Radioactivity

• Subatomic constituents of an atom

  • Proton
  • Neutron
  • Electron
• Arrangement of particles in an atom

  • Protons and neutrons are found in the atom's nucleus
  • Electrons are found in discrete energy levels around the nucleus
• Charge of the nucleus

  • Positive charge
  • The nucleus contains protons and neutrons
  • Protons have a positive charge
  • Neutrons have no charge
• Ways an atom's electron arrangement can be changed

  • Absorbing electromagnetic radiation
  • Emitting electromagnetic radiation
• Absorbing EM radiation

  1. Electrons move to higher energy levels

  2. They move away from the nucleus
• Emitting EM radiation

  1. Electrons move to a lower energy level

  2. They move towards the nucleus
• Reason atoms have no overall charge

  • The number of protons is equal to the number of electrons
  • Protons and electrons have equal and opposite charges, so charge cancels
• All forms of the same element have the same number of protons
• Atomic number

  The number of protons in an atom
• Mass number

  The total number of protons and neutrons in the atom
• Isotope

  An atom of an element that has a different number of neutrons, but the same number of protons
• The property that differs between isotopes is the mass of the atom
• Atoms turning into positive ions

  1. They lose one or more of their outer electrons

  2. Electrons are negatively charged, so the resultant charge of the atom is positive
• Relative masses of subatomic particles

  • 1 - proton
  • 1 - neutron
  • 1/1850 - electron
• Relative charge of a proton

  + 1
• Relative charge of an electron

  • 1
• Types of nuclear radiation


  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• Background radiation


  • Radiation that is always present
  • It is in very small amounts and so not harmful
• Sources of background radiation

  • Rocks
  • Cosmic rays from space
  • Nuclear weapon testing
  • Nuclear accidents
• Measuring and detecting background radiation

  1. Photographic film

  2. Geiger-Muller counter
• Photographic film used to measure radiation


  A photographic film turns dark when it absorbs radiation. This is useful for people who work on radiation as the more radiation they are exposed to, the darker the film becomes. Therefore the workers know when they have been exposed to too much radiation.
• Geiger-Muller tubes used to measure radiation

  When the Geiger-Muller tube absorbs radiation it produces a pulse, which a machine uses to count the amount of radiation. The frequency of the pulse depends on how much radiation is present. A high frequency would mean the tube is absorbing a large amount of radiation.
• Alpha particle

  • Two protons and two neutrons
  • It is the same as a helium nucleus
• What blocks beta radiation

  • A thin sheet of aluminium
  • Several metres of air
• What blocks gamma radiation


  • Several centimetres of lead
  • A few metres of concrete
• Most ionising radiation

  Alpha radiation
• Least ionising radiation

  Gamma radiation
• Effect of gamma emission on mass/charge of atom

  Both mass and charge remain unchanged
• Plum-pudding model of the atom

  A sphere of positive charge, with the negatively charged electrons distributed evenly throughout it
• Prior to the discovery of the electron, the atom was believed to be indivisible
• Experiment that led to the plum-pudding model being discarded

  Rutherford's alpha-Scattering experiment
• Currently accepted model of the atom

  The Bohr model
• Rutherford's experiment

  1. Alpha particles (charge +2) were fired at a thin sheet of gold foil

  2. Most particles went straight through

  3. Some particles were deflected by small angles (< 90º)

  4. A few particles were deflected by large angles (> 90º)
• Conclusions of Rutherford's experiment

  • Most of an atom is empty space
  • The nucleus has a positive charge
  • Most of the mass is concentrated in the nucleus
• Beta plus decay

  A proton turns into a neutron and a positron (in order to conserve charge)
• Beta minus decay

  A neutron changes into a proton and an electron
• Effect of alpha decay on atomic number and mass number


  • The atomic number decreases by 2
  • The mass number decreases by 4
  • A new element is made since the atomic number has changed
• Effect of beta minus decay on mass number and atomic number


  • The mass number stays the same as the total number of neutrons and protons hasn't changed (one has just turned in the other)
  • The atomic number increases since there is one more proton