Presentation Speech by Professor A. Westgren, Chairman of the Nobel Committee for Chemistry of the Royal Swedish Acadeny of Sciences
Your Majesties, Your Royal Highnesses, Ladies and Gentlemen.
In his famous treatise on air and fire, published in 1777, Scheele writes that
in some quarters at that time it was regarded as futile to make any more research
into what elements bodies might consist of. "A depressing prospect" he adds,
"for those whose greatest pleasure it is to study the composition of substances
found in nature." Scheele's own experience and the subsequent developments up
to our day have shown that, at the end of the 18th century, there certainly
still was enough to do for those who wanted to discover new elements. At least
as many elements as were then known still remained to be discovered.
In 1794, Scheele's friend from his days in Uppsala, the Åbo professor
Johan Gadolin published in the proceedings of the Academy of Sciences 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 Mendeleev 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 Mendeleev 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, Niels 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 remain 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, Nr. 92, 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 1936-1938, 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 Strassmann made, however, late in 1938, the epoch-making discovery
that is was not really a question of transuranium elements at all here. 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 Abelson 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 planet 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 235U, that is to say, when it
is split it develops great energy and gives off neutrons. In this way it came
to play an important part in the atomic-bomb project during the war, and methods
were developed for its production on a large scale.
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 all 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 uranium 92, but with actinium
89. Thus, corresponding to the lanthanides, there are the actinides, and a certain
agreement can be found member for member between these two series. Seaborg therefore
proposed for the new transuranium 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 233U, which gives off alpha-rays
and has a half-period of 120,000 years. This isotope, like 235U,
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.
Dr. McMillan. In 1934 Fermi showed that nuclear transmutations could be brought about by irradiating the heaviest elements with neutrons. Research into the reactions thus produced has, however, met with certain difficulties, and it took longer than was expected for the existence of the transuranium elements to be proved. You were the first to succeed in this enterprise. By your discoveries you have opened a field of research in which vast and fundamentally important scientific and technical gains have been made. Later, by your work on the accelerator problem you have also actively furthered the progress in this domain of chemistry.
Dr. Seaborg. At a time when the possibilities of finding
new elements appeared to be exhausted, you have produced a whole row of them
and thus extended the Periodic System beyond the limits which, one might say,
Nature seemed to have established. With great skill you have studied the chemical
characteristics of the new-found elements, and so made clear their atomic structure.
In times past, the hunting for new elements has been a favourite occupation
of many Swedish chemists and, to put it modestly, their efforts have not been
in vain. Quite a number of elements still unknown in the days of Scheele have
been discovered in this country. Such achievements have been appreciated here
and it is but natural that we rejoice in the fact that again a man of Swedish
blood has taken part, and this time a leading part, in fundamental and very
successful work of this kind.
Gentlemen. While proffering you our Academy's warmest congratulations, I will now ask you to receive the Nobel Prize for Chemistry for 1951 from the hands of His Majesty the King.
From Nobel Lectures, Chemistry 1942-1962, Elsevier Publishing Company, Amsterdam, 1964