Chapter 21: Nuclear Chemistry

1
Chapter 21 Nuclear Chemistry

• The study of nuclear reactions with an emphasis
  on their uses in chemistry and their effects on
  biological systems.

2
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.

3
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-to-proton ratio of the
  unstable nucleus.

4
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.

5
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 half-lives, 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.

6
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.

7

• 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 workers 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.

8
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?

9
Nuclear Reactor
10
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).