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212 Actinide Elements

atomicnumber89)isusuallyincludedindiscussionsabout tained by reduction of its tetrachloride with potassium he
the actinides. named thorium. (Later, in 1841, B. Peligot used the same
According to the International Union of Pure and Ap- method to prepare uranium metal for the first time.) Tho-
plied Chemistry (IUPAC), the name actinoid is prefer- rium constitutes 8.1 ppm of the Earth’s crust and is thus
able to actinide because the ending “-ide” normally indi- as abundant as boron. Converted by neutron irradiation
cates a negative ion. However, owing to wide current use, to 233 U, it could yield an amount of neutron-fissile ma-
“actinide” is still allowed. 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
I. DISCOVERY, OCCURRENCE, AND phosphates containing up to 30% ThO 2 . Monazite sands
SYNTHESIS OF THE ACTINIDES are widely distributed throughout the world. In Canada
thorium is recovered from uranothorite (a mixed thorium-
A. Naturally Occurring Actinides uranium silicate accompanied by pitchblende) as a co-
All of the isotopes of the actinide elements are radioac- product of uranium. Rarer minerals thorianite (90% ThO 2 )
tive, and only four of the primordial isotopes, 232 Th, 235 U, and thorite (ThSiO 4 ; 62% thorium) have been found in the
238 U, and 244 Pu, have a sufficient long half-life for there to western United States and New zealand. Natural thorium
be any of these isotopes left in nature. Only three actinide is 100% 232 Th.
elements and actinium were known as late as 1940. In ad- In 1913 protactinium was discovered by K. Fajans and
234m
dition to thorium and uranium, protactinium and actinium O. G¨ohring, who identified Pa as an unstable member
238
have been found to exist in uranium and thorium ores due of the U decay series. They named the new element bre-
to the 238 U [Eq. (1)] and 235 U [Eq. (2)] decay series: 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,
238 −α 234 −β − 234 −β − 234 was identified independently by two groups, O. Hahn and
92 U −→ 90 Th −→ 91 Pa −→ 92 U, (1)
L. Meitner, and F. Soddy and J. A. Cranston, as a prod-
235 −α 231 −β − 231 −α 227 uct of 235 U decay. Since the name brevium was obviously
92 U −→ 90 Th −→ 91 Pa −→ 89 Ac. (2)
inappropriate for such a long-lived radioelement, it was
It was not until 1971 that the existence of primordial 244 Pu
changed to protactinium, thus naming element 91 as the
in nature in trace amounts was shown by D. C. Hoffman
parent of actinium. Protactinium is one of the rarest of
and co-workers.
the naturally occurring elements. Although not worth ex-
Uranium was the first actinide element to be discov-
tracting from uranium ores, protactinium becomes con-
ered. M. H. Klaproth showed in 1789 that pitchblende con-
centrated in residues from uranium processing plants.
tained a new element and named it uranium after the then
Actinium was discovered by A. Debierne in 1899. Its
newly discovered planet Uranus. Uranium is now known
name is derived from the Greek word for beam or ray,
to comprise 2.1 ppm of the Earth’s crust, which makes
referring to its radioactivity. The natural occurrence of
it about as abundant as arsenic or europium. It is widely 227
the longest lived actinium isotope Ac, with a half-life
distributed, with the principal sources being in Australia,
of 21.77 years, is entirely dependent on that of its pri-
Canada, South Africa, and the United States. The two 235 227
mordial ancestor, U. The natural abundance of Ac
most important oxide minerals of uranium are uraninite −10
is estimated to be 5.7 · 10 ppm. The most concentrated
(U 3 O 8 ;50–90% uranium), a variety of which is called
actinium sample ever prepared from a natural raw material
pitchblende, and carnotite (K 2 (UO 2 )(VO 4 ) 2 · 3H 2 O; 54% 227
consisted of about 7 µgof Ac in less than 0.1 mg of
uranium). A very common uranium mineral is autu-
La 2 O 3 .
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 B. Synthetic Actinides
occur in nature but can be produced by thermal-neutron Stimulated by the discovery of the neutron in 1932 by
irradiation of 232 Th [Eq. (3)]:
J. Chadwick and the first synthesis of artificial radioactive
232 1 233 −β − 233 −β − 233 nuclei using α particle-induced nuclear reactions in 1934
0
90 Th + n → 90 Th −→ 91 Pa −→ 92 U. (3) by F. Joliot and I. Curie, many attempts were made to
This process converts thorium to fissionable fuel in a produce transuranium elements by neutron irradiation of
breeder reactor. uranium. In 1934, E. Fermi and later O. Hahn, L. Meitner,
Thorium was discovered by J. J. Berzelius in 1828 when and F. Strassmann reported that they had created transura-
he isolated a new oxide from a Norwegian ore then known nium elements. But in 1938, O. Hahn and F. Strassmann
as thorite. He named the oxide thoria, and the metal he ob- showed that the radioactive species produced by neutron