In 1934 Enrico Fermi began his first experiments involving the
bombardment of uranium with neutrons—experiments that were to change the
face of the world.
Fermi had found that slow neutrons, which had very little energy, were
easily absorbed by atomic nuclei—more easily than fast neutrons were
absorbed, and certainly more easily than charged particles were.
Often what happened was that the neutron was simply absorbed by the
nucleus. Since the neutron has a mass number of 1 and an atomic number
of 0 (because it is uncharged), a nucleus that absorbs a neutron remains
an isotope of the same element, but increases its mass number.
For instance, suppose that neutrons are used to bombard hydrogen-1,
which then captures one of the neutrons. From a single proton, it will
become a proton plus a neutron; from hydrogen-1, it will become
hydrogen-2. A new nucleus formed in this way will be at a higher energy
and that energy is emitted in the form of a gamma ray.
Sometimes the more massive isotope that is formed through neutron
absorption is stable, as hydrogen-2 is. Sometimes it is not, but is
radioactive instead. Because it has added a neutron, it has too many
neutrons for stability. The best way of adjusting the matter is to emit
a beta particle (electron). This converts one of the neutrons into a
proton. The mass number stays the same but the atomic number increases
by one.
The element rhodium, for example, which has an atomic number of 45, has
only 1 stable isotope, with a mass number of 103. If rhodium-103 (45
protons, 58 neutrons) absorbs a neutron, it becomes rhodium-104 (45
protons, 59 neutrons), which is not stable. Rhodium-104 emits a beta
particle, changing a neutron to a proton so that the nuclear combination
becomes 46 protons and 58 neutrons. This is palladium-104, which is
stable.
[Illustration: _Fermi’s laboratory in Rome in 1930._]
As another example, indium-115 (49 protons, 66 neutrons) absorbs a
neutron and becomes indium-116 (49 protons, 67 neutrons), which gives
off a beta particle and becomes tin-116 (50 protons, 66 neutrons), which
is stable.
There are over 100 isotopes that will absorb neutrons and end by
becoming an isotope of an element one higher in the atomic number scale.
Fermi observed a number of these cases.
Having done so, he was bound to ask what would happen if uranium were
bombarded with neutrons. Would its isotopes also be raised in atomic
number—in this case from 92 to 93? If that were so it would be very
exciting, for uranium had the highest atomic number in the entire scale.
Nobody had ever discovered any sample of element number 93, but perhaps
it could be formed in the laboratory.
In 1934, therefore, Fermi bombarded uranium with neutrons in the hope of
obtaining atoms of element 93. Neutrons were absorbed and whatever was
formed did give off beta particles, so element 93 should be there.
However, four different kinds of beta particles (different in their
energy content) were given off and the matter grew very confusing. Fermi
could not definitely identify the presence of atoms of element 93 and
neither could anyone else for several years. Other things turned up,
however, which were even more significant.
Before going on to these other things, however, it should be mentioned
that undoubtedly element 93 was formed even though Fermi couldn’t
clearly demonstrate the fact. In 1939 the American physicists Edwin
Mattison McMillan (1907- ) and Philip Hauge Abelson (1913- ),
after bombarding uranium atoms with slow neutrons, were able to identify
element 93. Since uranium had originally been named for the planet,
Uranus, the new element beyond uranium was eventually named for the
planet Neptune, which lay beyond Uranus. Element 93 is therefore called
“neptunium”.
[Illustration: _Lise Meitner_]
[Illustration: _Emilio Segrè_]
[Illustration: _Edwin M. McMillan_]
[Illustration: _Otto R. Frisch_]
[Illustration: _Glenn T. Seaborg_]
[Illustration: _Philip H. Abelson_]
What happened was exactly what was expected. Uranium-238 (92 protons,
146 neutrons) added a neutron to become uranium-239 (92 protons, 147
neutrons), which emitted a beta particle to become neptunium-239 (93
protons, 146 neutrons).
In fact, neptunium-239 also emitted a beta particle so it ought to
become an isotope of an element even higher in the atomic number scale.
This one, element 94, was named “plutonium” after Pluto, the planet
beyond Neptune. The isotope, plutonium-239, formed from neptunium-239,
was only feebly radioactive, however, and it was not clearly identified
until 1941.
The actual discovery of the element plutonium came the year before,
however, when neptunium-238 was formed. It emitted a beta particle and
became plutonium-238, an isotope that was radioactive enough to be
easily detected and identified by Glenn Theodore Seaborg (1912- ),
and his co-workers, who completed McMillan’s experiments when he was
called away to other defense research.
Neptunium and plutonium were the first “transuranium elements” to be
produced in the laboratory, but they weren’t the last. Over the next 30
years, isotopes were formed that contained more and more protons in the
nucleus and therefore had higher and higher atomic numbers. At the
moment of writing, isotopes of every element up to and including element
105 have been formed.
A number of these new elements have been named for some of the
scientists important in the history of nuclear research. Element 96 is
“curium”, named for Pierre and Marie Curie; element 99 is “einsteinium”
for Albert Einstein; and element 100 is “fermium” for Enrico Fermi.
Element 101 is “mendelevium” for the Russian chemist Dmitri Mendeléev,
who early in 1869 was the first to arrange the elements in a reasonable
and useful order. Element 103 is “lawrencium” for Ernest O. Lawrence.
“Rutherfordium” for Ernest Rutherford has been proposed for element 104.
And “hahnium” for Otto Hahn (1879-1968), a German physical chemist whose
contribution we will come to shortly, has been proposed for element 105.
Neptunium, however, was not the first new element to be created in the
laboratory. In the early 1930s, there were still 2 elements with fairly
low atomic numbers that had never been discovered. These were the
elements with atomic numbers 43 and 61.
In 1937, though, molybdenum (atomic number 42) had been bombarded with
neutrons in Lawrence’s laboratory in the United States. It might contain
small quantities of element 43 as a result. The Italian physicist Emilio
Segrè (1905- ), who had worked with Fermi, obtained a sample of the
bombarded molybdenum and indeed obtained indications of the presence of
element 43. It was the first new element to be manufactured by
artificial means and was called “technetium” from the Greek word for
“artificial”.
The technetium isotope that was formed was radioactive. Indeed, all the
technetium isotopes are radioactive. Element 61, discovered in 1945 and
named “promethium”, also has no stable isotopes. Technetium and
promethium are the only elements with atomic numbers less than 84 that
do not have even a single stable isotope.