Nobel Prize for Chemistry - Glenn Seaborg and Edwin McMillan
PRESENTATION OF AWARD
THE 1951 NOBEL PRIZE FOR CHEMISTRY.
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.
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
Majesty, Your Royal Highnesses, Ladies and Gentlemen.
© the Nobel Foundation 1952
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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