Atomic Structure

• Atom


  Positively charged nucleus (which contains neutrons and protons) surrounded by negatively charged electrons
• Subatomic Particles

  • Proton
  • Neutron
  • Electron

  
• Electron

  Relative Mass: 0 (0.0005), Relative Charge: -1
• Typical radius of an atom: 1 × 10−10 metres
• The radius of the nucleus is 10 000 times smaller than the radius of the atom
• Most (nearly all) the mass of the atom is concentrated at the nucleus
• Electron Arrangement


  Electrons lie at different distances from the nucleus (different energy levels). The electron arrangements may change with the interaction with EM radiation
• Isotopes


  Atoms of the same element, but with different masses, which have the same number of protons but different number of neutrons
• Elements


  All atoms of the same element have the same number of protons
• Neutral Atoms


  Have the same number of electrons and protons
• Atoms and EM Radiation


  1. When electrons change orbit (move closer or further from the nucleus)

  2. When electrons move to a higher orbit (further from the nucleus), the atom has absorbed EM radiation

  3. When the electrons falls to a lower orbit (closer to the nucleus), the atoms has emitted EM radiation

  4. If an electron gains enough energy, it can leave the atom to form a positive ion
• Plum pudding: 1897 JJ Thompson ->The overall charge of an atom is neutral, so the negative electrons were dispersed through the positive "pudding" to cancel out the charges
• Gold Foil Experiment- 1911 Rutherford (most of atom empty space)


  1. Most α particles went straight through, so most of atom is empty space

  2. Some α particles were slightly deflected, so nucleus must be charged, deflecting positive α

  3. Few α particles were deflected by >90°, so nucleus contained most of the mass
• 1913- Rutherford model

  Now there is a positive nucleus at the centre of the atom, and negative electrons existing in a cloud around the nucleus
• The positive charge of the nucleus could be subdivided into smaller particles, each with the same amount of charge – the proton
• 20 years after the 'nucleus' was an accepted scientific idea, James Chadwick provided evidence to prove neutrons existed
• Radioactive Decay


  Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay
• Activity


  The rate at which a source of unstable nuclei decays, measured in Becquerel (Bq)
• Count-rate


  The number of decays recorded by a detector per second, e.g. a Geiger-Muller Tube
• Forms of Decay


  • in order of highly ionising - to least & weakly penetrating to highly
  • Alpha (α, helium nucleus)
  • Beta Minus (β, electron)
  • Gamma (γ, radiation)
  • Neutrons
• Alpha Decay

  Causes both the mass and charge of the nucleus to decrease
• Beta Decay


  Does not cause the mass of the nucleus to change but does cause the charge of the nucleus to increase
• Gamma Decay does not cause the mass or charge to change
• Half-Life


  The time taken for half the nuclei in a sample to decay or the time taken for the activity or count rate of a sample to decay by half
• It cannot be predicted when any one nucleus will decay, but the half-life is a constant that enables the activity of a very large number of nuclei to be predicted during the decay
• A short half-life means the source presents less of a risk, as it does not remain strongly radioactive for long
• subatomic particles:

  
• A long half-life means the source remains weakly radioactive for a long period of time
• Net Decline


  The ratio of the initial number of radioactive nuclei to the number after X half-lives
• Contamination


  The unwanted presence of radioactive atoms on other materials, where the hazard is the decaying of the contaminated atoms releasing radiation
• Irradiation


  Exposing an object to nuclear radiation, but does not make it radioactive
• Scientific reports on the effects of radiation on humans need to be peer reviewed to ensure the measurements are accurate and the safety levels are appropriate