Physics 6.5

• Atom


  Contains protons, neutrons and electrons
• Atomic number (Z)


  The number of protons
• Mass number (A)


  The number of protons and neutrons
• Isotopes


  Elements with the same atomic number but different mass numbers
• Isotopes have the same number of protons but different numbers of neutrons
• If the nucleus of an atom is large enough or has an unusual ratio of protons to neutrons, then it becomes unstable
• It's impossible to predict when an unstable atom will decay or emit radiation, and it happens randomly
• Types of radiation


  • Alpha (α)
  • Beta (β)
  • Gamma (γ)
• Alpha particles


  • They are helium nuclei
  • They are the most ionising and therefore cause the most harm inside the body to living cells
  • They are the least penetrating type of radiation, absorbed by a thin sheet of paper or by skin
• Beta particles


  • They are high-energy electrons
  • They have medium ionising ability and cause little harm inside the body to living cells
  • They are absorbed by a few mm of aluminium or perspex plastic
• Gamma rays


  • They are high energy electromagnetic waves/short wavelength, high frequency
  • They are the least ionising, about 50 times lower than alpha particles and therefore cause the least harm to living cells in the body
  • They are the most penetrating, able to travel through several cm of lead
• Alpha decay


  1. Two protons and two neutrons are lost from the nucleus

  2. The proton number drops by 2

  3. A new element is formed that is two places to the left in the periodic table than the original element
• Beta decay


  1. A neutron changes into a proton plus an electron

  2. The proton stays in the nucleus

  3. The electron leaves the atom with high energy as a beta particle

  4. The nucleon number stays the same but the proton number increases by 1
• Sources of background radiation


  • Cosmic rays
  • Rocks and soil (including radon gas)
  • Radioactive materials in living things
  • Medical and dental examinations (x-rays)
  • Nuclear weapons testing
  • Nuclear power stations
• Greater exposure to radiation increases the likelihood of developing cancer, but is only dangerous in large amounts