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1951
Nobel Prize for Chemistry - Glenn Seaborg and Edwin McMillan PRESENTATION
OF AWARD THE 1951
NOBEL PRIZE FOR CHEMISTRY.
Professor
Arne Westgren,
Chairman of the Nobel Committee for Chemistry
In
1794, SCHEELE'S friend from his days in Uppsala, the Åbo professor Johan
Gadolin published in the proceedings of the Academy of Science a report on a
"Study of a black heavy kind of stone from Ytterby Stone Quarry at Roslagen".
In this mineral--later called gadolinite, after him--he had found a hitherto
unknown earth, the so-called yttria. Nine years later Berzelius, in a mineral
from Riddarhyttan in Västmanland (the so-called "Bastnäs
tungsten") discovered another earth, ceria. These
two discoveries together provided the starting-point for studies of the
so-called rare earth elements which went on throughout the 19th Century. Already
Gadolin had reckoned with the possibility that the yttria isolated by him was
not a simple substance and it proved indeed later to consist of several oxides.
Berzelius' ceria turned also out to be a mixture. The separation of the
different components in these compound earths has been no easy task, since they
are chemically very similar to one another. Little by little, however, it has
been possible to divide them up completely, and within this group alone as many
as 14 different elements have been isolated. Swedish chemists, chief among them
being Mosander and Cleve, have made very valuable contributions in this domain
of chemistry. Of the rare earth metals many--yttrium, terbium, erbium,
ytterbium, scandium, thulium, holmium--have been given names that show their
origin in various Swedish localities.
Besides
this group of closely connected rare earth metals many other elements were
discovered in the course of the 19th Century. A comprehensive survey of all the
known elements was provided in 1869 by the establishment of the periodic system.
At that time Mendelejeff and Lothar Meyer independently discovered that there
were clear evidences of periodicity in the chemical character of the elements
when they were arranged in the order of increasing atomic weights. From this
regularity Mendelejeff was able to conclude that certain gaps remained still to
be filled, and he could even predict all the most important properties of these
still undiscovered elements and their compounds. His predictions have been fully
confirmed by later discoveries. During
the years around 1920, Nils Bohr's investigations on the structure of atoms
threw new light on the periodic system. It was now possible, among other things,
to explain the chemical similarity between the rare earth elements. The positive
charge in the nucleus of the atom and the number of electrons surrounding it
rises by one unit for every step upwards in the element series. This additional
electron usually forms part of the outermost shell of the atom, and since the
chemical characteristics depend on the structure of the atom in just this part,
the successive members in the series of elements can for the most part be
clearly distinguished from one another in respect to their chemical properties.
But within the group of the rare earths it is not the outermost electronic shell
that is developed, nor the shell beneath it, but the one that underlies that.
The
result is that, through the whole series of these elements, the exterior parts
of the atomic structure remains virtually unchanged. Together they come to form
what might be called a group of quasi-isotopes. Since they are like lanthanum,
the first element in the series, they have been given the comprehensive name of
lanthanides. If,
said Bohr, there existed an extension of the series of elements beyond the
heaviest of them all, Nr92, uranium, then this would form a new series of very
closely associated elements. They would all resemble uranium and by analogy with
the lanthanides, would form a series of uranides. By
experiments which were carried out during the years 1936-38, Otto Hahn and Lise
Meitner believed they could confirm Fermi's statement that the transuranium
elements are formed by irradiating the heaviest elements with neutrons. But
these synthetic elements were not like uranium, but appeared to be homologues of
elements so dissimilar to one another as rhenium, the platinum metals, and gold.
Hahn and Strassman made, however, late in 1938 the epoch-making discovery that
is was not really a question of transuranium elements here at all. The heavy
atoms were found to split up into substances belonging to the middle of the
elemental series and this brought the whole problem into a new stage.
The
first transuranium element of which there was definite proof was produced by
McMillan and Abelsson in May 1940 at the University of California, by
irradiating uranium with neutrons with the aid of the cyclotron built by
Lawrence. It was obtained as a disintegration product of a [[beta]]-radiating
uranium isotope, which has a half life of 23 minutes. Hahn and Meitner had also
discovered this body, but their preparation was too weak for its
daughter-product to be demonstrated. The Americans were able to investigate this
thoroughly, and showed that it forms an isotope of element 93, that is to say, a
transuranium element. They called it neptunium
after the plane Neptune, whose orbit lies next outwards after Uranus in the
solar system. By irradiating uranium with rapid neutrons or with heavy-hydrogen
nuclei deuterons, other neptunium isotopes were soon produced in Berkeley. In
1940 McMillan and Seaborg and their fellow-workers had already reported that
when neptunium disintegrates it gives rise to an element 94. By analogy with the
way in which names had been found for neptunium and uranium, this second
transuranium element was called plutonium,
after the planet Pluto, which has its orbit next outside that of Neptune. The
first isotope of this element, which has a half-period of 24,000 years and thus
is relatively stable, is what is called an atomic fuel. This plutonium isotope
reacts with slow neutrons in the same way as the uranium isotope
After
these problems, conditioned by the war, had been solved, Seaborg, as leader of a
comprehensive group of able colleagues, completed the studies of the
transuranium elements. In doing this, he has written one of the most brilliant
pages in the history of the discovery of chemical elements. Not
less than four more transuranium elements have been produced. The chemical
characteristics of these new elements have been established by developing a
refined ultra-microchemical experimental technique. Bohr's prophesy that in the
transuranium elements we are dealing with a group of substances of the same sort
as the rare earth metals, has thus been confirmed. However, this new series of
closely associated elements does not begin with 92 uranium, but with 89
actinium. Thus, corresponding to the lanthanides, there are the actinides, and a
certain agreement can be found between every member in these two series. Seaborg
therefore proposed for the new trans-uranium elements 95 and 96 the names americium
and curium, in analogy with their
corresponding rare earths europium and gadolinium (after Europe and Gadolin
respectively). The two transuranium elements most recently discovered, berkelium
and californium, correspond to terbium and dysprosium in the lanthanides.
