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02/13/2001
Heavy Stuff and Stability Islands
Temptation Island is apparently a TV show designed to foster
instability and disorder in a world that seems to me to have quite
enough of both already. On the other hand, the Periodic Table,
with its neat and orderly rows and columns of the elements, is a
bedrock of chemistry. When it was proposed by the Russian
Dmitri Mendeleyev in the mid 1800s, there were only 60 known
chemical elements. Each element has an atomic number, which
is simply the number of electrons orbiting the nucleus. Element
number 1 is hydrogen, with just one electron. Uranium, with its
92 electrons, is number 92. Mendeleyev left empty spaces in his
table and, sure enough, these empty spots were filled in later as
the missing elements were discovered.
If you''re a nuclear physicist, you might prefer to call the atomic
number the number of protons in the nucleus. That''s ok since the
protons are positively charged and the number of protons in the
nucleus equals the number of electrons outside the nucleus. That
way everything is neatly balanced. The nucleus also contains
neutrons, which have no charge. This is important because it
allows a chemical element to exist in several different forms
known as "isotopes".
The simplest example again is hydrogen. You have ordinary
hydrogen composed of just one proton and one electron. You
also have the hydrogen found in so-called "heavy" water. This
isotope of hydrogen has a proton and a neutron in its nucleus and
is also known as deuterium. Since the proton and neutron weigh
about the same, deuterium is twice as heavy as ordinary
hydrogen. It still only has one electron and acts chemically like
ordinary hydrogen. For completeness, there is a third form of
hydrogen, known as tritium, which has two neutrons and a
proton in the nucleus but still only one electron. (We can forget
here about the quarks and gluons and stuff that make up protons
and neutrons. They just complicate things unnecessarily.)
When I was in high school, there were only 92 elements, the
heaviest one being uranium. By the time I graduated from
college, there were two more and the whole world knew about
element number 94, plutonium. That was the critical ingredient
in "Fat Man", the atomic bomb dropped on Nagasaki. I''ve been
to the museum at Sandia in New Mexico and seeing the replicas
of Fat Man and Little Boy, the bomb dropped on Hiroshima, is a
sobering experience.
The discovery of plutonium and neptunium, element number 93,
was the beginning of a concerted effort to create other new, even
heavier elements. The effort has been quite successful. I just
checked the Periodic Table in my Encarta 97 CD encyclopedia
and found that the heaviest element listed was number 111.
What prompted this column, however, was an article by Sonja
Bisbee Wulff in the American Chemical Society publication
Chemistry, Autumn 2000 issue, which perhaps should have been
described as Winter 2001 since it just arrived in the mail. At any
rate, what excited me was that the heaviest element is now
number 118, seven more than in my Encarta 97.
You probably are thinking, "What''s the big deal?" The big deal
is not just that more elements have been created but there''s an
island involved. This island may not have the scintillating
qualities of "Temptation Island", which I haven''t seen and don''t
plan to. Quite the opposite of Temptation, with what I gather is
its attempts to inject instability into committed relationships, the
island I''m talking about could better be called "Stability Island".
It is this island of stability for which heavy element researchers
have been striving for decades.
Let''s set the stage by considering the property of radioactive
elements known as the half-life. Who said anything about
radioactivity and why worry about half-life? Well, have you
ever tried pushing two magnets together with the north poles of
each facing each other? Similarly, protons, with their positive
charges tend to repel each other. Maybe I''ll renege on my
statement to forget quarks and gluons for a moment and just
mention that we''ve discussed in an earlier column why the
nucleus doesn''t just fly apart. However, as you put more and
more protons together, as in uranium, the protons really want to
push away from each other and the nucleus become unstable. It
splits up, ejects various particles and turns into some other
element or elements, i.e., we have radioactivity. The half-life is
the time for half of a given sample of the element to decay. If
you have a pound of plutonium, 82 million years later you only
have half a pound left. The half-life is 82 million years and is a
huge problem for our future descendants. Disposing of nuclear
waste, particularly plutonium and other elements with long half-
lives, has to be done in such a way that succeeding generations
will know and be able to deal with the nuclear waste disposal
sites.
The reason for bringing up the half-life concept is that the half-
lives of the elements heavier than plutonium have generally been
zillions of times smaller than the half-life of plutonium. Half-
lives of a second or small fractions of a second are the rule. But
theorists have predicted that if one could make elements that
were superheavy enough, there could be an "island of stability"
for which the half-lives would be much longer.
Enter the Russians. The economic situation in Russia is
disastrous but they still manage some first rate science. Two
years ago, in January 1999, at the Joint Institute for Nuclear
Research near Moscow, they bombarded plutonium with calcium
and claimed to have made the heaviest element to date, number
114. Number 114 have 114 protons and 175 neutrons. But the
real joy was to be found when they measured the half-life. It was
30 seconds! You are no doubt thinking that''s nothing to cheer
about. However, to a nuclear physicist who''s had to deal with
half-lives of tiny fractions of a second, 30 seconds is like an
eternity. They''ve reached an island of stability.
The reason I noted that the Russian element 114 had 175
neutrons is that the theorists predict that element 114 should be a
so-called "doubly magic" element. Their calculations indicate
that if you could cram not 175, but 184 neutrons into element
114 you would have an isotope that should be at the peak of
Stability Island. No predictions are given for the value of the
half-life of this magic element and speculations that it might be
so stable that someday new materials might be made of 114 are
probably overly optimistic. Nevertheless, you can bet that there''s
going to be a lot of effort to ram in those neutrons and if half-
lives of months or years are obtained, the researchers will be
ecstatic!
The Russian work was performed in collaboration with the
Lawrence Livermore National Laboratory in California. Within
6 months of the 114 achievement, workers at Lawrence Berkeley
National Laboratory, also in California, had jumped over 114 to
form element 118, which promptly decayed to form another new
guy, number 116. They did it by bombarding lead with krypton.
To give you some idea of the difficulty involved in these types of
experiments, the Berkeley group took 11 days of experimenting
to find only 3 atoms of 118, which quickly tossed out a helium
nucleus to become 116. Three atoms ain''t many! It''s a tribute to
the wizardry of these nuclear guys and gals who can detect and
measure the properties of these few fleeting atoms.
The Berkeley group uses a cyclotron to make these new
elements. This atom smasher allows particles to whiz around a
doughnut shaped apparatus and periodically get goosed to higher
and higher energies. When these particles smash into the target
element, the energies are so high that the repulsion of all those
protons is overcome and a new nucleus is formed. This nucleus
is normally unstable and decays to another unstable element, as
with 118 going to 116.
It seems only fitting that one of the superheavy elements now
bears the name of mendelevium. When Mendeleyev constructed
his Periodic Table, he could not have anticipated it would grow
to almost twice as many elements as the 60 that were known to
exist at the time. I just went on the Web to try to determine if I
should have said twice and not "almost twice". I didn''t find any
more elements after 118. I did find out what happens when 118
goes to 116, however. Element 116 quickly goes to our "stable"
30-second 114. I knew you were dying to find out!
Those Russians aren''t stuck on superheavy stuff. They also are
fiddling with the superlight, namely hydrogen. Remember that I
said there are three forms or isotopes of hydrogen? Well, I
hadn''t read the Chemistry article thoroughly enough. It seems
that last year the Russians managed to goose a couple more
neutrons into the hydrogen nucleus, making it 4 neutrons and 1
proton. I don''t know what they call it but I suspect we now have
quintium! Who knows what comes next?
Allen F. Bortrum
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