Chapter 11 - Nuclear Chemistry
Overview Chapter 11 provides an introduction to nuclear chemistry. Exercises take students
from the history of radiochemistry through concepts of radioactive decay to a discussion of
nuclear fission and fusion. Applications include medical uses of radioactivity, body imaging,
radiocarbon dating, and production of power.
Chapter Goals
• To be able to write and balance equations for nuclear reactions
• To understand and be able to list the characteristics of the three common kinds of nuclear
radiation
• To be able to explain half-life and calculate quantity of radioisotope remaining after a given
number of half-lives
• To understand the properties of ionizing radiation and their potential for harm
• To understand the common methods and units of measuring radiation
• To be able to explain transmutation reactions and balance equations
• To understand the differences between nuclear fission and nuclear fusion
• To understand how nuclear reactors are designed and operated safely
Lecture Outline
11.1 Nuclear Reactions
• Atoms having the same atomic numbers (Z) but different mass numbers (A) are called
isotopes.
• An isotope which undergoes spontaneous nuclear reactions (decays with emission of
radiation) is called a radioisotope.
-The term “nuclide” refers to a nucleus; “radionuclide” refers to the nucleus of a radioisotope.
• Chemical reactions involve changes in distribution of outer-shell electrons.
• Nuclear reactions involve changes in an atom’s nucleus. Usually, a different element is formed.
• Isotopes have the same chemical behavior but may undergo different nuclear reactions.
• Rates of nuclear reactions are unaffected by temperature, pressure, or catalysts.
• Nuclear reactions proceed the same whether the nuclide is free or part of a compound.
• Energy changes of nuclear reactions are millions of times greater than chemical reactions.
• In some nuclear reactions, mass may be destroyed and converted into energy.
11.2 The Discovery and Nature of Radioactivity
• French physicist Henri Becquerel accidentally discovered radioactivity after placing a piece of
uranium-containing mineral atop a photographic plate.
• Marie and Pierre Curie investigated radioactivity, isolating uranium, polonium and radium.
• English scientist William Rutherford distinguished between alpha and beta radiation.
• Radiation consists of three fundamental types:
-Alpha (α) particles are identical to the nuclei of helium atoms.
-Alpha particles have little penetrating ability.
-Alpha particles can easily be shielded against, but may still be dangerous.
-Beta (β) particles are identical to electrons and have moderate penetrating ability.
-Gamma (γ) rays are high-energy electromagnetic waves similar to X-rays. They have high
penetrating power and are very difficult to shield against.
• Radioactivity is the emission of an α or β particle or a γ ray from a nucleus.
11.3 Stable and Unstable Isotopes
• Radiation is emitted when a radionuclide spontaneously changes to a more stable nuclide.
• Many natural radioisotopes exist in the earth’s crust, but thousands of artificial radioisotopes have been made.
11.4 Nuclear Decay
• In most nuclear decay, atoms of one element release radioactivity and become atoms of another element.
• In alpha emission, a nucleus emits a particle identical to a helium nucleus (made up of two
protons and two neutrons). Thus, the atomic mass of the original nuclide decreases by four
amu and the atomic number decreases by two.
• In beta emission, there is no apparent change in mass but the atomic number increases by one.
-In effect, a neutron in the nucleus has changed into a proton and an electron (the beta
particle emitted).
• Gamma emission may occur alone or in concert with alpha or beta emission. By itself, the
gamma emission results in no change in nuclear mass or charge.
• Gamma rays are very difficult to shield against and may be very dangerous but are also very
useful in medicine and industry.
• In some cases, a proton in the nucleus is converted into a neutron and a positive electron (or
positron). Positron emission results in a decrease in atomic number of one, but no change in
atomic mass number.
• Electron capture may convert a proton into a neutron, increasing the atomic number by 1.
11.5 Radioactive Half-Life
• The length of time needed for one-half of a sample of radionuclide to decay (change into
another nuclide) is called the half-life.
-Some radionuclides have very long half-lives, measured in millions or billions of years.
-Other radionuclides have very short half-lives, sometimes measured in thousandths of a
second.
• Half-life is unaffected by pressure, temperature, or presence or absence of a catalyst.
Chemistry in Action -Medical Uses of Radioactivity
• Nuclear medical techniques include:
-In vivo studies carried out by injecting a sample of radionuclide-labeled agent into the body
and following its metabolism. This technique allows determination of some
measurements such as blood volume which could not safely be measured otherwise.
• Therapeutic procedures use radiation to kill diseased tissue.
-The radiation may be delivered extraneously or from radionuclides which have been
administered to the patient.
-Very tiny quantities of radionuclides can be absorbed by tissues and used to assess the
metabolic condition of that tissue.
-Some radioactive nuclides can be administered and then absorbed by tumors to slow the
growth of the tumor.
