atomic structure & radioactivity

• radius of an atom

  1 x 10 (^-10) metres
• energy levels which are further from the nucleus are at a higher energy than those which are closer to the nucleus
• isotopes are atoms of the same elements with different number of neutrons
• 1897: scientists discovered that atoms contain tiny negative particles called electrons - this showed that atoms are not spheres that can be divided
• radioactive decay - the process of isotopes with unstable nucleus’ giving out radiation to become stable
• radioactive decay is a random process
• activity - rate at which source of unstable nuclei decays
• activity is measured in becquerels (1 becquerel = 1 decay per second)
• if an atom absorbs electromagnetic radiation an electron can move from a lower energy level to a higher energy level so the atom can now emit electromagnetic radiation and the electron returns back to the lower energy levels
• plum pudding model - an atomic ball of positive charge with negative electrons embedded in it
• to measure the activity of a radioactive source we can use a Geiger muller tube
• count rate - number of decays recorded each second by a detector
• niels bohr

  • electrons orbit the nucleus at specific distances
  • bohrs work agreed with the results of experiments by other scientists
  • we now call the orbits energy levels/shells
• alpha scattering experiment results

  1. most of the alpha particles went straight through the foil - therefore atoms are mainly empty space
  2. some alpha particles were deflected - therefore the centre of an atom must have a positve charge that repelled the alpha particles
  3. some alpha particles bounced straight back - therefore the mass of the atom must be concentrated in the centre (nucleus)
• james chadwick discovere that the nucleus contains neutrons which have no charge
• precautions with radiation

  • shielding

  gloves can stop alpha, lead aprons can stop gamma and beta radiation

  • monitoring
• alpha particles

  • same as a helium nucleus
  • 2 protons and 2 neutrons
• beta particles

  • when a neutron changes into a proton and ekectron
  • this electron is ejected from the nucleus where it is formed at very high speed
• gamma radiation

  • type of electromagnetic radiation from the nucleus
• range in air

  • alpha particles are very large so can travel around 5 cm in air before they collide with air particles and stop
  • beta particles can travel further, they reach around 15 cm before stopping
  • gamma radiation travels several metres in air before stopping
• penetrating power

  • alpha particles are stopped by a single sheet of paper
  • beta particles are stopped by a few mm of aluminium
  • gamma radiation can be stopped by several cms of lead
• ionising power - when radiation collided with atoms causing atoms to lose electrons and form ions

  • alpha particles are very strongly ionising
  • beta particles are quite strongly ionising
  • gamma radiation is weakly ionising
• alpha decay

  • atomic number decreases by 2
  • mass number decreases by 4
• beta particle

  • atomic number increases decreases by 2
  • mass number stays the same
• ionising radiation can increase the risk for cancer in humans
• half life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample in halve
• half life is also the time it takes for the count-rate from a sample contains the isotope half its initial level
• irradiation - exposing an object to nuclear radiation (alpha, beta, gamma, or neutrons)
• in irradiation the object does not become reactive after being irradiated (comes in contact with radiation not radioactive isotope)
• radioactive contamination - when unwanted radioactive isotopes end up another other materials, this is hazardous as the radioactive atoms decay and emit ionising radiation