Special Focus on Nuclear Chemistry

Nuclear chemistry : An introduction

NUCLEAR chemistry is the study of the chemical and

physical properties of elements as influenced by changes

in the structure of the atomic nucleus. Modern nuclear

chemistry, sometimes referred to as radiochemistry, has

become very interdisciplinary in its applications, ranging

from the study of the formation of the elements in the

universe to the design of radioactive drugs for diagnostic

medicine. In fact, the chemical techniques pioneered by

nuclear chemists have become so important that biolo-

gists, geologists, and physicists use nuclear chemistry as

ordinary tools of their disciplines. While the common

perception is that nuclear chemistry involves only the

study of radioactive nuclei, advances in modern mass

spectrometry instrumentation has made chemical studies

using stable, nonradioactive isotopes increasingly impor-

tant.

There are essentially three sources of radioactive ele-

ments. Primordial nuclides are radioactive elements whose

half-lives are comparable to the age of our solar system and

were present at the formation of Earth. These nuclides are

generally referred to as naturally occurring radioactivity and

are derived from the radioactive decay of thorium and

uranium. Cosmogenic nuclides are atoms that are con-

stantly being synthesized from the bombardment of plan-

etary surfaces by cosmic particles (primarily protons

ejected from the Sun), and are also considered natural in

their origin. The third source of radioactive nuclides is

termed anthropogenic and results from human activity in

the production of nuclear power, nuclear weapons, or

through the use of particle accelerators.

Lasers focus on a small pellet of fuel in attempt to create

a nuclear fusion reaction (the combination of two nuclei to

produce another nucleus) for the purpose of producing

energy.

Marie Curie was the founder of the field of nuclear

chemistry. She was fascinated by Antoine-Henri Becquerel's

discovery that uranium minerals can emit rays that are

able to expose photographic film, even if the mineral

is wrapped in black paper. Using an electrometer

invented by her husband Pierre and his brother Jacques

that measured the electrical conductivity of air (a

precursor to the Geiger counter), she was able to

show that thorium also produced these rays—a pro-

cess that she called radioactivity. Through tedious

chemical separation procedures involving precipita-

tion of different chemical fractions, Marie was able to

show that a separated fraction that had the chemical

properties of bismuth and another fraction that had the

chemical properties of barium were much more radio-

active per unit mass than the original uranium ore.

She had separated and discovered the elements polo-

nium and radium, respectively. Further purification of

radium from barium produced approximately 100 mil-

32 CHEMICAL BUSINESS <> DECEMBER 2012

ligrams of radium from an initial sample of nearly

2,000 kilograms of uranium ore.

In 1911 Ernest Rutherford asked a student, George de

Hevesy, to separate a lead impurity from a decay product

of uranium, radium-D. De Hevesy did not succeed in this

task (we now know that radium-D is the radioactive isotope

210 Pb), but this failure gave rise to the idea of using

radioactive isotopes as tracers of chemical processes.

With Friedrich Paneth in Vienna in 1913, de Hevesy used

210 Pb to measure the solubility of lead salts—the first

application of an isotopic tracer technique. De Hevesy

went on to pioneer the application of isotopic tracers to

study biological processes and is generally considered to

be the founder of a very important area in which nuclear

chemists work today, the field of nuclear medicine. De

Hevesy also is credited with discovering the technique of

neutron activation analysis, in which samples are bom-

barded by neutrons in a nuclear reactor or from a neutron

generator, and the resulting radioactive isotopes are mea-

sured, allowing the analysis of the elemental composition

of the sample.

In Germany in 1938, Otto Hahn and Fritz Strassmann,

skeptical of claims by Enrico Fermi and Irène Joliot-Curie

that bombardment of uranium by neutrons produced new

so-called transuranic elements (elements beyond ura-

nium), repeated these experiments and chemically iso-

lated a radioactive isotope of barium. Unable to interpret

these findings, Hahn asked Lise Meitner, a physicist and

former colleague, to propose an explanation for his obser-

vafions. Meitner and her nephew. Otto Frisch, showed that

it was possible for fhe uranium nucleus to be split into two

smaller nuclei by the neutrons, a process that they termed

" fission ." The discovery of nuclear fission eventually led

to the development of nuclear weapons and, after World

War II, the advent of nuclear power to generate electricity.

Nuclear chemists were involved in the chemical purifica-

tion of plutonium obtained from uranium targets that had

been irradiated in reactors. They also developed chemical

separation techniques to isolate radioactive isotopes for

industrial and medical uses from fhe fission products

wastes associated with plutonium production for weapons.

