The Synthetic Elements

11 Pages·1950·4.735 MB·English

by  I. Perlman and G. T. Seaborg

THE S THETIC
ELEMENTS
There are nin,e of including the newly manufactured
the~n,
berkelium. Four have filled gaps in the periodic table
of 9 2 elements and five have extended it beyond uranium
by I. Perlman and G. T. Seaborg
THE urge to take our material world riodic similar-ities among the known ele As one might expect, a new element
apart and identify its ultimate units ments. When the elements were ar did not necessarily appear for the first
of construction is at least as old as ranged in the sequence of their atomic time in pure form, nor dtd it assert its
the early Greek philosophers. vVe have weights, it was found that elements oc singularity. Some research workers may
come a long way from their conClusion curring at certain intervals on the list have handled substances in relatively
that all substances are variations of an resembled one another in chemical pure form and failed to recognize them
Olympian brew composed of only four properties. This resulted in the construe as new elements. More often new ele
ingredients-fire, water, earth and air- tion of a periodic table of the elements, ments were reported which proved to be
but the end of the quest is not yet in which in turn disclosed some gap_s. It identical with previously known ele
sight. At the moment the list of identified was logical to assume that the_m issing ments or mixtures. D.ming the 1920s and
elements stands at 97. The physicists, of chemical properties should be attributed 1930s a number of workers reported the
course, have broken these down into to still undefected elements. discovery of elements 43, 61, 85 and 87.
protons, n.eutrons and units which may \ With the discovery of X-rays and of They gave these elements such names as
be even more basic. In this article, how- the atomic nucleus at the turn of the masw'ium, illinium, florentium, alaba
ever, we shall stop short of the subatomic century, a more meanin_gful picture of mine, virginium and moldavium, and to
world and confine ourselves to the ele- the differences among eTements began tlJis day these names appeal' in some
ments, considering protons and neub·ons to emerge. We ·now know that the dis tables of elements. It is fairly certain,
only as they affect the clements' stability. tinguishing mark of each element-what however, that none of these elements
From the strictly chemical point of view, determines its chemical properties-is can exist in natme in quantities detecta
the elements may be considered the the number of elecb·ons it possesses, and ble by the methods of investigation then
blocks of which our universe is built. that this in turn is uniquely determined employed. Actually the unambiguous
Our particular concern in this ruticle by the number of positive charges or identification of three of the four ele
will be the so-called synthetic elements, protons in the nucleus. The electrons ments (the exception: element 87) had
which for all practical purposes are not are attached to the nucleus in successive to await. their preparation by artificial
found in nature but are created only by shells, and as we go up the periodic table means, and even element 87 can be pre
the alchemy of the modern laboratory. from the lower to the higher elements pared more readily by transmutation
Let us make clear at the beginning we find that the electrons closest to the than from natural somces.
what we mean when we say that the nucleus are attached more ana more
synthetic elements are missing in nature. firmly to the atom. The dislodging of Stahle and Unstable Isotopes
Actually tiny amounts of some of them, one of these inner elecb·oris is imme
such as plutonium, have been detected diately (ollowed by an outer electron To understand the transmutation of
in the earth, and all of them doubtless falling into the vacancy, and the energy elements, we must turn to considerations
existed in-considerable amounts at the released by this event appears as an of nuclear stability. An element, as we
primordial creation of the elements. But X-ray. The wavelength of the X-ray is have hoted, is uniquely characterized by
without e:cception they are so unstable characteristic of the element. It was the number of protons in the nucleus.
that the original atoms must have disap- H. G. J. Moseley of England who dis But the number of neutrons associated
peared long ago; any such atoms now covered this relationship and thereby with a given number of protons may
found in nature are created only rarely was able to arrange the elements accord vary. This results in the existence of vari
by spontaneous nucleru· reactions due to ing to atomic number and to telrprecise ous species of t:J;te same element, known
cosmic-ray bombardment or naturalm- ly which elements were missing. Gradu as isotopes, which differ in weight and
dioactivity. ally most of the gaps between hydrogen stability but not appreciably in chemical
The idea that all matter could be re- (atomic number 1) and manium (atom properties. Relatively few of the possible
duced to a limited number of chemically ic number 92) were filled by the-discov isotopes of any element are stable; in
indivisible elements began to take form ery of new elements. By 1925 only four fact, of the 1,000 isotopes of the 97 ele
in the 19th· century. It developed prin- elements remained to be found: those of ments known to date, only about 275 are
cipally from the discovery of certain pe- atomic numbers 43, 61, 85 and 87. stable. Several hundred unstable nuclei
16 Nobel Prize Winning Authors Volume III Scientific American
may yet be prepared, but probably few
stable nuclei remain to be discovered.
The paucity of stable nuclei is largely
explained by the interconversion of pro
tons and neutrons; a proton will change
into a neutron or vice versa if there is a
slight imbalance from a norm character
istic of each region of the periodic table.
