Nuclear Chemistry Chapter 19

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Nuclear Chemistry Chapter 19
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19.1 - A little review

• Elements are made up of only one kind of atom
• These atoms are made up of three subatomic
  particles
• Proton
• Neutron
• Electron
• 12 A
• 6 C Z C A number of protons (atomic
  number)
• Z mass number

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Review

• Isotopes same number of protons, but different
  number of neutrons
• Different mass number than what is on the
  periodic table
• Isotopic notation C-14 means carbon with a
  mass of 14 amu

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Antoine Becquerel - 1896

• Discovered radioactivity
• The spontaneous decay of an unstable nucleus with
  accompanying emission of radiation.
• Unstable isotopes decompose
• A radioactive nucleus spontaneously decomposes
  (decays) with the evolution of energy
• Hypothesis was that a fluorescent compound, which
  glows when sunlight strikes it, also emits X-rays
• Reasoned that the sun had nothing to do with the
  emittance of X rays

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Ernest Rutherford - 1919

• Did was alchemists had been trying to do for
  centuries
• He artificially converted one element into
  another
• He converted nitrogen into oxygen
• This process, called transmutation, means
  changing one element into another
• The atomic number of the product nucleus differs
  from that of the reactant
• Demonstrated that the nucleus of an tom could be
  experimentally manipulated

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Curies (Marie, Pierre, and their daughter Irene)

• Discovered radium, polonium, and the positron
• First radioactive isotopes to be made in the
  laboratory were prepared in 1934 by Irene and her
  husband Frederic
• Achieved this by bombarding certain stable
  isotopes with high-energy alpha particles
• A stable nucleus is converted to one that is
  radioactive which in turn decays to stable
  products

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Radioactivity

• Property of matter whereby an unstable nucleus
  spontaneously emits small particles and/or energy
  in order to attain a more stable nuclear state
• The process is called radioactive decay
• An isotope that contains an unstable nucleus is
  called a radioactive isotope or radioisotope

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Nuclide protons neutrons mass number
nucleons

• nuclide - atom with a specific number of protons
  and neutrons in its nucleus.
• There are 264 stable nuclides in nature, others
  are radioactive
• radionuclide - unstable isotope that undergoes
  nuclear decay.
• All isotopes of elements with 84 protons are
  radioactive specific isotopes of lighter
  elements are also radioactive. ( E.g. 1H)

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Nuclear reactions differ from ordinary chemical
reactions

• Atomic numbers of nuclei may change (elements may
  transmute to other elements)
• Protons and neutrons react inside of the nucleus
• Matter is converted to energy and huge amounts of
  energy are released
• Reactions involve a specific isotope of an
  element isotopes of an element react differently

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Radiation

• Radiation is energy in motion. Not only does
  radiation come from elements in the form of
  radioactivity, some come from our natural
  environment, others from human activities and
  devices.

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Nuclear Stability

• As the atomic number increases, more neutrons
  are needed to help bind the nucleus together, so
  there is a high neutron proton ratio.
• Nuclei of heavy elements are unstable due to
  the large of nucleons present by undergoing
  radioactive decay unstable nuclei can form more
  stable nuclei.
• Nuclei with both even numbers of both protons
  neutrons are more stable than those with odd
  numbers

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Graph of Stable Isotopes
Decrease Z A
Need to Increase Z
Need to Decrease Z
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Types of Radioactive Decay

• Alpha particle, a, emission
• 4
• 2 He
• a particles - high energy and low speed charged
  particles
• alpha particles lose energy quickly. A hand or
  thin piece of paper stops it. Quickly become
  ordinary helium.
• Ordinary helium nucleus is given off
• The atomic number decreases by two units the
  mass number decreases by four units

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• 2) beta, ß, emission
• 0
• -1 e
• ß particles high energy and high speed
  charged electrons
• Beta particles are high speed electrons that
  travel close to the speed of light and can
  penetrate a hand but not concrete.
• Produces an electron
• The product nucleus has the same mass number as
  the reactant, but its atomic number is one unit
  larger

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• 3) gamma, ?, emission gamma emission accompanies
  other types of decay
• ? particles - high energy photons, very
  penetrating
• Gamma rays come from the nucleus of the atom of a
  radioactive isotope. They are the most energetic
  and most penetrating of all radiation.
• Gamma emission changes neither the mass number
  nor the atomic number, it is ordinarily omitted
  from the nuclear equation

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• 4) positron, 01e, emission - same mass, but
  opposite charge of electron
• identical to an electron except that it has a
  charge of 1 rather than -1

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• 5) neutrons
• 1
• 0 n
• Very penetrating, easily passing through most
  materials because of their zero charge

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• 6) Electron capture - ß particle is captured
  instead of emitted
• also known as K-electron capture
• An electron in the innermost energy level (n1)
  falls into the nucleus
• Same as positron emission mass number remains
  unchanged whereas the atomic number decreases by
  one unit
• More common with heavy nuclei, presumably because
  the n1 level is closer to the nucleus

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Radioactive Decay Series

• Many heavy elements undergo several sequential
  emissions before forming a more stable nuclei

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NUCLEAR BOMBARDMENT REACTIONS

