topic p4- atomic structure

• developing the model of the atom

  1. tiny sphere that can't be broken
  2. electron discovery
  3. plum pudding model- sphere of positive charge with negative electrons stuck in it
  4. alpha scattering experiment
  5. nuclear model- positively charged nucleus surrounded by a cloud of negative electrons
  6. niel's bohr's- theoretical calculations
  7. bohr model- electrons orbit the nucleus at a fixed position
• radioactive decay

  when an unstable nucleus decays into another element and gives out radiation to become more stable
• isotopes

  atoms with the same number of protons but different number of neutrons
• alpha

  • consists of- 2 neutrons, 2 protons
  • absorbed by- sheet of paper
  • range in air- few centimetres
  • ionising power- strong
  • example of use- smoke detectors
• beta

  • consists of- fast moving electrons from the nucleus
  • absorbed by- sheet of aluminium
  • range in air- few metres
  • ionising paper- moderate
  • example of use- material thickness testing
• gamma

  • consists of- electromagnetic radiation from the nucleus
  • absorbed by- thick sheets of lead
  • range in air- longer distances
  • ionising power- weak
  • example of use- medical tracers
• nuclear equations

  • show radioactive decay
  • mass and atomic numbers have to balance
  • atom before decay -> atom after decay + radiation emitted
• alpha decay decreases the charge and mass of the nucleus

  238 234 4

  U -----> Th + He

  92 90 2
• beta decay increase the charge of the nucleus

  14 14 0

  C --------> N + e

  6 7 -1
• gamma rays don't change the mass of the nucleus or the charge
• half-life

  • the time taken for the number of radioactive nuclei in an isotope to halve
• you can measure half-life using a graph

  
• activity

  the rate at which a source decays measured in becquerels (Bq)
• count rate

  the number of radiation counts reaching a detector per second
• radioactive contamination

  getting unwanted radioactive atoms onto or into an object
• irridation

  the exposure of an object to nuclear radiation
• half-life

  time taken for the number of nuclei of an isotope in a sample to halve
• risks

  • cells can be killed
  • cells can be damaged
  • damaged cells can multiply and become cancer