• Diagnosis of cancer may be done by injecting a sample of a radio-labeled compound and
tracing its uptake by tissue.
11.6 Radioactive Decay Series
• Radioactive decay results in release of radioactivity and formation of a new nuclide.
• Each radionuclide undergoes a specific number and type of steps in a decay series.
-The type and amount of radioactivity released gives a specific “signature” for that nuclide
which can be detected by radiation monitors.
11.7 Ionizing Radiation
• The term ionizing radiation refers to high energy radiation.
• Ionizing radiation is not dangerous every time it occurs, but the danger increases with
subsequent exposure and is especially damaging to rapidly-dividing cells such as cancer, bone
marrow, immune system, hair follicles, or gonadal tissue.
• The effects of ionizing radiation vary with the radiation’s energy, distance from the tissue, and
shielding (for example, inside as opposed to outside the body).
-Alpha particles are stopped by clothing and skin, but if present in food samples, may enter
the body and present a much greater hazard than alpha particles outside the body.
-Beta particles can be stopped or diminished by a layer of wood, paper or clothing.
-Gamma rays have no mass or charge, so interact little with tissue. This allows them great
penetrating ability. Most gamma rays pass through tissue with no damage, but high
intensities (from very powerful or very close sources) may pose a hazard.
• Humans are constantly exposed to low levels of radiation, some of which is ionizing. Certain
occupations require special shielding or precautions to prevent damage.
11.8 Detecting Radiation
• The simplest method of detecting radiation is by using photographic film.
-Film badges are used by lab workers as monitors of radiation exposure.
• Geiger counters are used to detect radiation in large areas or to check the intensity of
contamination. Geiger counters are often used to monitor radiation in commercial installations.
• Liquid scintillation counting allows detection of very small amounts of radiation. LSC allows
technicians to use very small amounts of radioactivity and to measure it accurately.
• Technicians working with radionuclides undergo testing and special record keeping
procedures as well as using survey equipment to assure minimal exposure.
Chemistry in Action -Irradiated Food
• Exposure to ionizing radiation may be used to kill harmful bacteria in food.
• The food does not become radioactive during irradiation, and this process helps avoid use of
chemical preservatives as well as contamination by human contact.
-In most cases, food is exposed to high levels of gamma rays for treatment.
11.9 Measuring Radiation
• Intensity of radiation is expressed in several ways:
-Curie: (Ci) Amount of radiation equal to 3.7 x 1010 dps
-Roentgen: (R) Amount of radiation producing 2.1 x 109 charges/cm3 of dry air
-Rad: 1 rad = 1 R
-Rem: Amount of radiation producing the same damage as 1 R of X-rays
-Sievert: 1 Sv = 100 rem
-Becquerel: 1 disintegration per second
• The average person receives about 0.27 rem/year.
• 80% of the radiation received by an average person comes from background sources.
11.10 Artificial Transmutation
• Most of the known radioisotopes are produced by artificial transmutation.
• A natural transmutation in the upper atmosphere converts 14N to 14C.
• Many transmutations brought about in laboratories by nuclear bombardment yield isotopes
which provide useful information about matter.
Chemistry in Action -Body Imaging
• X-ray imaging allows technicians to view tissue regions of different densities, such as bone
and flesh. Radioisotope imaging allows technicians to label an organ or tissue by injecting a
radiopharmaceutical agent, then scanning the organ with an external radiation detector which
acts as a camera. X-ray imaging works especially well with bone or dense tissue, but not as
well with soft tissue. X-ray imaging is has been the traditional method of producing images
of inner body spaces for years.
• CAT scans use thousands of X-ray tomography images to allow clinicians to visualize slices
of internal organs.
• Positron emission tomography allows technicians to visualize brain tissue in action, allowing
a determination of the parts of the brain which are actually involved in a mental process.
• Magnetic resonance imaging (MRI) uses magnetic and radio-frequency fields to interact
with nuclei in the body, yielding excellent images of soft tissues.
11.11 Nuclear Fission and Nuclear Fusion
• Nuclear fission splits a heavy nucleus into two or more lighter nuclei.
-Fission of a 235U nucleus releases neutrons which may cause other nuclei to undergo fission,
releasing more neutrons which may result in a nuclear chain reaction.
-A “critical mass” of fissionable material must be present in order for a nuclear chain reaction to occur.
-In a fission reaction, some mass is converted to energy, resulting in release of an enormous
amount of energy.
• Nuclear fusion occurs when two light nuclei fuse to form a heavy nucleus. As in fission,
some mass is converted to energy, resulting in the release of tremendous amounts of energy.
-Fusion is the source of energy from our sun and other stars.
-Nuclear fusion could be an important source of energy, but it’s difficult to start and contain a
fusion reaction in a laboratory or commercial installation.