The nucleus is much more stable if it has
an even number of neutrons or protons.
As a result each element with an odd
uumber of protons has only one or two
stable isotopes.
The margin by which an isotope may
be stable or unstable is indeed small
when compared with the total amount
of energy involved in binding together
the components of a nucleus. In mod
erately heavy nuclei the total binding
energy is about 1,000 million electTon
volts, yet if one nucleus is bound more
firmly by even .01 mev than another
with one more proton and one less neu
tron, the second nucleus wi.ll decay into
the first. Such small irregularities in nu
clear binding may deprive some odd
numbered elements of the possibility of
having even one stable isotope. Conse PLUTONIUM hydroxide is isolated CURIUM is so intensely radioactive
quently such an element, barring some in a few crystals at the bottom of a that it glows hy its own light in a
freakish factor that prevented its most capillary tube. Sample w~s prepared water solution. Curium was discov
nearly stable isotope from decaying, at University of Chicago in 1942. ered at University of Chicago in 1944.
would not be found in natme.
It may be of interest to speculate how
different our lives would be, if indeed
we would be here at all, should certain
elements be unstable. One element with
only a single stable isotope, for example,
is iodine. This element, as a constituent
of thyroxine, the hormone of the thyroid
gland, is vital ns a regulator of growth\
and development and of the metabolic
rate. It is difficult to visualize what form
vertebrate animal life would have taken
had this element been missing. Anot!J!;lr
element with but one stable isotope is
gold. As well as we can measure it, this
isotope, Au107, is considerably less than
one mev more stable than the artificial
and highly unstable mercury isotope
Hg197• If this situation were reversed,
Fort Knox might still be used to store
something, but il would not be gold.
The Missing Elements
Below lead, clement 82, only two ele
ments are missing in nature. These are
elements 43 (technetium) and 61 (pro
methium) which, as we shall see, may
be prepared artificially in radioactive
form just as one may prepare radioa<.:tive
isotopes of all of the elem!;lnts. The form
of instability responsible for the absence
of elements 43 and 61 involves neutron
proton interconversions. It is called beta
instability, meaning that the nucleus
emits beta particles, i.e., clccb·ons, in 'J...,
•.-1
attaining stability.
Among the heavier elements another PROi\'IETiflUM nitrate is isolated TECHNETIUM is prepared in the
type of instability sets in. This type is a as clustered crystals. The crystals in compound ammonium pertcchne
consequence of what may loosely be the original photomicrograph have tate. In original photomicrograph
considered an overstuffing of positive been enlarged by about 30 diameters. crystals were enlarged 16 diameters.
ScicntificA mcrican.com Nobel Prize Winning Authors Volume III 17
charge in the nucleus. The nucleus has
an urge to get rid of protons. The mecha
nism for relieving its condition is the
emission of alpha particles, or helium
nuclei, composed of two protons and two
neutrons. The reason this complex par
ticle rather than a pwton is emitted is
simply that the helium nucleus is such a
stable structme that it is more economi
cal of energy to rid the nucleus of pro
tons in this fashion than individually. By
the time bismuth, element 83, is reached,
alpha instability sets in as a general con
dition. (Some nuclei do not exhibit their
alpha instability, however,' because their
beta-decay rate is much faster than their
alpha-decay rate.) These alpha-emitters
have vastly varying lifetimes: some as
short as a microsecond, others compara
ble with the age of the earth. The point
to· be made here is that there are only
three nuclei above bismuth sufficiently
long-lived to have sm:vived geological
time-thorium, 232, uranium 235 and
uranium 238. These isotopes are respon
HOT LABORATORY at the University of California provides facilities for sible for the fact that the earth still has
working with highly radioactive elements, synthetic and otherwise. Elements small amounts of the elements between
are chemically manipulated behind a lead wall by remote control (below). 83 and 92, for the latter arise as products
of the decay of uranium and thorium.
Thus the existence of this small island of
relative stability at thorium and manium
is the only factor preventing the termina
tion of the periodic table at bismuth.
As these three nuclei decay, they
maintain their various products in equi
librium with them, in amounts that de
pend on their relative stability or half
lives. For example, radium 226 is one of
the decay products of U2"8• Since there
spective half-lives of these isotopes are
1,600 years and 4.5 billion years, the
two are found together in the ratio of
one part of radium to three million parts
of uranium, or a third of a gram of ra
dium to a ton of uranium. Two of the
elements with extremely short half-lives,
elements 85 .and 87, are almost missed
completely in the radioactive-decay se
ries; element 87 occurs in uranium only
in the fantastically low concentration of
a few parts per billion billion. In the case
of element 85, the amount that has been
detected in nature is much smaller. Such
small quantities, of course, cannot be iso
lated and are measurable only tlu·ough
their radioactivity.