• Transmutation - Change of one element to another
  as a result of bombardment by high-energy
  particles (e.g. neutrons, electrons, and other
  nuclei).
• Rutherford prepared 1st synthetic nuclide, 17O,
  in 1919 Irene Curie prepared 1st radioactive
  nuclide, 30P, in 1934.
• All trans-Uranium elements (Z gt 92) are both
  synthetic (man-made) and radioactive.
• Nuclear transmutations can show a-, ß-, and?
  ?-emissions as well as production of protons,
  neutrons, and other isotopes

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RATE OF RADIOACTIVE DECAY

• Different isotopes decay at different rates
  rates vary from ms to days to years.
• Radioactive decay is a first order rate
  process all radioactive substances have a
  characteristic half-life

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APPLICATIONS OF RADIOACTIVE ISOTOPES

• Nuclear power plants
• Medical diagnosis and treatment e.g. PET scan
  monitors glucose metabolism in brain using C-11
  isotope I-131 measures activity of thyroid
• Carbon dating (measure amount of C-14 remaining
  in a sample)
• Synthesis of new elements
• Irradiation of food - preserves food destroys
  parasites
• Nuclear Weapons (Atomic bombs and H bombs)

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Review - Parts of an Atom

• Complete the Table

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Nuclear equations

• Must be balanced with respect to nuclear charge
  (atomic number) and nuclear mass (mass number)

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Practice with decay. Complete the following
nuclear reactions and identify the types of decay.

• 1327Al 01n ? ___ 00?
• 1328Al ? -10e ___ 00 ?
• 1327Al 24He ? ____ 01n

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90232Th ? ? ____
88228Ra ? ?- ____
89228Ac ? ?- ____
90228Th ? ? ____
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Applications

• Large number of radioactive nuclei have been used
  both in industry and in many areas of basic and
  applied research
• Medicine
• Chemistry
• Commercial applications

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19.2 - Rate of radioactive decay

• Radioactive decay is a first-order process (see
  chapter 11 for more information)
• A reaction whose rate depends upon reactant
  concentration raised to the first power
• Following equations apply
• Rate kX
• X0
• ln --- kt
• X
• K 0.693 / t1/2

K first-order rate constant T1/2 half-life X
amount of radioactive species present at time
t X0 amount of radioactive species present at
time t 0
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Other ways to write the equation

• Because of the way in which rate of decay is
  measured, it is often described by the activity
  (A) of the sample, which expresses the number of
  atoms decaying in unit time
• A kN
• A activity
• Can be expressed in terms of the number of atoms
  decaying per second, or becquerels (Bq) 1 Bq 1
  atom/s
• May be cited in disintegrations per minute, or
  curies (Ci) 1 Ci 3.700 x 1010 atom/s
• K first-order rate constant
• N number of radioactive nuclei present

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One last equation

• The activity of a sample is directly proportional
  to the amount of C-14 so we can write an
  equation as
• A0
• ln --- kt
• A
• A0 original activity
• A the measured activity today
• T age of the sample

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19.3 Energy

• Energy change accompanying a nuclear reaction can
  be calculated from the relation

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Finding Energy

• Einsteins - ?E ?mc2
• ?E Energy from matter
• Energy products energy reactants
• ?m mass in kilograms
• Change in mass mass products mass reactants
• Compare mass of each nucleon with total mass of
  nucleus
• c speed of light 3 x 108 m/s

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What does this mean?

• In a spontaneous nuclear reaction, the products
  weigh less than the reactants so m is negative
• If E is negative, then the energy of the products
  is less than that of the reactants and energy is
  evolved to the surroundings
• In an ordinary chemical reaction, m is
  immeasurably small, with a nuclear reaction the
  change in m can be calculated from a table of
  nuclear masses (table 19.3 on page 558)

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Nuclear binding energy

• It is always true that a nucleus weighs less than
  the individual protons and neutrons of which it
  is composed
• Binding energy is a measure of an atoms
  stability
• Greater Binding Energy means greater stability,
  the more difficult it would be to decompose the
  nucleus into protons and neutrons

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Binding Energy
Iron-56 is very stable

• Greater Binding Energy means greater stability

Fission Elements Lots of Energy
Fusion Elements LARGE Energy
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19.4 and 19.5 - NUCLEAR FISSION AND FUSION

• Fission - A nuclear reaction that releases energy
  as a result of splitting of large nuclei into
  smaller ones.
• Nuclear Power plants use fission to split U-235
  to produce energy
• U-235 is bombarded with slow neutrons - this
  produces smaller nuclei as well as more neutrons
  and energy.

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Fission Reactions

• Bombardment by slow neutrons results in splitting
  the nucleus into smaller nuclides and more
  neutrons.

92235U 01n
92236U
56141Ba 3692Kr 3 01n
and ENERGY
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• Fusion - A nuclear reaction that releases energy
  as a result of the union of smaller nuclei to
  form larger ones.
• Fusion generates even more energy than fission
  and creates little radioactive waste, so it would
  provide a wonderful source of energy.
• but, fusion requires very high temps (tens of
  millions of degrees Celsius) in order for nuclei
  to overcome strong repulsive forces magnetic
  fusion reactors are being designed and tested.

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Fusion Reactions

• Light Nuclides join to make Heavier Isotopes
• Deuterium (H-2) combines with Tritium (H-3)

• 12H 13H? 24He 01n

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Fusion on the Sun
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Primary problems with nuclear power plants

• 1) safety (Chernobyl and Three Mile Island had
  cooling system failures that led to reactor
  meltdowns. Chernobyl also did not have
  containment building around reactor.)
• 2) nuclear waste - some products will remain
  radioactive for thousands of years.