p4 - atomic structure and radioactivity

• Democritus' idea on atoms:

  smallest unit of matter

  separated from each other by empty space

  small spheres
• John Dalton's idea on atoms:

  atoms are solid spheres

  different type of spheres make up different elements
• Development of nuclear model:


  a series of experiments: alpha particles were fired at a thin sheet of gold foil

  most particles passed through, but some were deflected off course

  hypothesis: dense region of positive charge at centre of atom that repelled alpha particles

  nuclear model - central positive nucleus, surrounded by negative electrons
• issue and solution with nuclear model


  issue: atoms should collapse as negative electrons would be attracted to positive nucleus, causing them to rush inwards

  solution: neil bohr suggested that electrons orbit around the nucleus in shells so atom does not collapse
• alpha radiation


  same as helium nucleus - 2 protons, 2 neutrons

  doesn't have any electrons

  overall charge of +2

  relative large - can't penetrate very far into other materials, travels a few cm in the air, absorbed by a single sheet of paper

  strongly ionising - easily knocks electrons off any atom they collide with
• beta particles

  atom's neutron decays into proton and electron - protons stays in nucleus but electron emitted out at high speed

  quite tiny - moderately ionising

  penetrate moderately far - several meters in air, 5mm of aluminium to stop
• gamma rays


  waves of electromagnetic radiation

  often emitted after alpha and beta radiation

  no mass nor charge - able to pass through materials. rather than collide with atoms, makes them weakly ionising

  penetrate really far - travels long distances, thick sheet of lead or multiple metres of concrete to stop
• emission of a neutron

  if nucleus has too many neutrons - making it unstable - nucleus emits neutron out of nucleus
• activity


  overall rate of decay of all the unstable isotopes in our sample

  measured in Becquerels, Bq

  1 Bq = 1 decay per second
• 2 definitions of half-life


  1. the time taken for the number of radioactive nuclei in a same to halve
  2. the time taken for the number of decays or activity to halve
• geiger-muller tube and counter


  records all the decays that reach them each second