It is only by the grace of an odd com
bination of unusual circumstances, by
the way, that fissionable U235 still exists
in the earth in sufficient quantity to pro
vide us with nuclear chain reactors and
atomic energy. U2:H> has a half-life of
only .7 billion years, which means that
more than 90 per cent has disappeared
through radioactive decay during the
tl1!'ee billion years of the earth's age.
Thus only by the slenderest of margins
does enough U235 remain in natural
uranium to operate a nuclear reactor or
BEHIND LEAD WALL is equipment for manipulating radioactive clements. to make its separation from U238 feasi
At upper left is a mirror in which the manipulations may he olJServed. Some ble. And this is only half the story. Re
stages in the isolation of curium were carried out in this laboratory. cent studies of alpha radioactivity have
18 Nobel Prize Winning Authors Volume liT Scientific American
shown that U235 falls into a categ01·y of is the hazard in handling these radio
nuclei whose half-lives are longer than active substances when they are made in
would be predi_cted from their decay visible amounts. A good ·example is the
energy-the principal factor influencing curium isotope Cm212, the principal iso
the half-life. Even among this group in tope of curium that has been used for
which alpha decay is "forbidden," U235 experimentation up to this time. This
is something of a freak. For its particular isotope has a half-life of only five months,
decay energy it might be expected to and if it were possible to make one milli
decay about 10 times more rapidly than gram of it, its alpha radioactivity would ·
it does; while even if its decay were only be 3.5 curies. This would be a consider
twice as rapia as it actually is, it would able amount of radioactivity to work
essentially have disappeared from the with. If it were spread uniformly over
earth by this time. the entire state of New York (we de
Above manium, alpha-decay half liberat(!ly refrain from spreading it here
lives again become quite short, so the in California), radioactivity could be de
transuranium elements of primordial tected on every square· foo_t of ground.
origin are no longer present although Thus the experimenter has no alterna
there is every reason to believe that such tive but to work with extremely small
elements were formed at the same time amounts of such isotopes. Ultramicro
as the more stable ones. The longest chemical methods have been developed, ONE ELEMENT, the synthetic rare
lived tninsuran.ium isotope known to however, which permit almost any type earth promethium, has a characteris
date, rfeptunium 239, has a half-life of of chemical and physical measurement tic X-ray spectrum with two lines.
only two million years, which is almost to be made on only a few micrograms to
100 times too short to have permitted a milligram of an element.
the element to persist through geological
time. Element 43
Radiochemistry The first synthetic element to be
created was technetium, element 43,
What about the chemical behavior of wh.ich filled the gap in the periodic ta
these unstable elements? Basically the ble between molybdenum and ruthe
chemistry of a radioactive substance is nium. Technetium was definitely identi
no diiferent from that it would have if it fied for the fhst time by C. Perrier and
were not radioactive. There is one prac E. Segre of Italy in 1937. A sample of
tical difference in handling it, however, molybdenum that had been irradiated
and this is that we can detect it even with deuterons in the University of Cal
when it is present" in vanishingly small ifornia cyclotron was sent to them. From
concentrations. If it were not for th.is it they isolated a chemical fraction
facility, most unstable nuclei would re which, on the basis of its radioactive
main undiscovered, for they are fotmcl, behavior, was distinct from all other
or can be prepared only in unweighable known elements. Its chemistry con
amounts. formed with what might have been ex
Today. the techniques of the new pected from element 43, an element in
branch of study called radiochemistry the series known as Group VII. Such a
make it possible to obtain a great deal group, as already indicated, is made up .THREE ELEMENTS in succession,
of information about chemical properties of elements in the periodic table that are neodymium, promethium and samar
and to carry through chemical separa chemically similar to one another be- ium, have successive pairs of lines.
tions with amounts of material far too cause they have the same number of
small to handle by the usual methods of electrons in the outer shell. Further ex
chemistry. By the mere analysis of a ploratory work showed that element 43
substance's radioactivity it is possible to was somewhat closer in properties to
obtain semiquantitative . information rhenium, the next heaviest element in
about its solubility, its oxidation-reduc Group VII, than to manganese, the .next
tion potentials, its formation of complex lowest element in the group. Ten years
ions and many other properties. later Segre suggested that element 43
Once the element has been identified be named technetium ( Tc), derived
there is a great incentive for manufac from the Greek technikos, signifying the
turing it in visible amounts so tha:t one element's artificial or "technical" origin.
can study its spectra, its crystal structure From the behavior of a certain isotope
and many other properties that are in of technetium, Tc00, with a half-life of
accessible to radiochemical methods. six hours, it was deduced that this iso
With the advent of the nuclear reactor tope must have another form, or nuclear
the synthesis of these radioactive ele isomer, with a long half-life. Thus one of
ments is no longer difficult, provided the the conditions for obtaining macroscopic
transmutations can be effected with neu amounts of the element was fulfilled,
trons. There are problems, however, in namely, that a long-lived isotope must
cotmection with the elements' great ra exist. The other condition, a method of
dioactivity. It is desirable to work with preparing the element in quantity, was
an isotope with a relatively long half realized when it was shown that the six
life, for if the half-life is short it may be hour Tc00 is a fission product and there- TWO ELEMENTS, neodymium and
difficult to produce the element at a fore can be made in an atomic pile. The samarium, have X-ray lines with a
faster rate than it decays. Also important fission yield of technetium is high; Tc911 gap between them fo1· promethium.
