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Actinide Elements 213

irradiation of uranium were in fact fission fragments re- cium and curium failed, believing that they would have
sulting from the nuclear fission of uranium! Thus, the early chemical properties similar to uranium, neptunium, and
search for transuranium elements led to one of the greatest plutonium. Once it was recognized that these elements,
discoveries of the 20th century. according to G. T. Seaborg’s actinide concept, might have
The first transuranium element, neptunium, was discov- properties similar to europium and gadolinium, the use of
ered in 1940 by E. M. McMillan and P. H. Abelson. They proper chemical procedures led to success. By analogy to
were able to chemically separate and identify element 93 europium (named after Europe) and gadolinium (named
formed in the following reaction sequences [Eq. (4)]: after Johan Gadolin, a Finnish rare-earth chemist), for el-
ements 95 and 96 the names americium after the continent
−β − −β −
238 1 239 239 239
92 U + n → 92 U −−→ 93 Np −−−→ 94 Pu. (4) of America and curium to honor Pierre and Marie Curie
0
23 min 2.3 days
were proposed. The elements with the atomic numbers
They showed that element 93 has chemical properties sim- 97 and 98 at first could not be produced by irradiation
ilar to those of uranium and not those of an eka-rhenium as with neutrons, because β − decaying isotopes of curium
suggested on the basis of the periodic table of that time. To were not known. By 1949 sufficient amounts of 241 Am
distinguish it from uranium, element 93 was reduced by and 242 Cm had been accumulated to make it possible to
SO 2 and precipitated as a fluoride. This new element was produce elements 97 and 98 in helium-ion bombardments.
named neptunium after Neptune, the planet discovered af- The α particle-emitting species produced in the bombard-
ter Uranus. In 1952, trace amounts of 237 Np were found ments could be identified as isotopes of elements 97 and
in uranium of natural origin, formed by neutron capture 98, which were named berkelium and californium after
in uranium. the city and state of discovery.
It was obvious to the discoverers of neptunium that Elements 99 and 100, named einsteinium and fermium
239
Np should β decay to the isotope of element 94 with to honor Albert Einstein and Enrico Fermi, were unex-
mass number 239, but they were unable to identify it. pectedly synthesized in the first U. S. thermonuclear ex-
However, up to the end of 1940, G. T. Seaborg, E. M. plosion in 1952. The successive capture of numerous neu-
−
McMillan, J. W. Kennedy, and A. C. Wahl succeeded in trons by 238 U and subsequent β decay chains ended in
identifying 238 Pu in uranium, which was bombarded with the β stable nuclides 253 Es and 255 Fm. From tons of coral
deuterons produced in the 60-in. cyclotron at the Univer- collected at the explosion area, hundreds of atoms of the
sity of California in Berkeley [Eq. (5)]: new elements could be separated and positively identi-
fied. Further attempts to produce still heavier elements
−β
238 2 1 238 238
1
92 U + H → 2 n + 93 Np −−−→ 94 Pu. (5) in underground nuclear tests or in high-flux nuclear re-
0
2.1 days 257
actors failed. Fm is the heaviest nuclide which can be
Element 94 was named plutonium after the planet discov- produced using neutron-capture reactions, owing to the
ered last, Pluto. In 1941, the first 0.5 µg of the fissionable very short half-lives of the heavier fermium isotopes and
isotope 239 Puwereproducedbyirradiating1.2kgofuranyl their spontaneous fission instead of β − decay. To pro-
9
nitrate with cyclotron-generated neutrons. In 1948, trace duce element 101, mendelevium, only about 10 atoms of
amounts of 239 Pu were found in nature, formed by neutron 253 Es were made available for a bombardment with he-
capture in uranium. In chemical studies, plutonium was lium ions in the Berkeley 60-in. cyclotron. For the first
shown to have properties similar to uranium and not to os- time an element was discovered in “one-atom-at-a-time”
mium as suggested earlier. The actinide concept advanced experiments on the basis of only 17 produced atoms re-
by G. T. Seaborg, to consider the actinide elements as a coiling from the einsteinium target. The discoverers of
second f transition series analogous to the lanthanides, element 101, A. Ghiorso, B. G. Harvey, G. R. Choppin,
systematized the chemistry of the transuranium elements S. G. Thompson, and G. T. Seaborg, suggested the name
and facilitated the search for heavier actinide elements. mendelevium in honor of the Russian chemist Dmitri I.
The actinide elements americium (95) through fermium Mendeleev, who was the first to use a periodic system of
(100) were produced first either via neutron or helium-ion the elements to predict the chemical properties of undis-
bombardments of actinide targets in the years between covered elements.
1944 and 1955. The synthesis of element 102 was even more compli-
Element 96, curium, was produced in 1944 by the bom- cated, because a fermium target to apply the bombardment
bardment of 239 Pu with helium ions in the Berkeley 60-in. with helium ions was not available. In order to make use of
cyclotron, and soon after it was found that 241 Pu, formed lighter target elements, heavier ions had to be accelerated.
from 239 Pu by two successive neutron captures in a nuclear The discovery of element 102 was first reported in 1957
reactor, decays under β particle emission to give 241 Am. by an international group working at the Nobel Institute
−
Earlierattemptstoproduceandchemicallyseparateameri- of Physics in Stockholm. The name nobelium in honor of