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Nuclear Physics. Chp 30. The Atom. A nucleus of equal mass positive protons and neutral neutrons, surrounded by almost massless, negative electrons Atomic number = # protons Atomic mass = # protons + neutrons Most atoms are neutral, so electrons = protons

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Presentation Transcript
the atom
The Atom
  • A nucleus of equal mass positive protons and neutral neutrons, surrounded by almost massless, negative electrons
    • Atomic number = # protons
    • Atomic mass = # protons + neutrons
    • Most atoms are neutral, so electrons = protons
    • The nucleus, although containing the most mass, takes up very little space in the atom
isotopes
Isotopes
  • Different versions of an element
    • Only the atomic mass and # neutrons change (changing protons would change the element)
    • Atomic mass as given on the periodic table is an average of all possible isotopes (this is why it’s a decimal)
    • Use AZX to show isotopes
      • A = atomic mass
      • Z = atomic number
      • X = element symbol
strong nuclear force
Strong Nuclear Force
  • The force that holds an atomic nucleus together
    • Must be very strong to hold like charges together (they normally repel each other)
      • Even stronger than electricity
    • Only works over a very short range though
    • Energy must be added to take a nucleus apart (need to overcome that force)
e mc 2
E = mc2
  • Mass is a form of energy
    • That means if mass changes, energy is released or absorbed
    • For atoms smaller than iron, they have less mass when they combine than when separate (fusion)
    • For atoms larger than iron, they have less mass when they separate than when they are held together (fission)
    • Iron is stable and undergoes neither fission or fusion
radioactive decay
Radioactive Decay
  • Alpha
    • A particle of 2 protons and 2 neutrons are emitted
    • Most massive, but easiest to stop
  • Beta
    • A neutron turns into a proton and an electron, the electron is emitted and the proton stays
    • Fairly easy to stop because its charged
  • Gamma
    • Massless energy is released
    • Hardest to stop and most dangerous
  • Substances often undergo the release of many of these particles in stages until a stable isotope is reached
half life
Half Life
  • The time required for half the atoms in a radioactive sample to decay
  • The time it takes is unique and constant for each isotope
  • If an isotope has a short half life, it decays more quickly, and therefore is more dangerous
  • Used to “date” objects
    • Carbon – 14 has a half life of 5730 yrs (good for living things)
    • Uranium – 238 has a half life of 4.5 billion years (good for planets)
nuclear reactors
Nuclear Reactors
  • Currently all based on fission of uranium - 235
  • Needs a neutron to start the process, then 3 new neutrons are created
    • Each of these can start a new reaction (chain reaction)
  • Reactions are kept in check by mixing in U-238, which doesn’t react and having control rods, which can drop down and absorb neutrons to stop the reaction
reactors cont d
Reactors, Cont’d
  • The heat created by U-235 fusion, heats water which creates steam to turn turbines and generate electrical energy
  • Creates tons more energy than coal burning, but does leave radioactive byproducts that must be properly disposed of
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