Nuclear Transformation

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Nuclear Transformation

• Prentice-Hall Chapter 25.2
• Dr. Yager

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Objectives

• Describe the type of decay a radioisotope
  undergoes.
• Solve problems that involve half-life.
• Identify the two ways transmutation can occur.

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Radon in the Environment

• Radon-222 is a naturally occurring radioisotope
  found in the soil in some areas which can collect
  in a closed house and pose a health risk.

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• The protons and neutrons of a nucleus are called
  nucleons.
• A nuclide is a general term applied to a
  specific nucleus with a given number of protons
  and neutrons.
• Nuclides can be represented in two ways. One way
  shows an elements symbol with its atomic number
  and mass number.

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Nuclear Strong Force

• In 1935, the Japanese physicist Hideki Yukawa
  proposed that a force between protons that is
  stronger than the electrostatic repulsion can
  exist between protons.
• Later research showed a similar attraction
  between two neutrons and between a proton and a
  neutron.
• This force is called the strong force and is
  exerted by nucleons only when they are very close
  to each other.

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• All the protons and neutrons of a stable nucleus
  are held together by this strong force.
• Although the strong force is much stronger than
  electrostatic repulsion, the strong force acts
  only over very short distances.
• Although forces due to charges are weaker, they
  can act over greater distances.

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• In smaller nuclei, the nucleons are close enough
  for each nucleon to attract all the others by the
  strong force.
• In larger nuclei, some nucleons are too far apart
  to attract each other by the strong force.
• If the repulsion due to charges is not balanced
  by the strong force in a nucleus, the nucleus
  will break apart.

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The Strong Force

• In the nucleus, the nuclear force acts only over
  a distance of a few nucleon diameters.
• Arrows describe magnitudes of the strong force
  acting on the protons.

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What determines the type of decay a radioisotope
undergoes?

• The neutron-to-proton ratio determines the type
  of decay that occurs.
• If there are too many neutrons relative to
  protons, a neutron will turn into a proton
  (b-decay)
• If there are too many protons, a proton will turn
  into a neutron (positron emission)
• A positron is a particle with the mass of an
  electron but a positive charge.

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• Approximately 350 isotopes of 90 elements are
  found in our solar system. About 70 of these
  isotopes are radioactive.
• An additional 1,500 isotopes have been made in
  the laboratory, most of which are unstable
  (radioactive).
• For elements with atomic number lt 83, most
  isotopes are stable. These nuclei are in a
  region called the band of stability.

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Predicting Nuclear Stability

• Except for 1H and 3He, all stable nuclei have
• neutrons gt protons
• A nucleus with N/Z (neutrons/protons) that is too
  large or too small is unstable.
• small nuclei N/Z 1
• large nuclei N/Z 1.5
• Nuclei with even numbers of protons neutrons
  are more stable
• No atoms with atomic number gt 83 and mass number
  gt 209 are stable

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Isotope Trivia

• Calcium has six stable isotopes (40Ca 48Ca)
• Tin has 10 stable isotopes (most of any element)
• Heaviest stable isotope
• 209Bi bismuths only stable isotope w/126
  neutrons

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Half Life

• A half-life (t1/2) is the time required for
  one-half of the nuclei of a radioisotope sample
  to decay to products.
• After each half-life, half of the existing
  radioactive atoms have decayed into atoms of a
  new element.

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Half-lives can range from fractions of a second
to billions of years.
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Decay of Uranium-238
Stable Isotope
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Carbon-14 Dating

• The ratio of Carbon-14 to stable carbon in the
  remains of an organism changes in a predictable
  way that enables the archaeologist to obtain an
  estimate of its age.

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• The half-life (t½) of C-14 is 5,730 years.
• How long is three half-lives?
• 3 x (5,730 years) 17,190 years
• How much of 8 grams are left after three
  half-lives?
• 8 g x ½ x ½ x ½ 1 g

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Limitations of C-14 Dating

• Two factors limit dating with carbon-14.
• C-14 cannot be used to date objects that are
  completely composed of materials that were never
  alive, such as rocks or clay.
• After four half-lives, the amount of radioactive
  C-14 remaining in an object is often too small to
  give reliable data.
• C-14 is not useful for dating specimens that are
  more than about 50,000 years old.

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Potassium-40 Useful for Dating

• Anything older than 50,000 years must be dated on
  the basis of a radioactive isotope that has a
  half-life longer than that of carbon-14.
• Potassium-40, which has a half-life of 1.28
  billion years, represents only about 0.012 of
  the potassium present in Earth today.
• Potassium-40 is useful for dating ancient rocks
  and minerals.

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Transmutation Reactions

• The conversion of an atom of one element to an
  atom of another element is called transmutation.
• Transmutation can occur by radioactive decay.
• Transmutation can also occur when particles
  bombard the nucleus of an atom.

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The first artificial transmutation reaction
involved bombarding nitrogen gas with alpha
particles.
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Transuranium Elements

• The elements in the periodic table with atomic
  numbers above 92, the atomic number of uranium,
  are called the transuranium elements.
• All transuranium elements undergo transmutation.
• None of the transuranium elements occur in
  nature, and all of them are radioactive.

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• Transuranium elements are synthesized in nuclear
  reactors and nuclear accelerators.

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• During nuclear decay, if the atomic number
  decreases by one but the mass number is
  unchanged, the radiation emitted is
• a positron.
• an alpha particle.
• a neutron.
• a proton.

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• During nuclear decay, if the atomic number
  decreases by one but the mass number is
  unchanged, the radiation emitted is
• a positron.
• an alpha particle.
• a neutron.
• a proton.

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• When potassium-40 (atomic number 19) decays into
  calcium-40 (atomic number 20), the process can be
  described as
• positron emission.
• alpha emission.
• beta emission.
• electron capture.

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• When potassium-40 (atomic number 19) decays into
  calcium-40 (atomic number 20), the process can be
  described as
• positron emission.
• alpha emission.
• beta emission.
• electron capture.

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• If there were 128 grams of radioactive material
  initially, what mass remains after four
  half-lives?
• 4 grams
• 32 grams
• 16 grams
• 8 grams

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• If there were 128 grams of radioactive material
  initially, what mass remains after four
  half-lives?
• 4 grams
• 32 grams
• 16 grams
• 8 grams

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• When transmutation occurs, the _____ always
  changes.
• number of electrons
• mass number
• atomic number
• number of neutrons

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• When transmutation occurs, the _____ always
  changes.
• number of electrons
• mass number
• atomic number
• number of neutrons

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• Transmutation occurs by radioactive decay and
  also by
• extreme heating.
• chemical reaction.
• high intensity electrical discharge.
• particle bombardment of the nucleus.

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• Transmutation occurs by radioactive decay and
  also by
• extreme heating.
• chemical reaction.
• high intensity electrical discharge.
• particle bombardment of the nucleus.