Today, many of these same chemical separation tech-

niques are being used by nuclear chemists to clean up

radioactive wastes resulting from the fifty-year production

of nuclear weapons and to treat wastes derived from fhe

production of nuclear power.

In 1940, at the University of California in Berkeley,

Edwin McMillan and Philip Abelson produced fhe first

manmade element, neptunium (Np), by the bombardment

of uranium with low energy neutrons from a nuclear accel-

erator. Shortly thereafter, Glenn Seaborg, Joseph Kennedy,

Arthur Wahl, and McMillan made the element plutonium by

bombarding uranium targets with deuterons, particles de-

rived from the heavy isotope of hydrogen, deuterium ( 2 H).

Both McMillan and Seaborg recognized that the chemical

properties of neptunium and plutonium did not resemble

those of rhenium and osmium, as many had predicted, but

more closely resembled the chemistry of uranium, a fact

that led Seaborg in 1944 to propose that the transuranic

elements were part of a new group of elements called the

actinide series that should be placed below the lanthanide

series on the periodic chart. Seaborg and coworkers went

on to discover many more new elements and radioactive

isotopes and to study their chemical and physical proper-

ties. At the present, nuclear chemists are involved in trying

to discover new elements beyond the 112 that are presently

confirmed and to study the chemical properties of these

new elements, even though they may exist for only a few

fhousandths of a second.

Nobel laureate Glenn T. Seaborg was among those who

discovered many radioactive elements and isotopes.

As early as 1907

Bertram Boltwood had

used the discovery of ra-

dioactive decay laws by

Ernest Rutherford and

Frederick Soddy to ascribe

an age of over two billion

years to a uranium min-

eral. In 1947 Willard Libby

at the University of Chi-

cago used the decay of '̂̂

C to measure the age of

dead organic matter. The

cosmogenic radionuclide,

•̂̂ C, becomes part of all living matter through photosynthe-

sis and the consumption of plant mafter. Once the living

organism dies, the ^̂ C decays at a known rate, enabling a

date for the carbon-containing relic to be calculated.

Today, scientists ranging from astrophysicists to marine

biologists use the principles of radiometric dating to study

problems as diverse as determining the age of the universe

to defining food chains in the oceans. In addition, newly

developed analytical techniques such as accelerator mass

spectrometry (AMS) have allowed nuclear chemists to

extend the principles of radiometric dating to nonradioac-

tive isotopes in order to study modern and ancient pro-

CHEMICAL BUSINESS •> DECEMBER 2012 33

cesses that are affected by isotopic frac-tionation. This isotopic fractionation re-sults from temperature differences in theenvironment in which the material wasformed (at a given temperature, the lighterisotope will be very slightly more reactivethan the heavier isotope), or from differentchemical reaction sequences.

The newest area in which nuclear chem-ists play an important role is the field ofnuclear medicine. Nuclear medicine is arapidly expanding branch of health carethat uses short-lived radioactive isotopesto diagnose illnesses and to treat specificdiseases. Nuclear chemists synthesize drugs from radio-nuclides produced in nuclear reactors or accelerators thatare injected into the patient and will then seek out specificorgans or cancerous tumors. Diagnosis involves use of theradiopharmaceutical to generate an image of the tumor ororgan to identify problems that may be missed by x rays orphysical examinations. Treatment involves using radioac-tive compounds at carefully controlled doses to destroytumors. These nuclear medicine techniques hold muchpromise for the future because they use biological chem-istry to specify target cells much more precisely thantraditional radiation therapy, which uses radiation fromexternal sources to kill tumor cells, killing nontarget cellsas well. Additionally, the use of nuclear Pharmaceuticals

containing the short-lived isotope ^̂ C has allowed nuclearchemists and physicians to probe brain activity to betterunderstand the biochemical basis of illnesses ranging fromParkinson's disease to drug abuse.

Bibliography

• Hoffman, D.C.; Ghiorso, A.; and Seaborg, Glenn T., eds. (2000).The Transuranium People: An Intimate Glimpse. London: ImperialCollege Press.

• Morrissey, D.; Loveland, W.T.; and Seaborg, Glenn T. (2001).Introductory Nuclear Chemistry. New York: John Wiley & Sons.

• Rydberg, J.; Liljenzin, J,-0 and Choppin, Gregory R. (2001).Radiochemistry and Nuclear Chemistry, 3rd edition. Woburn,MA; Butterwoth-Heinemann •

Nuclear chemistry(Contd. from page 31)

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34 CHEMICAL BUSINESS <• DECEMBER 2012

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