from R&D Innovator Volume 3, Number 12
My Romance with the Transuranium
Seaborg, winner of the 1951 Nobel Prize in Chemistry, for
co-discovering plutonium and nine other transuranium elements, is
(at 82 years old) associate director-at-large of the Lawrence
Berkeley Laboratory. Recently,
element number 106 was proposed to be named seaborgium, the only
element named after a living scientist.
things you just take for granted—for instance, the periodic
table of elements. Questioning this well-worn chart certainly was not the thing
to do. But data
forced me to question it and, thank goodness, I did.
after becoming a chemistry graduate student at the University of
California—Berkeley in 1934 I fell in love with the transuranium
elements. These were
the undiscovered elements with atomic numbers greater than 92 (the
atomic number of uranium), the heaviest naturally-occurring
(the transuranics and I) were first introduced at the weekly
chemistry seminar on nuclear science held in venerable Gilman
Hall. Actually, I was
introduced to what were thought
to be the transuranium elements.
I read articles by Enrico Fermi about the induced
radioactivities observed when elements such as uranium were
bombarded with neutrons. Since
some were published in their native Italian, they were a challenge
induced radioactivities were of special interest to me because
these were supposedly due to the formation of new elements with
atomic numbers greater than 92.
and his coworkers tried to chemically identify these new
radioactivities which were, of course, produced in trace (unweighable)
quantities, so radiochemistry methods were needed.
For guidance, we predicted the chemical properties with the
periodic table as it was then known.
The heaviest natural elements, thorium, protactinium, and
uranium (atomic numbers 90, 91, and 92), were placed in that table
just below the sixth period “transition elements” — hafnium,
tantalum, and tungsten (in these elements, the "5d"
electron shell is being filled).
Thus it was assumed that the 6d electron shell was being
filled in these heaviest elements, and the chemical properties of
the transuranics, the undiscovered elements 93, 94, 95, and 96,
would be homologous with the 5d elements immediately above them in
the periodic table, rhenium, osmium, iridium, and platinum.
The limited chemical identification experiments of Fermi
and coworkers seemed consistent with this view.
The work of Otto Hahn, Lise Meitner, and Fritz Strassman in
Berlin seemed to further confirm it.
did we know then how we were being misled by accepting what was
easiest to accept. I bought this interpretation “hook, line, and sinker."
In fall 1936, I described the work and interpretation of
Otto Hahn and coworkers during a required graduate student talk to
the chemistry faculty, staff, graduate students, and visiting
in January 1939, the bubble burst!
At the physics journal club meeting, we heard something
Bohr, who had arrived in New York the previous week, brought news
from Otto Hahn’s laboratory that the neutron-bombardment of
uranium produced isotopes of light elements, like barium and
meaning was simple: the uranium had been split approximately in
half, and all the radioactive “transuranium” isotopes studied
by Hahn, Strassman, and Meitner during the previous four years
were actually isotopes from the middle of the periodic table.
was exciting! After
the seminar, I walked the Berkeley streets for hours, chagrined
that I hadn’t recognized that the “transuranium elements” in
which I had been so interested were nothing of the kind.
I felt stupid for failing to admit the possibility.
Subsequent work showed that practically all of the
radioactivities that had been ascribed to transuranium elements
were actually due to fission products!
poetic justice, the actual discovery of the first transuranic
resulted from experiments aimed at understanding the fission
process. In 1940, E.M.
McMillan and P.H. Abelson showed that a radioactive product of the
bombardment of uranium with neutrons was an isotope of element 93,
with a mass number 239 (23993).
The isotope 23993,
a negative beta-particle emitter, should decay to the product 23994, but they were unable to observe this daughter product due
to its long half-life.
then started looking for a shorter-lived isotope of element 94
through the deuteron bombardment of uranium.
When McMillan was called to MIT for war work, I continued
this quest with the help of my graduate student Arthur C. Wahl and
another instructor in chemistry at Berkeley, Joseph W. Kennedy.