By
irradiating different sorts of heavy atoms with neutrons, protons, deuterons,
helium nuclei, or, most recently, carbon nuclei, a great number of isotopes have
been produced from the six transuranium elements. The study of these isotopes'
formation and properties has yielded a wealth of scientific material.
A
great many, originally isolated, observations on the radioactive transmutation
series were made during the work on the great plutonium project. Thanks above
all to Seaborg's activities it has been possible to bring these observations
together into a comprehensive wholeness. In this way there was discovered an
entirely new radioactive series which, from its most long-lived member is now
called the neptunium family. The
mass numbers of the three radioactive families which were previously known have
the form 4n (thorium series) 4n + 2 (uranium series) and 4n + 3 (actinium
series). Here the neptunium series fills a gap with mass numbers of the form 4n
+ 1.
During
his studies on the reaction of slow neutrons with thorium, Seaborg and his
colleagues made a discovery which opened important technical prospects. They
obtained a uranium isotope U233, which gives off a-rays
and has a halfperiod of 120,000 years. This isotope, like U235, can
be used as an atomic fuel. Thorium, which is more plentiful in nature than
uranium, will therefore probably play a role as a basic material in the
production of atomic energy.
The
Swedish Academy of Sciences is of the opinion that these discoveries in the
realm of the chemistry of the transuranium elements, of which I have here tried
to give a brief account, are of such importance that McMillan and Seaborg have
together earned the 1951 Nobel Prize for Chemistry. ©
the Nobel Foundation 1952 ACCEPTANCE
SPEECHES M.
MCMILLAN:
"Your
Majesty, Your Royal Highnesses, Ladies and Gentlemen. ©
the Nobel Foundation 1952 M.
SEABORG: Your
Majesty, Ladies and Gentlemen: I
am very thankful that I and my co-workers have been able to conduct researches
that the Swedish Royal Academy of Science believes deserve the bestowal of the
Nobel Prize. I can only hope that the new elements that we have found will be
used for the good of mankind. And finally, I would like to thank the Academy for
honoring me and my co-workers in the manner that they have. ©
the Nobel Foundation 1952 BIOGRAPHY
SUBMITTED BY DR. SEABORG TO THE NOBEL COMMITTEE GLENN
THEODORE SEABORG.
Glenn
T. Seaborg was born in Ishpeming, Michigan, on April 19, 1912, of Swedish
ancestry. His father, Herman Theodore Seaborg, was the son of Swedish
immigrants; his mother, the former Selma Olive Erickson had come to the United
States from Grängesberg, Sweden. When he was ten, his family moved to Home
Gardens, California (now part of South Gate near Los Angeles) where his parents
still reside. There after finishing grammar school in 1925 he attended the David
Starr Jordan High School in Los Angeles, from which he was graduated in 1929. He
entered the University of California at Los Angeles in September, 1929, earning
his way through school by working at various jobs. By 1931 he had been accepted
as an assistant in the University's chemistry laboratory preparing samples and
doing some research and teaching. During his last two years, through his courses
at the university, nuclear physics and chemistry captured his imagination and he
concluded that he would pursue this field. In 1934 Seaborg received his A.B.
degree and transferred to the University of California at Berkeley. There in
1937 he earned his Ph.D., his thesis subject being the inelastic scattering of
fast neutrons. The following two years he was laboratory assistant to Dr.
Gilbert Newton Lewis, then Dean of the College of Chemistry on the Berkeley
campus. In
1939 he received his appointment from the University of California as an
Instructor, and in 1941 he was promoted to the rank of Assistant Professor. From
1942 to 1946, he was on leave of absence from the University of California
acting as chief of the section working on transuranium elements at the Manhattan
Project's wartime Metallurgical Laboratory at the University of Chicago. Shortly
after he joined the Manhattan Project in 1942 he married Helen L. Griggs, then
secretary to Nobel Laureate E.0. Lawrence. They now have four children Peter
Glenn, Lynne Annette, David Michael, and Stephen Keith.
While
still on leave from the University of California, he was promoted from the rank
of Assistant Professor to that of full Professor (1945). In May, 1946, he
returned to the University of California to assume his position in the Chemistry
Department and to take responsibility for the direction of the nuclear chemical
research in the Radiation Laboratory of the University. Among
his major contributions are his discoveries, with several collaborators, of the
transuranium elements plutonium
(94), americium (95), curium (96),
berkelium (97), and californium (98), and the
study of their chemical properties and their position in the periodic table. He
is at present engaged in research work at Berkeley on the transuranium elements. |
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