ScicntificAmerican.com Nobel Prize Winning Authors Volume III 19
ASTATINE PROMETHIUM
1 2
H He
I
~~s
3 4 5 6 7 8 9 10
II 11 N:_,
'i"l~·i 12p
11 12 13 14 15 16 17 18
~r'';~~
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 ·35 36
K C':JSc Ti V Cr Mn Fe Co Ni Cu Zn.JGa Ge As Se Br Kr.l
4s 3d 4p
37 38 '39 40 46 47 48 49 50 51 52 53 54
Rb Pd Ag C~ ln Sn .Sb Te I X~
~4~~5~
55 56 57 58 59 60 1 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 ··8 6
Cs sc:,L~Ce Pr Nd • Sm Eu Gd Tb Dy Ho Er Tm Yb LJ Hf Ta W Re Os· lr Pt Au H!J Tl Pb Bi Po · E':J
6s 5d 4f 5d 6p
I I
L - - ---•--f- -i--l-i--i-- ---------------------------___ I
1
88 89 90 91' 92 .
Ra . Ac Th Pa U ~~~ ' · : ..
.J.J •
7s 6d 5f ?
..
1I ------------------------------------_________ .:•. :
7p
6d
20 Nobel Pri~e Winnjng Authors Volume III ScienLilic American
constitutes 6.2 per cent of the eventual hers 58-71 inclusive). They are closely Ridge National) Laboratory. They sug
fission products in apile, and it can be allied in chemical properties to one an gested that the element be named pro
calculated that the fission of one gram other and to their prototype, lanthanum methium ( Pm) to draw a parallel be
of U233 produces 26 milligrams of Tc99• (element 57). The rare earths always tween mankind's newly acquired nuclem·
The element can also be made by ina- occur together and have a marked pre power and the acquisition of fire, which
diating molybdenum with neutrons to dilcction for remaining together under according to Greek mythology was
form Mo99, which decays to Tc99; so two most chemical treatments. The classical stolen from the gods and given to man
methods for production in quantity arc method for separating them is repeated by Prometheus.
available. recrystallizations, a most laborious and The longest-lived known isotope of
When it became possible to produce material-consuming process. promethium, Pm147, has a half-life of
suitable amounts of long-lived Tc90, it When the rare emths were finally 3.7 years; nevertheless it has been iso
tw·ned out to have a half-life of close to lined up according to atomic number, lated in the pure state. At the Oak Ridge
one million years. With this half-life the the space for element 61 remained un National Laboratory G. W. Parker and
element becomes relatively easy to han- filled. Obviously the place to look for P. W. Lantz obtained promethium from
dle. A number of milligrams have now element 61 was in rare-earth ores, but a fission-product mixture, and B. H.
been isolated and many of its chemical the difficulty of separating rare earths Ketelle and G. E. Boyd isolated some
properties have been investigated. Like from one another was an obstacle. Al from neub'on-irradiated neodymium.
the brilliant violet permanganates fa- though a number of claims to its dis
miliar to chemists, the corresponding covery were made, the question of the ElemenL85
pertechnetates are also brightly colored. existence or nonexistence of element 61
Technetium has been prepared in the in nature was still unsettled when the Let us now consider the third of the
metallic state and combined in a number possibility of /.)reparing it artificially fom gaps in the periodic table, element
of compounds; its optical emission spec- presented itsel . Several groups of in 85. The synthesis of this element actually
truro and X-ray spectrum have been vestigators irradiated neighboring rare came second chronologically, before
recorded; its mass number has been earths and produced some new sow·ces promethium. Its manufacture presented
proved to be 99 by means of the mass of radioactivity, some of which undoubt a somewhat different problem from that
spectrograph. edly were isotopes of element 61. Bv.t of technetium and promethitun. Tech
It is almost certain that there can be which radioactivities belonged to ele netium can be made by irradiating
na primordial technetium in nature, ment 61 and which to new isotopes of molybdenum, the next lowest element,
since all possible isotopes of the element known rare earths? This question re with neutrons or deuterons; promethium
seem to have half-lives which m·e far too mained unanswered because the meth similarly can be made from neodymium,
short. The only processes that come to ods of fractionating rare earths were of the next element below it. But in the
mind as possible sources for the con- insuperable difficulty and .the radioac case of element 85 the next lowest ele
tinuing formation of technetium in na- tivities observed had short half-lives. ment, polonium, itself exists only in trace
turc arc ( 1) the spontaneous fission of Two developments in the Manhattan amounts in natme. Hence polonium can
U238, a very rare event, and (2) the ac- Project during the war conspired to al not serve as the starting material. To
tion of stray neutrons on molybdenum low the unambiguous discovery of ele make element 85 one must start with the
and U235• If we assume that the yield of ment 61. Foremost was the development element two numbers below it: bismuth.
