Nuclear-Chemistry by liwenting

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									Nuclear Chemistry
          10.1 Radioactivity
• Radioactivity: process in which an
  atomic nucleus emits charged
  particles and energy
• Radioisotope: any atom containing an
  unstable nucleus
During nuclear decay, atoms of one element
can change into atoms of a different element
altogether
Uranium – 238 decays into Thorium – 234
(also a radioisotope)
• Nuclear Radiation: charged particles
  and energy that are emitted from the
  nuclei of radioisotopes
• Common nuclear radiation types
  – alpha particle
  – beta particle
  – gamma rays
            Alpha Decay
• Alpha Particle (): a positively
  charged particle made up of 2
  protons and 2 neutrons (the SAME
  as a HELIUM NUCLEUS)
 Common symbol = 
Example of alpha decay of uranium –
 238
   Dangers of Nuclear Radiation

• Least penetrating type of nuclear
  radiation, can be stopped by sheet of
  paper or clothing
               Beta Decay

• Beta Particle (): an electron
  emitted by an unstable nucleus
  – Written as: 
• Assigned atomic # of -1 mass of 0
  (zero)
• How can a nucleus (which is positive),
  emit a negatively charged particle?
During beta decay, a neutron
decomposes into a proton and an e-
• Proton stays trapped in the nucleus,
  e- released
Example of beta decay of thorium –234
• Product isotope has 1 proton more
  and 1 neutron fewer than the
  reactant isotope




• Mass number of the isotopes are
  equal because the emitted beta
  particle has essentially NO MASS
• Beta particles pass through
  paper, but stopped by thin sheet
  of metal
            Gamma Decay

• Gamma Ray (): a penetrating ray of
  energy emitted by an unstable nucleus
           Gamma Decay

• NO mass and NO charge
• During Gamma Decay:
• Atomic number and mass number of
  the atom remains the same
• Energy of nucleus decreases
• Gamma decay often accompanied by
  alpha or beta decay
• Example of thorium – 234 emitting
  both beta particles and gamma rays
  as it decays:
• Gamma rays much more
  penetrating – takes several
  centimeters of lead or several
  meters of concrete to stop
  gamma radiation
     Effects of Nuclear Radiation

• Background Radiation: nuclear
  radiation that occurs naturally in the
  environment
• When nuclear radiation exceeds
  background levels, it can damage the
  cells and tissues of your body
     Effects of Nuclear Radiation

• Nuclear radiation can ionize atoms
  (when cells are exposed to nuclear
  radiation, the bonds holding together
  proteins and DNA molecules may
  break cells may no longer function
  properly)
     Effects of Nuclear Radiation

• , , and  are all forms of ionizing
  radiation
• The extent of the damage of
  external nuclear radiation is
  dependent on the penetrating power
  of the radiation…
• Beta particles cause more damage
  than alpha particles, but less than
  gamma rays
• Gamma rays can penetrate deeply into
  the human body, potentially exposing
  all organs to ionization damage
     Detecting Nuclear Radiation
• Although you can’t see, hear, or feel
  the radioactivity
  around you, scientific instruments can
  measure nuclear radiation
• Geiger Counters
• Film Badges
10.2  Rates of Nuclear Decay
            Nuclear Decay
• By studying the radioactive nuclei of an
  object we can determine how old the object
  is.
• Because most materials contain at least
  trace amounts of radioisotopes, scientists
  can estimate how old they are based on
  rates of nuclear decay.
                  Half-Life
• Half-life: the time required for one half of a
  sample of a radioisotope to decay
  – After one half-life, half of the atoms in a
    radioactive sample have decayed, while the
    other half remain unchanged
  – After two half-lives, half of the remaining have
    decays, leaving one quarter of the original
    sample unchanged
          Half-Life Example
• Iodine Half-life= 8.07 days
  – After one half-life (8.07 days) half of the
    original sample remains
  – After 2 half-lives (16.14 days) one quarter of
    the original remains
  – After 3 half-lives (24.21 days) one half of one
    quarter remains, or 1/8 (one eighth)
  – …and so on
           Half-Lives Vary
• Half-lives can vary from fractions of a
  second to billions of years
• Unlike chemical reaction rates, which vary
  with the conditions of a reaction, nuclear
  decay rates are constant!!!
          Radioactive Dating
• Method used for determining the age of
  objects using the half-lives of Carbon – 14
• Radiocarbon dating: determining the age
  of an object by comparing its carbon-14
  levels with carbon-14 levels in the
  atmosphere.
         Radioactive Dating
• Carbon-14 has a half-life of 5,730 years.
• Carbon-14 is formed in the upper
  atmosphere when neutrons produced by
  cosmic rays collide with nitrogen-14 atoms.
• The radioactive carbon-14 undergoes beta
  decay to form nitrogen-14.
      Using Carbon-14 to Date
• Living organisms absorb the carbon (CO2)
  from the atmosphere, but when they die
  they stop absorbing it and the levels do not
  change.
• From this point levels start to decrease as
  the radioactive carbon decays.
• The levels in the object are then compared
  with levels in the atmosphere.
Example: if an object has half the amount of carbon-14 in it
as in the atmosphere, then we know the object is about 5,
730 years old (which is one half-life for carbon-14)
            Carbon-14 Dating
• Carbon-14 or radiocarbon dating can be
  used to date any carbon-containing object
  less than 50,000 years old.
  – After this point, there is too little carbon-14 left
    to be measurable
• Objects older than this use radioisotopes
  with longer half-lives
• The older the object the lower the levels of
  radioisotopes present
10.3 Artificial Transmutation
             Transmutation
• Transmutation: the conversion of atoms of
  one element to atoms of another.
• It involves a nuclear change, not a chemical
  change.
• Transmutations can either occur naturally
  (nuclear decay) or artificially.
• Scientists can perform artificial
  transmutations by bombarding atomic
  nuclei with high-energy particles such as
  protons, neutrons, or alpha particles.
        Transuranium Elements
• Transuranium Elements: Elements with
  atomic numbers greater than -92 (uranium)
  – All transuranium elements are radioactive and
    generally not found in nature
• Scientists can create a transuranium element by
  the artificial transmutation of a lighter
  element
• Useful transuranium elements
  – Americium-241: used in smoke detectors
  – Plutonium-238: energy source for space probes
        Particle Accelerators
• Sometimes transmutations will not occur
  unless the bombarding particles are moving
  at extremely high speeds.
• To achieve these high speeds scientists use
  particle accelerators.
         Particle Accelerator
• These accelerators move charged particles
  at speeds very close to the speed of light
• The particles are then guided toward a
  target, where they collide with atomic
  nuclei and transmutations are allowed to
  occur
• These collisions have also lead to the
  discovery of more subatomic particles
  – Quarks: protons and neutrons are made up of
    these even smaller particles
Large Hadron Collider (LHC)
10.4  Fission & Fusion
                  Question

