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Nuclear Physics

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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Nuclear Physics

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  1. Nuclear Physics Chp 30

  2. 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

  3. 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

  4. 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)

  5. 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

  6. 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

  7. 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)

  8. 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

  9. 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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