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 AZX 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 = 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 Alpha Beta Gamma 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 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 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