Chapter 21: Nuclear Chemistry
The study of nuclear reactions with an
emphasis on their uses in chemistry
and their effects on biological systems.
Reactivity
• In nuclear reactions, the nuclei of unstable isotopes,
called radioisotopes, gain stability by undergoing
changes accompanied by the emission of large
amounts of energy.
• The process by which materials give off such
energy, in the form of waves (rays), is called
radioactivity.
• Waves and particles emitted are called radiation.
• Types of Radioactive Decay
– Beta: an electron ejected from or captured by the nucleus.
– Positron: positively charged particle of same mass as an
electron ejected from the nucleus
– Neutron: particle given off during fission process
– Alpha: Helium (He) nucleus with no electrons
– Gamma: High energy wave (no mass/no charge).
• Sample Exercises 21.1 and 21.2 pg 896 and 897.
Patterns of Nuclear Stability
• The stable nuclei on a
neutron-versus-proton
plot are located in a
region called the band
of stability. Unstable
nuclei undergo
spontaneous
radioactive decay.
The type of decay
that occurs depends
on the neutron-toproton ratio of the
unstable nucleus.
Nuclear Transmutations
• Naturally occurring
– Have already talked about several
• Radioactive Decay
– Another is the production of N-14 from naturally
occurring C-14 (half life of 5715 years)
• Earliest artificial transmutation was performed
in 1919 by Ernest Rutherford by bombarding
nitrogen gas with alpha particles.
• Elements with Atomic Number above 92, the
transuranium elements, all undergo
transmutations. None of them occurs in
nature. These elements have been
synthesized in nuclear reactors and
accelerators.
Rate of Radioactive Decay
• Every radioisotope has a characteristic rate of decay
measured by its half-life
• A half-life is the time required for one-half of the nuclei of a
radioisotope sample to decay to products.
– After one half-life, half of the original radioactive atoms have decayed into
atoms of a new element.
– How many are left after two half-lives?
• Half-life equation: Activity final = Activity initial (1/2) time passed/half-life
• Half-lives may be as short as a fraction of a second or as long
as billions of years.
• Many artificially produced radioisotopes have very short halflives, a feature that is a great advantage in nuclear medicine.
– The rapidly decaying isotopes do not pose long-term biological radiation hazards to
patients.
• When radioisotopes decay, they may decay to another
element that is also unstable
• These elements that are unstable have half-lives of their own.
– Elements that are unstable and undergo further decay until a stable nucleus configuration
is reached are called radioactive intermediates.
Detection of Radioactivity
• Ionizing radiation: radiation with enough energy to knock
electrons off some atoms of the bombarded substance to
produce ions. Ionizing radiation penetrates a thin window at
end of detector. Gas becomes ionized, free electrons are
produced. Each time this occurs, current flows. Current flows
drive a counter or cause and audible “click”
– One such device, a Geiger-counter, uses a gas-filled metal tube to detect
radiation.
• Geiger-counters are used primarily to detect beta/positron and gamma radiation.
• Neutron Detectors: The fast neutrons from fission are slowed
down (thermalized) by the material that surrounds the
detector. The thermal neutrons then interact with a material in
the detector tube that has a high cross section for absorption.
After the neutron is captured, free electrons are given off. The
gas becomes a conductor and causes current flow through
the detector. The current drives a counter or causes an
audible “click”.
• Alpha Radiation: A scintillation counter uses a
specially coated phosphor surface to detect
radiation. Ionizing radiation striking the phosphor
surface causes flashes of light. The number of
flashes are detected electronically, converted into
electronic pulses, then measured and recorded.
Similar to what takes place inside some television
tubes that are coated with phosphor on the inside.
• Film Badges and Dosimeters: Film badges consist
of several layers of photographic film which darken
when exposed to radiation
– Workers wear the badges the entire time at work, the badges are “developed” at
regular intervals to monitor the worker’s exposure to ionizing radiation
– Dosimeters are hand held devices that are designed for short duration use
only. They consist of a filament that has been charged and is calibrated
such that at full charge the dosimeter indicates zero exposure. As the
worker is exposed to ionizing radiation the charge in the dosimeter is
reduced and the filament moves across a scale indicating how much
ionizing radiation has been received by the worker.
Nuclear Power: Fission and Fusion
• When the nuclei of certain isotopes are bombarded
with neutrons, they undergo fission, the splitting of a
nucleus into smaller fragments.
– Isotopes – What is the difference between U – 238 and U –
235?
– An example of U – 235 fission
• Fusion occurs when nuclei combine to produce a
nucleus of greater mass.
– In solar fusion, hydrogen nuclei (protons) fuse to make helium
nuclei.
– An example, shows that the reaction also requires two beta
particles. What is a beta particle?
Nuclear Reactor
Two Steps Involved in Nuclear Reactors
• Step 1 – Neutron Moderation
– Neutrons produced from fission move so fast they will pass right
through a nucleus without being absorbed.
– Water and carbon (graphite) are good moderators because they slow
the neutrons (close to elastic collisions) so the chain reaction can be
sustained.
• Step 2 – Neutron Absorption
– To prevent the reaction from going too fast some of the slowed
neutrons must be trapped before they hit fissionable atoms.
– Carried out by control rods made of materials such as Cadmium.
– Some unintended absorbers are created by the fission process and
impact reactor operation (Xenon).
• Despite other dangers, a nuclear reactor cannot produce a
nuclear explosion. The fuel elements are widely separated
and cannot physically connect to produce the critical mass
required. Once a nuclear reactor is started, however, it
remains highly radioactive for many generations (nuclear
waste discussion today, half- life discussion on Friday).