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Chapter 21: Nuclear Chemistry

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Title: 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).
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