• What holds the nucleus together?
• It’s full of positive particles, so why don’t
  they push each other away?
• What keeps the protons and neutrons
  together?
• Clearly, there must be an attractive force
  that binds the particles
                   Answer
• Strong Nuclear Force: the attractive force
  that binds protons and neutrons together in
  the nucleus
  – Over very short distances, the strong nuclear
    force is much greater than the electric forces
    among protons
Forces in the Atom
             Electric Force
• Question: What determines the strength of
  the electric force?
• Answer: The number of protons
              Electric Force
• The greater the number of protons, the
  greater is the electric force that repels the
  protons
• Larger nuclei have a stronger repulsive
  force than a smaller nuclei
• As a result, the nucleus will become
  unstable (or radioactive) when the strong
  nuclear forces can’t overcome the repulsive
  electric forces among protons.
  Nucleus Size & Radioactivity
• Because of the size issue, there is a point
  beyond which all elements are radioactive.
• Once they become large enough, the
  repulsive forces overcome. This occurs
  with all nuclei with 83 or more protons.
• Therefore, all elements with an atomic
  number greater than 83 are radioactive!
                 Fission
• FISSION: the splitting of an atomic
  nucleus into two smaller parts
• In nuclear fission, tremendous amounts of
  energy can be produced from very small
  amounts of mass
   Converting Mass into Energy
• During a fission reaction, some of the mass
  of the reactants is lost!
• The Law of Conservation of Mass says this
  is illegal, highly illegal!
• This “lost” mass is converted into energy!
  Converting Mass into Energy
• Since we bent the law a little…we use a
  revised version of the law: Law of
  Conservation of Mass and Energy
  – It basically says: The total amount of mass and
    energy remains constant!!!
                   E=     mc 2

• 30 years before the discovery of fission,
  Albert Einstein introduced the mass-energy
  equation.
• E=mc2describes the relationship between
  mass and energy:
  – E = energy
  – m = mass
  – c = the speed of light (3.0 x 108m/s)
• It shows that the conversion of a small
  amount of mass releases an ENORMOUS
  amount of energy.
            Lots of Energy!!!
• Example: the explosion of the first atomic
  bomb contained 5kg of plutonium, but
  created an explosion equivalent to18, 600
  tons of TNT!!!
• So, since we bent the law a little, just a
  little, we use a revised version of the law:
  – This law is referred to as the Law of
    Conservation of mass and energy.
• It basically says:
  – THE TOTAL AMOUNT OF MASS AND
    ENERGY REMAINS CONSTANT!
             Chain Reaction
• Nuclear fission reactions act like rumors
  being spread throughout school:
• One person tells a few friends, they tell a
  few friends, and on and on…
• During a fission reaction each reactant
  nucleus splits into 2 smaller nuclei and
  releases 2-3 neutrons.
• If one of these neutrons is absorbed by
  another nucleus, fission can result again,
  releasing more neutrons.
   Triggering a Chain Reaction
• CHAIN REACTION: neutrons released
  during the splitting of an initial nucleus
  trigger a series of nuclear fissions.
• Uncontrolled chain reactions occur when
  each released neutron is free to cause other
  fissions
Chain Reaction
             Chain Reaction
• Nuclear weapons are designed to produce
  uncontrolled chain reactions
• In order for a chain reaction to keep going,
  the nucleus that splits needs to produce one
  neutron that causes the fission of another
  nucleus
  – The material reacting uncontrolled needs to
    have a critical mass.
  – CRITICAL MASS: the smallest possible mass
    of a fissionable material that can sustain a chain
    reaction.
                   Fusion
• Another type of nuclear reaction can release
  huge amounts of energy is fusion:
• FUSION: a process in which the nuclei of
  two atoms combine to form a larger
  nucleus.
• Just like fission, a small fraction of the mass
  is converted into energy
          Example of Fusion
• The sun and stars are powered by the fusion
  of hydrogen into helium
  – Fusion requires extremely high temperatures
    where matter exists as plasma.
• This is a problem for scientists wanting to
  use fusion for an energy source
  – They cannot get high enough temperatures and
    have trouble containing plasma here on Earth
Fusion in the Core

								
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