Tc99 from the spontaneous fission of of methods for.separating the rare earths Cousequently it is necessary to add not
U238 is the same as that from the neu- ' . by means of synthetic ion-exchange one but two charges, transmuting ele
tron-induced fission of U235, and that the resins. It was found that neighboring ment 83 to 85. This can be done by irra
half-life of spontaneous fission is 1010 rare earths could be largely separated in diating bismuth with accelerated ions of
years, then each kilogram of uranium one pass tll!'ough a glass column filled helium, which has two protons. The syn
when dug from the ground would con- with resin. Furthermore, the rare earths thesis was first accomplished by D. R.
tain only a few millionths of one micro- came off the column in a definite order, Corson, K. R. MacKenzie and Segre at
gram of Tc99• The other mechanisms for inversely as the atomic numbei·, so that the University of California (to which
producing Tc99 would not yield much it was fairly safe to assume that element Segre had come from Italy) . They
more. 61 would follow samarium, element 62. named element 85 astatine, from the
The second important discovery was Greek word meaning "unstable.!' The
Element-61 that a relatively long-lived isotope of -ine ending mean8 that the element is a
element 61 occurs as a result of uranium halogen, ·i.e., a member of the chlorine
The next synthetic element we shall fission. The first positive identification of family.
consider is the rare earth promethium. clement 61· came in 1945 from the ex The particular isotope of astatine first
The rare-earth elements are_ a group periments of J. A. Marinsky and L. E. made was At211, an alpha-particle emit
from cerium to lutetium (atomic num- Clendenin at the Clinton (now Oak ter. As a matter of fact, it exhibits a phe-
THE PERIODIC TABLE at the bottom of the opposite graphic terminology they are designated l, 2, 3, 4, 5 and
page pt·esents the 97 natm·al and synthetic elements in 6. The spectrographic suhshells are designated s, p, d
horizontal rows to show similarities in their chemical and f. The maximum number of electrons in any s sulJ
properties. Elements of similar chemical properties arc shell is two, in any p suhshell six, in any d subshell 10
connected by the lines running from top to bottom. and in any f suhshell14. The number of electrons in each
AJJove the sym]Jol of each element is its atomic num]Jer, suhshell is indicated hy a superscl'ipt; for example 6p"
i.e., the number of positive charges in the nucleus or in ~he oute~·most suhshell of astatine (At) indicates that
the number of electrons bound by them. The nine syn there arc five electrons in the p suhshell of shell 6. In
thetic clements are indicated in red. In each horizontal the case of astatine all the subshells arc filled except the
row is one or mOl'e half-brackets designated ls, 2s, 2p last. The case of promethium (Pm) and the other rare
and so on. Each of these brackets denotes the filling of eat·ths, however, is more comp.lcx. _In the rare-earth
a shell of electrons-more properly a subshcll-in the series from cerium (Ce) to lutetium (Lu) the number
succession of the elements. The electron shell structures of 5d and 6s electrons l'Cmains the same; in successive
of two synthetic clements are giveu in the schematic elements electrons are added in the 4/ suhshell. Prome
drawings at the top of the page. In X-ray terminology thium thus has four 4/ electrons. The transuranium
the shells are designated K, L, M, N, 0 and P. In spectra· elements appear to belong to a second rare~earth series.
ScientificArnerican.corn Nobel Prize Winning Authors Volume III 21
nomenon known as "branched decay," with thorium, 'which breaks down by a drogen to manium, are now completely
which means that some atoms break number of steps through various isotopes filled. We turn next to the transuranium
down in one way and some in another, of radium, actitlium, polonium and other elements.