We succeeded in observing the isotope 23894,
which we could chemically identify but most importantly, we found
that the chemical properties of element 94 weren’t like those
predicted from the periodic table of that time (i.e., not like
osmium), but were chemically similar to uranium.
Joined by physicist Emilio Segrč, we soon identified 23994
and, most importantly, demonstrated that it was fissionable by
slow neutrons. Following
McMillan's suggestion for naming element 93 “neptunium” (after
Neptune, the first planet beyond Uranus), Wahl and I suggested
“plutonium” (after Pluto, the next planet) for element 94.
The chemical symbols would be "Np" and "Pu".
Had the Wrong Properties
chemical properties of neptunium and plutonium were found to be
similar to those of uranium and quite unlike those of rhenium and
osmium, which, according to the existing periodic table, they
should have resembled. Thus
we concluded that a new series of 14 rare-earth-like elements,
starting at uranium, would be the “uranide” (uranium-like)
series, just as the 14 rare-earth elements were known as the
“lanthanide” (lanthanum-like) series.
after Pearl Harbor and the U.S. entry into World War II, I moved
to the wartime Metallurgical Laboratory of the University of
Chicago. Here, we
solved many of the problems attendant with plutonium-239
production, and I turned my attention to the quest for the next
two transuranium elements, 95 and 96.
I was joined in the endeavor by my colleagues Albert
Ghiorso, Ralph A. James, and Leon O. (Tom) Morgan.
weren’t successful until I suggested that we needed a bold
revision of the periodic table.
I wrote a secret report in July, 1944, suggesting that
thorium, protactinium, and uranium be removed from the body of the
periodic table and placed as the beginning of a
"transition" series, analogous to the lanthanide
(rare-earth) elements, in a separate row at the bottom.
Thus the 14 elements beginning with thorium (elements
90-103), would become the “actinide” elements (by analogy with
the “lanthanide” elements).
They would then show the necessary element-by-element
analogy with the lanthanide elements (58-71).
element 95 would be chemically similar to the lanthanide element
europium (63) and element 96 would be chemically similar to
gadolinium (64). Using
this concept, in 1944 and 1945 we synthesized and chemically
identified elements 95 and 96, by analogy with their rare earth
homologues, europium (element 63) and gadolinium (element 64). The new elements were subsequently named americium (95) and
bold revision of the periodic table was a hard sell.
When I showed it to some world-renowned inorganic chemists,
I was advised not to publish it--such an act would "ruin my
I did publish it after the war, and it became a guide for the
chemical identification of most of the subsequent members of the
actinide series. The
series was predicted to end at element 103, and the subsequent
investigations confirmed this.
element 104 (now known as rutherfordium), we jumped back up to the
body of the periodic table, and rutherfordium took its place under
hafnium (element 72). (This
spot had been occupied by thorium before I moved it to a separate
row at the bottom of the periodic table).
Then we proceed across the periodic table to undiscovered
element 118, which will be a noble gas or liquid.
form of the periodic table is accepted throughout the world and is
now ubiquitous in wall charts and chemistry books.
I am, needless to say, proud that U.S. chemists recommended
that element 106, which is placed under tungsten (74), be called
‘seaborgium.’ I was looking forward to the day when chemical
investigators will refer to such compounds as seaborgous chloride,
seaborgic nitrate, and perhaps, sodium seaborgate. Unfortunately, however, this name is under dispute by the
Commission on Nomenclature in Organic Chemistry of the Union of
Pure & Applied Chemistry (IUPAC) because I’m still alive. Rejection of this name would be the first in history that the
uncontested discoverers of an element are denied the privilege of
naming it. IUPAC may
make a final decision on this at its meeting in the United Kingdom
in August, 1995.
sometimes you just have to do some remodeling.
The periodic table, one of the beauties of scientific
thought—is not Gospel but rather a guideline based on the best
available evidence at the time. Essentially, what I did was to follow the fundamental
principle of Mendeleev's table: that elements tend to chemically
resemble those above them in the table.
Instead of deciding that this rule no longer applied in the
transuranic region, I remodeled the table to follow its essential
The moral I draw from all this is that you must have the courage to follow the data--even if it goes against the known “truth.”