Unstable Nuclei and

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Section 4.4

• Unstable Nuclei and
• Radioactive Decay

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Objectives

• Explain the relationship between unstable nuclei
  and radioactive decay.
• Characterize alpha, beta, and gamma radiation in
  terms of mass and charge.

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Radioactivity

• Chemical Reactions
• Atoms of one element do not change into atoms of
  another element during a chemical reaction.
• Why?

4
Radioactivity

• Chemical reactions involve the rearrangement of
  atoms not their destruction.
• Chemical reactions involve only an atoms
  electrons the nucleus remains unchanged.

5
Radioactivity

• Nuclear Reactions
• These are reactions that involve an atom of one
  element changing into an atom of another element.
  These reactions involve the change of an atoms
  nucleus.

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Radioactivity

• In the late 1890s, radioactivity was discovered.
• Radioactivity is a process in which some
  substances spontaneously emit radiation.
• Radiation is the name for the energy (rays) and
  particles emitted by radioactive material.
• Scientists discovered that by emitting radiation,
  atoms of one element could change into atoms of
  another element.

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Radioactivity

• Why does radioactivity occur?
• Radioactive atoms emit radiation because their
  nuclei are unstable.
• Unstable systems gain stability by losing energy.
• Unstable nuclei lose energy by emitting radiation
  in a spontaneous process called radioactive
  decay.
• Radioactive decay continues until stable,
• non-radioactive atoms form. They are often of
  a different element.

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Types of Radiation

• Research, begun in the late 1880s, used
  electrically charged plates. Scientists were
  able to identify 3 types of radiation.

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Types of Radiation

• Alpha Radiation
• Deflected to negatively charged plates.
• Made up of alpha particles 2 protons and 2
  neutrons with a 2 charge.
• Symbol for an alpha particle is 42a or 42He (the
  helium nucleus)
• Example of alpha decay

10
Alpha Radiation

• Note what happens to the atomic number of an
  element upon emission of an alpha particle.
• The atom giving up the alpha particle has its
  atomic number reduced by two. This results in
  the atom becoming a different element. In the
  example, Radium-226 undergoes alpha decay to form
  Radon-222.

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Alpha Radiation

• 22688Ra --gt 22286Rn 42He
• This type of equation is known as a nuclear
  equation.
• The atomic numbers and mass numbers of all
  particles involved are shown.
• Note that both atomic number and mass number are
  conserved in a nuclear equation.

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Practice Problems

• Complete the following nuclear equations for
• alpha decay
• 24195Am ----gt
• _____ ----gt 42He 23592U
• 23592U ----gt
• _____ ----gt 42He 20684Po

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Types of Radiation

• Beta radiation
• Deflected towards the positively charged plate
• Consists of fast-moving electrons called beta
  particles. They have a 1- charge.
• Symbol for a beta particle is 0-1ß or 0-1e.
• Example of beta decay

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Beta Radiation

• The atom giving up the beta particle has its
  atomic number increased by one.
• This results in the atom becoming a different
  element. In the example, carbon-14 undergoes beta
  decay to form nitrogen-14, according to the
  equation
• 146C ---gt 0-1e 147N.
  
• Note that, in the equation, atomic numbers and
  mass numbers are conserved.

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Beta Radiation

• Since beta radiation is a type of nuclear
  radiation, the beta particle is emitted by the
  nucleus of an atom.
• How, you may ask, is it possible for an electron
  to come from a nucleus??
• Scientists believe that neutrons are actually
  composed of 2 particles - an electron and a
  proton.
• In beta decay, the electron is emitted and the
  proton is left, increasing the atomic number.

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Practice Problems

• Complete the following nuclear equations for
• beta decay.
• 1. 6027Co ----gt
• 2. _____ ---gt 0-1e 23191Pa
• 3. 9740Zr ---gt
• 4. _____ ---gt 0-1e 6630Zn

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Types of Radiation

• Gamma Radiation
• Also called gamma rays
• It is high-energy electromagnetic radiation. It
  has no charge. It also has no mass so its
  emission does not change the element.
• However, gamma radiation often accompanies alpha
  and beta emission, which do change the element's
  identity.
• Symbol for a gamma ray is 00 ? or just ?
• Gamma rays account for most of the energy lost
  during the radioactive decay process.

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Types of Radiation
19
Unstable Nuclei

• Why are some atoms unstable?
• The primary factor in determining stability is
  the ratio of neutrons to protons.
• Atoms containing either too many or too few
  neutrons are unstable.
• Unstable nuclei lose enough energy and emit
  enough particles during radioactive decay to
  eventually form a nucleus with a stable neutron
  to proton ratio.