Nuclear Chemistry

1
Nuclear Chemistry

• Targets
• I CAN Utilize appropriate scientific vocabulary
  to explain scientific concepts.
• I CAN Distinguish between fission and fusion and
  where each process may be used.
• I CAN Construct balanced nuclear equations
  (alpha, beta, gamma, neutron, etc.).
• I CAN Complete a decay series for a given
  element.
• I CAN Perform calculations to determine how much
  of a radioactive substance will remain based on
  the half-life and time elapsed.
• I CAN Perform calculations to determine how much
  time has elapsed based on starting and ending
  amounts in a half-life problem.

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• Changes in matter originating in the nucleus of
  an atom
• All atoms have-
• p and n0 (nucleons)
• p number is the atomic number Z
• p n0 mass number A

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The Nucleus

• The nucleus is comprised of the two nucleons (a
  nuclear particle) protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together is
  the mass of the atom.
• Nuclide, is the nucleus of an atom having a
  specific atomic number and atomic mass

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Isotopes

• Elements with the same atomic , but different
  mass due to gaining or losing of neutrons
• Ex There are three naturally occurring isotopes
  of uranium
• Uranium-234
• Uranium-235
• Uranium-238

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Radioactivity

• It is not uncommon for some nuclides of an
  element to be unstable, or radioactive.
• We refer to these as radionuclides
• Radionuclides are unstable and spontaneously emit
  particles and electromagnetic radiation (EM)
• Radioisotopes- atoms containing these
  radionuclides
• Transmutation- process by which one element is
  converted to another element by the spontaneous
  emission of radiation

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Large Nuclei

• Nucleus is held together by strong force
• Large nuclei tend to be unstable because the
  force is not strong enough to hold it together
• These nuclei break apart and decay
• All nuclei with 83 or more protons are
  radioactive
• Almost all elements with more than 92 protons
  dont exist naturally

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Neutron-Proton Ratios

• Any element with more than one proton (i.e.,
  anything but hydrogen) will have repulsions
  between the protons in the nucleus.
• A strong nuclear force helps keep the nucleus
  together
• Neutrons play a key role stabilizing the nucleus.
• Therefore, the ratio of neutrons to protons is an
  important factor.

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Neutron-Proton Ratios

• For smaller nuclei (Z ? 20) stable nuclei have a
  neutron-to-proton ratio close to 11.

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Neutron-Proton Ratios

• As nuclei get larger, it takes a greater number
  of neutrons to stabilize the nucleus.

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Stable Nuclei

• Nuclei above this belt have too many neutrons.
• They tend to decay by emitting beta particles.

11
Stable Nuclei

• Nuclei below the belt have too many protons.
• They tend to become more stable by positron
  emission or electron capture.

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What is Radioactivity?

• When an unstable nucleus emits one or more
  particles or energy
• When these particles are emitted, the element
  changes to another isotope or to a different
  element
• Nuclear radiation refers to radiation resulting
  from nuclear changes

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Types of Nuclear Radiation

• Alpha particles
• Beta particles
• Gamma rays

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Alpha Particles

• Symbol is
• Actually particles made of 2 p and 2 n0, same
  as a He nucleus
• 2 charge, most massive of all nuclear radiation,
  mass 4 amu
• Do not travel far, can be stopped by a sheet of
  paper
• Can be very dangerous inside human body illness
  and disease

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Alpha Particle Emission
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Beta Particles

• Symbol is Greek letter, beta (ß)
• High speed negatively charged particles that come
  from the nucleus hmmm
• Neutron (neutral) actually decays to form a p
  and an e-, e- is ejected from nucleus
• Travel farther than a, but can be stopped by
  3 mm of Al, or 10 mm of wood
• Can cause damage inside cells

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Beta Decay


mk M
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Gamma rays

• Gamma Radiation, ?
• Not made of matter, no charge
• Waves of electromagnetic energy (photons)
• Usually emitted from nucleus when alpha or beta
  decay occurs
• A nucleus de-excites by emitting a high energy
  gamma ray photon
• High energy, can be stopped by 60 cm of Al or 7
  cm of Pb
• Cause less damage than alpha because it has no
  charge

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Gamma Radiation
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Damage Caused
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Penetrating Power
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Nuclear Decay

• When unstable nuclei emit alpha or beta
  particles, what changes?
• Number of protons or neutrons
• New element or new atomic mass

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Change During Alpha Decay

• Nucleus gives up 2 p and 2 n0
• All mass numbers add up, all atomic numbers add
  up

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Change During Beta Decay

• Neutron is changed to a proton and an electron
  and emitted
• Increases atomic number due to conversion of a
  neutron to a proton and an electron
• So neutron is lost and proton is gained

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Gamma Emission

• Loss of a ?-ray (high-energy radiation that
  almost always accompanies the loss of a nuclear
  particle)
• 238 U ? 238Th 4He 0? (gamma
  ray)
• 92 90 2
  

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Classwork/Homework

• WS with example problems

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Decay Rates

• Half-life the time in which half a radioactive
  substance decays
• After 1 half-life, half of the substance is
  unchanged
• After 2 half-lives, ¼ of the substance is
  unchanged
• Half-life table on p. 226

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Half-Life

• Measure how quickly a substance decays
• Can be anywhere between nanoseconds to billions
  of years, depending on nuclear stability
• C-14 is used to find the age of relatively recent
  materials
• C-14 is taken in in tiny fractions while alive in
  some molecules of CO2
• C-14 goes through beta decay, so ratio is
  compared in living and nonliving things

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Half-Life Example

• The half-life of iodine-131 is 8 days.
• If you start with 36 grams of I-131, how much
  will be left after 24 days?
• 36 g ? 1 half-life ? 8 days ? 18 g
• 18 g ? 2 half-lives ? 16 days ? 9 g
• 9 g ? 3 half-lives ? 24 days ? 4.5 g

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Classwork/Homework

• WS Nuclear Decay
• Half-Life problems on the back