atomic structure (topic 4 )

• what did JJ thompson discover?

  electrons

  • this disproved daltons theory
  • he then created the plum pudding model which consited of spheres of positive charge with negative particles embedded in them
• The risks when using radiation :

  • it can enter living cells and ionise atoms and molecules within them - this can lead to tissue damage
  • lower doses tend to cause minor damage without killing cells, this can cause mutant cells which divide uncontrolably - this is cancer
  • higher doses tend to kill cells completely causing radiation sickness (leading to tiredness, vomiting and hair loss ) if alot of cells get killed at once
• How are gamma sources used in medical tracers?

  • certain radiocative isotopes can be injected into people (or they can just swallow them) and their progress arounf the body can be followed using an external detector
  • isotopes which are taken into the body like this are usually gamma (never alpha) so that the radiation passes out of the body without causing much ioniastion. they should have a short halflife so the radioactivity inside the patient quicly disapears
• Radotherapy:

  • since high doses of radiation will kill all living cells it can be used to treat cancers
  • gamma rays are directed carefully at just the right dosage to kill the cancer cells without damaging too many normal cells, Radiation emmiting implants (usually beta-emmiters ) can be put next to or inside tumors
  • however a fair bit of damage is done to normal cells which makes the patient feel very ill
• What are isotopes ?


  an atom with the same number of protons but a different number of neutrons (the atomic number is the same but the mass number changes )
• all elements have different isotopes but here are usually only one or 2 different stable ones
• unstable isotopes tend to decay into other elements and give out ratiation as they try to become more stable. this is known as radioactive decay
• what are the 3 types of ionising radiation ?

  • alpha
  • beta
  • gamma
• ionisinf radiation is radiaiton that knocks electrons off of atoms creating positive ions, the ionising power of a radion source is how easily it can do this
• alpha radiation

  • when an alpha particle is emmited from the nucleus
  • an alpha paricle is 2 protons and 2 neutrons (a helium nucleus)
  • they do not penetrate very far into materials and are stopped quickly, they can only travel a few cm inn air and are absorbed by a sheet of paper
  • due to their size they are highly ionising
• beta radiation

  • a beta particle is a fast moving electron released by the nucleus
  • they have virtually no mass and a charge of -1
  • moderately ionising
  • penetrate moderatley far into materials before colliding and have a range in air of a few metres, they are absorbed by a sheet of aluminium
  • for every beta particle emmited a neutron in the nucleus has turned into a proton
• gamma radiation

  • gamma rays are waves of electromagnetic radiation released by the nucleus
  • they penetrate far into materials without being stopped and will travel a long distance through the air
  • they are weakly ionising as they tend to pass through rather than collide with atoms
  • they can be absorbed by thick layers of lead or metres of concrete
• what is half life?

  the time taken for the number of radioactive nuclei in an isotope to halve
• how can radiation be measured ?

  Geiger-Muller tube and counter which records the count rate (the number of radiation counts reaching it per second)
• what is nuclear fission ?

  a nuclear reaction that is used to release energy from large and unstable atoms by splitting them into smaller atoms
• what is nuclear fusion ?

  2 light nuclei collide at a high speed and join (fuse) to create a larger heavier nucleus
• nuclear fusion :

  • the heavier nucleus that is created does not have as much mass as the 2 seperate smaller nuclei, some of the mass og the lighter nuclei is converted to energy and released
  • fusion releases alot of energy (more that fission)
  • so far scientists havnt found a way of using fusion to generate energy as fusion reactors are really difficult and expensive to build
• rutherfords alpha scattering experiment lead to the plum pudding model being disproved as it showed that atoms are mostly empty space.

  the new model developed was called the nuclear model and it consisted of a positivley charged nucleus surrounded by a cloud of negative elctrons
• niehls bohr proved that electrons orbit the nucleus at certain distances (called energy levels)
• in 1932 james chadwick proved the existence of the neutron
• what is backround radiation ?

  the low level radiation that is around us all the time
• what does backround radiation come from ?

  • radioactivity of naturally occuring isotopes which are all around us
  • radiation from space (cosmic rays)
  • radiation due to human activity
• what is irradiation?

  (an object) being exposed to radiation
• what is radioactive contamination?

  unwanted radioactive particles getting onto or into an object