93 has chemical properties sim-
ilar to those of uranium and not those of an eka-rhenium as
suggested on the basis of the periodic table of that time. To
distinguish it from uranium, element 93 was reduced by
SO
2
and precipitated as a fluoride. This new element was
named neptunium after Neptune, the planet discovered af-
ter Uranus. In 1952, trace amounts of
237
Np were found
in uranium of natural origin, formed by neutron capture
in uranium.
It was obvious to the discoverers of neptunium that
239
Np should
β
decay to the isotope of element 94 with
mass number 239, but they were unable to identify it.
However, up to the end of 1940, G. T. Seaborg, E. M.
McMillan, J. W. Kennedy, and A. C. Wahl succeeded in
identifying
238
Pu in uranium, which was bombarded with
deuterons produced in the 60-in. cyclotron at the Univer-
sity of California in Berkeley [Eq. (5)]:
238
92
U
+
2
H
→
2
1
n
+
238
Np
− −
−→
1
0
93
2.1 days
−β
238
94
Pu.
(5)
Element 94 was named plutonium after the planet discov-
ered last, Pluto. In 1941, the first 0.5
µg
of the fissionable
isotope
239
Pu were produced by irradiating 1.2 kg of uranyl
nitrate with cyclotron-generated neutrons. In 1948, trace
amounts of
239
Pu were found in nature, formed by neutron
capture in uranium. In chemical studies, plutonium was
shown to have properties similar to uranium and not to os-
mium as suggested earlier. The actinide concept advanced
by G. T. Seaborg, to consider the actinide elements as a
second
f
transition series analogous to the lanthanides,
systematized the chemistry of the transuranium elements
and facilitated the search for heavier actinide elements.
The actinide elements americium (95) through fermium
(100) were produced first either via neutron or helium-ion
bombardments of actinide targets in the years between
1944 and 1955.
Element 96, curium, was produced in 1944 by the bom-
bardment of
239
Pu with helium ions in the Berkeley 60-in.
cyclotron, and soon after it was found that
241
Pu, formed
from
239
Pu by two successive neutron captures in a nuclear
reactor, decays under
β
−
particle emission to give
241
Am.
Earlier attempts to produce and chemically separate ameri-

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