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Nuclear Chemistry Chapter 19

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Nuclear Chemistry. Chapter 19. 19.1 - A little review ... Nuclear transmutations can show a, , and ?-emissions as well as production of ... Nuclear power plants ... – PowerPoint PPT presentation

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Title: Nuclear Chemistry Chapter 19


1
Nuclear Chemistry Chapter 19
2
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

3
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

4
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

5
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

6
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

7
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

8
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)

9
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

10
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.

11
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

12
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13
Graph of Stable Isotopes
Decrease Z A
Need to Increase Z
Need to Decrease Z
14
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15
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

16
  • 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

17
  • 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

18
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19
  • 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

20
  • 5) neutrons
  • 1
  • 0 n
  • Very penetrating, easily passing through most
    materials because of their zero charge

21
  • 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

22
Radioactive Decay Series
  • Many heavy elements undergo several sequential
    emissions before forming a more stable nuclei

23
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

24
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

25
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)

26
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27
Review - Parts of an Atom
  • Complete the Table

28
Nuclear equations
  • Must be balanced with respect to nuclear charge
    (atomic number) and nuclear mass (mass number)

29
Practice with decay. Complete the following
nuclear reactions and identify the types of decay.
  • 1327Al 01n ? ___ 00?
  • 1328Al ? -10e ___ 00 ?
  • 1327Al 24He ? ____ 01n

30
90232Th ? ? ____
88228Ra ? ?- ____
89228Ac ? ?- ____
90228Th ? ? ____
31
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

32
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33
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
34
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

35
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

36
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37
19.3 Energy
  • Energy change accompanying a nuclear reaction can
    be calculated from the relation

38
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

39
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)

40
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

41
Binding Energy
Iron-56 is very stable
  • Greater Binding Energy means greater stability

Fission Elements Lots of Energy
Fusion Elements LARGE Energy
42
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43
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.

44
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
45
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46
  • 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.

47
Fusion Reactions
  • Light Nuclides join to make Heavier Isotopes
  • Deuterium (H-2) combines with Tritium (H-3)
  • 12H 13H? 24He 01n

48
Fusion on the Sun
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51
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.
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