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  The nucleus, although containing the most mass, takes up very little space in the atom

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 A Z X to show isotopes  A = atomic mass   Z = atomic number X = element symbol

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

 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

    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

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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 planets) – 238 has a half life of 4.5 billion years (good for

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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

 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