both sequences yielding alpha particles. heavy clements until it finally becomes
One part of a sample of astatine emits stable lead. The second series starts with Beyond Uranium
alpha particles directly, thereby decay uranium and goes through a number of
ing to bismuth 207; the other part first transformations into other isotopes of The seardt for tnmsuraniurn elements,
captures an electron and .becomes polo these elements until it, too, degenerates a quest born of scientific curiosity, was
nium 211, but the latter is extremely to stable lead. The third series, starting destined to be the trigger for a series
short-lived and promptly gives off a very with actino-manium, or U235, proceeds of events which within a decade were
energetic alpha particle. As a result, through actinium, from which the series to rock the world and burst upon the
At211 is observed to decay with two derives its name, and finally ends as still consciousness of every literate human
alpha pru·ticles of widely differing ener another stable isotope of lead. being. These events, of course, were the
gies. When observed with proper radia Soon after the significance of these discoveries that led to the exploitation
tion-measming equipment, this isotope decay processes became understood, and of nuclear energy, in particular as a
is as distinctive as a cat with two heads. it was realized that a number of isotopes weapon of mass destruction. Other fun
The nuclear properties of·astatine iso of clements between uranium and lead damental scientific discoveries undoubt
topes have been well charted and they should be found as decay prodncts, at edly have had equal or greater effect on
lead to the conclusion that probably ,no tempts were made to locate some isotope mankind's mode of existence in the past,
species of this element will have a half of element 87 in a decay sequence. Ele but none literally exploded in his face
life greater than several hours. The ment 87 of course stands above lead, as has this one.
chemistry of astatine has been investi element 82, so it should be found some In 1934 Frederic and Irene Joliot
gated on the tracer scale by the methods where in one of these series. It soon be Curie of France made the exciting ob
of radiochemistry. Its behavior is that of came obvious, however, that no isotope servation that an ordinary stable ele
a halogen considerably more electro of element 87 would result from the ment could be made radioactive by ir
positive than iodine, just as iodine is known breakdowns in the main pathway radiating it with alpha particles of
more electropositive than the next light of any of the three decay series. But in natural origin. This discovery of artificial
est halogen, bromine.. One means of 1914 Stefan Meyer, V. F. Hess and F. A. radioactivity immediately stimulaled
separating ast'atine from solutions is by Paneth of Austria noted that actinium, research toward preparing radioactive
electroplating or chemical plating. There Ac227, which was known as a beta forms of many elements. Two other ex
is no easy way to prepare astatine in visi emitter, also decayed occasionally by tre-mely important developments were
ble runmmts, for its longer-lived isotopes alpha emission. Since .actinium is ele taking place at about the same time.
can only be made by the use of a parti ment 89, its alpha-decay product must One was the development by E. 0.
cle accelerator such as a cyclotron. Even be an isotope of element 87, in this case Lawrence at the University of California
these have half-lives of only a few hours, 87223• It was not until 1939, however, of the cyclotron, which was soon able to
so that work with macroscopic quanti that Mlle. M. Perey of France suc accelerate charged pruticles to energies
ties will be exceedingly difficult because ceeded, by very meticulous radiochemi far beyond those of naturally occurring
of the intense radioactivity. cal separations, in obtaining a 21-min alpha particles. This discovery made it
ute beta-particle emitter which she possible to bombard and transmute the
Element 87 \ proved to be the alpha-decay product of heavier elements for the first time, for
Ac227• She later named this new clement alpha particles from natural sources can
The foruth and final gap in the peri francium in honor of her native land. peneh·ate the nuclei of only the lightest
odic table was element 87. According The three natmal radioactive series, elements. The second development wa.c
to ·its place in the table, tlus element as we have observed, almost miss ele the discovery by James Chadwick of .
should be a member of the alkali family, ment 87 completely. But when the mti England of the neutron, an uncharged
which includes sodium, potassium and ficial tl'ansmanium element neptunium particle capable of entering any nucleus
cesium. As in the case of the other miss was synthesized, a fourth series, starting easily. Neutrons will literally fall into
ing elements, there had been a number with that element, was discovered. And any nuclei at which they are directed.
of claims to the discovery of element tlus series yields an isotope of francium, Since neutrons could be prepared by
87 by conventional chemical methods; Fr22\ in its main decay sequence. This diJ:ecting radium alpha-particles at a
the substance so identified had been isotofe arises from the alpha decay of light element such as beryllium, it be
variously called virginium and molda Ac22 • It has a half-life of only five min came possible for anyone who could ac
vium. But we now are virtually certain utes, decaying by alpha emission to an quire 100 milligrams or so of radium to
that there can be no stable isotope of astatine isotope of .02-second half-life. produce and study the transmutation of
element 87, and furthermore the longest Subsequently-it was found that Fr221 is elements. Most prominent in such stu
lived known radioactive isotope of the also obtained as a decay product in a dies was a group working with Enrico
element has a half-life of only about 20 sequence starting from the artificial iso Fermi of Italy. They soon found a meam
minutes, so it could not have been de tope thorium 233 (see page 45) . of preparing transuranium elements by
tected by conventional chemical meth The short half-lives and inaccessibility making use of the great avidity of nuclei
ods. of the francium isotopes have discour for neutrons.
Actually element 87 was first discov aged chemical investigation of the ele It was already known that if the heavi
ered unmistakably through its radio ment. It appears to behave like m1 al est stable isotope of an element captured
activity, and it was found as a product kali element in solution; one item of a neutron, the nucleus became unstable
of the decay of a heavy element. To note is that francium has great volatility and decayed to the next higher element
understand how it was identified, we when the solution is evaporated to dq by beta emission; this method, as we
must examine briefly the various proc ness and brought to a temperature of have seen, can be used to produce tech
esses by which the elements at the heavy several hundred degrees. This is a prop netium from molybdenum and pro
end of the periodic table decay. There erty of alkali elements that begins to be methium from neodymium. Suppose
are three separate decay series among prominent with cesium and is accen this process were applied to uranium, the
the natural heavy elements, known re tuated with francium. heaviest element. U238 should captme a
spectively as the thorium, uranium and Thus the gaps in the classical periodic neutron and become a heavier isotope,
actinium series. The first series starts system, covering the elements from hy- una, which would be beta-unstable and
22 Nobel Pri1.c Winning Authors Volume III Scicnti fie American
decay to clement 93- a brand-new ele
ment outside the periodic table! When
this experiment was tried, the experi
menters experienced, a shock: instead of
observing just one or ~wo radioactivities URANIUM
from the product, they found a be~
wilclering array of radioactivities. For
some time it was thought that these ac
tivities must represent a number of new
transuranium elements. Not tmtil several
years later was it recognized that the
activities came from fission products.
Thus the discovery of fission was a by
product of the search for b·ansuranium
elements.
With poetic justice the actual dis
covery of the first transmanium ele
ment in tmn resulted from experiments
aimed at understanding the fission proc
ess. Several experimenters, including
E. M. McMillan of the University of
California, measmed the energies of
the two main fission fragments by ob
serving the distances they traveled from
each other as ~ result of their mutual
RADIUM
recoil when the nucleus exploded. Mc
Millan noted that there was another
radioactive product of the reaction, with
a half-life of 2.3 days, which did not re
coil, at least not sufficiently to escape
FRANCIUM
from the thin layer of fissioning uranium.
He suspected. that tlus was a product
formed by neuh·on capture, which does
not release much energy, rather than by NATURAL FRANCIUM is tl1c result of rare branched decay of actinium 227.
fission. McMillan and P. H. Abelson Ninety-~e per cent of Ac227 .decays by heta emission to thorium 227.
early in 1940 deduced by chemical An almost negligible amount decays by alpha emission to francium 223.
means that this product was smely an
fsotope of element 93, 1.1rising by beta
decay from U239• The latter had a half
life of 23 minutes·. Element 93 was givem
the name neptunium (Np) because it
was beyond uranium, just as the planet
Neptune is beyond Uram~s;
About this time the possibility of a
nuclear chain.reaction and the produc
tion of transuranium elements for mili
tary use began to take shape. Witl1 the
war ah·eady in progress, further work
on the transuranium elements and re
lated subjects was conducted by physi
cists and chemists under self-imposed
secrecy, at first informally and finally as
an organized program.
A neptunium isotope of great practical
interest is Np237, discovered in 1942 by
A. C. Wahl and Seaborg at the Univer
sity of Qalifornia. It is very long-lived ACTINIUM
(half-life: two million years) and can be
made in appreciable amounts as a by
product in the uranium pile. Because it
is relatively innocuous, it can be handled
RADIUM
experimentally in principle like any nor
mal element.
It was not obvious a pt'iol'i what the
electronic configuration and chemical
properties of neptunium might be.
FRANCIUM
Uranium was known to have some simi
larity to tungsten and it was thought that
element 93 might be a homologue of tl1e
next element above tungsten, rhenium. SYNTHETIC FRANCIUM is made in appreciable quantity by irradiating
Yet there was also a possibility that ele thorium 232 with neutrons. The irradiation forms Th233, the starting point
ment 93 might be a member of a new of this table. The decay proceeds through five other isotopes to francium 221.
ScicntificArnerican.com Nobel Prize Winning Authors Volume TIT 23
,.
transition series among the heavy ele element's electrons, whether this is ac to the quadrivalent state. The next state,
ments, similar to the rare-earth group. complished by the specific method of pentavalent plutonium, is colorless; the
It turned out that neptunium bears no adding oxygen or by any otl1er means. highest oxidation state, six, is bright yel
resemblance to rhenium. It is much more "Oxidation state" is a somewhat more low. It is unfortunate that plutonium,
closely allied to its neighboring element rigorous term for what-we used to call because of its radioactivity, may never
uranium. The evidence is mounting that the "valence" of an element.) Only a be suitable material for classroom use,
all of the b:ansuranium clements belong few rare earths can be induced to as for it has superb attributes as a teaching
to a new transition series analogous to sume oxidation states other than the material.
the rare-earth group. The transuranium trivalent, and these with difficulty. The
eleme1its parallel the rare earths in elec heavier elements, notably uranium, nep Element 94
tronic conRgw-ation and have some twlium, pluto:nium and americium, are
stwng resemblances to them in chemical distinctly multivalent, witl1 tl1e trivalent A_fter neptunium plutonium was of
properties. Just as lanthanum is the pro state becoming progressively more stable ..:ow-se the next element discovered; its
totype element for the rare-earth series, along the series. Thus trivalent thorium name derives from the fact that Pluto is
so actinium is the prototype for the cannot be obtained in aqueous solution; the next planet beyond Neptune. The
heavy-element series. l-Ienee the new uranium can be reduced to this state work of McMillan and Abelson had
group may be called the actinide series. only with difficulty; plutonium can be shown that Np230 decayed by emission
The members of this series known in na reduced to it fairly readily; for americi of beta particles; therefore the product
ture-thorium, protactinium and urani um it is the principal state, and for curi should be the next higher element, num
um-had not appeared to be related um it is the only one known. ber 94. However, the new element de
·chemically, but when the transuranium The effort brought to bear on under cayed so slowly that it could not be
elements were studied latent similarities standing plutonium chemistry has ele definitely detected through its radioac
in the whole group began to appear. Be vated it to the status of one of the com tivity. By the end of 1940 Seaborg, Mc
cause of certain differences in chemical mon elements, insofar as knowledge of Millan, J. W. Kennedy and A. C. Wahl
properties the theory that these ele its chemical properties is concerned. did discover element 94" by a somewhat
ments belong in one series is not ac The pronounced multivalent nature of different approach. By irradiating w-ani
cepted by all chemists. It is possible to an element such as plutonium makes it um with deuterons they made a new
answer the objections on the basis of a of great interest in chemical studies, for isotope of neptunium which also de
detailed analysis of the evidence, but this single element affords means of ob cayed to plutonium, but in this case tht
this is not the place for such a discus serving most of the phenomena of inor plutonium was sufficiently short-lived tc.
sion. ganic chemistry. Plutonium is perhaps allow its detection. This isotope of plu
Let us note a few points here, how unique in having four different oxidation tonium has proved to be Pu2a~, with an
ever, on the chemical properties of states, which coexist in easily measurable alpha-decay half-life of 90 years, while
these heavy elements as a group and concenh·ations in aqueous solutions. The that for Pu239, the first isotope made, is
their comparison with the rare earths. color changes from one oxidation state 24,000 years.
The rare-earth elements are predomi to another afford a fitting visual accom Armed with the information on the
nantly trivalent, or in what the chemist paniment to the curious existence of the chemistry of the new transuranium ele
now calls the "plus three oxidation state." many states. Plutonium in its trivalent ments, Kennedy, Seaborg, Segn\ ·and
(Most chemists now use the term "oxi state in solution is a beautiful pme blue; Wahl in 1941 were able to identify
dation" to signify the removal or neutral- \ it changes to green or ambe1: ( depen.ding Pu239 from strongly irradiated uranium
ization through bond formation of an upon solution conditions) when oxidized and were able to prove that Pu239 ,v-ould
·--·~---,---,.---·,-,.----.----,---.----r----r----..,r------r---,----,----,-----,---·--
~:l-~-~:~J~~:~C~:~~~~--+-2-06_I+-I-2-0-7+-2-08·-jl-20--9-+--21-0-+-2-1-1-+1 --21-2-+-21-3-~ i :'o ,~ l~
, 216 217 21s : ,. 1 --2-22
~~R=:~=~=~~-~--~=-~-:=I~UM~:~---i~~-------t-+--~1---l---+-l--t-j--+--1!--l-j- +~ ----i----1--~-t-!~! ~~~-~~~J!,- ----------~-- --~l
--r
i I
- -------+--+---------+-:-------r-----R'
!
! ~---'---~+- -+ I
THORIUM I ---r-- I +---!------
ACTINIUM J__j i 1 _ J _ Ac222
_RA__D_1u-M---+-·T ! , 1 -~ Ra222
!
FRANCIUM _ t ! f~to + _ J. ~~ EFrm2t& "~~':-- -~~·
~~::.~~ON ~;;;,. A;,;-~ ~--
I L F
_j;...--.,.....
Po~06 p~t -~ P~• J .-
POLONIUM ,....... - 4(1- Pom
sr06 1 ___
BISMUTH -~~ B_!!•ol'~~"' ~ ~· ...J......--~ rl--~
-LE_A_D----.. ~ PbZtO ~--' ___ 1 Pb,,.,.,_ I I I -
-+··- ,....... .,
THE URANIU:M: SERIES of radioactive elements is the nucleus (numbers at the top). In nature (red sym·
mapped on the basis of atomic number or number of bols ctnd arrows) the series hegins with uranium 238,
protons in the nucleus (column of elements at left) which decays through 13 other isotopes to stable lead
and atomic weight or number of protons and neutrons in 206. The series has now been enla1·ged ( blaclc symbols
24 Nobel Prize Winning Authors Volume Ill Scientific American