Topic 6 - Radioactivity

• Atom

  A positively charged nucleus made of positive protons and neutral neutrons, surrounded by negatively charged electrons that orbit the nucleus at different fixed distances
• The nuclear radius is a lot smaller than the radius of the atom
• Almost all the mass of the atoms lies in the nucleus
• Subatomic Particle

  • Proton
  • Neutron
  • Electron
  • Positron
• Size of atom: ~0.1 nanometers, 10^-10
• Isotopes and Elements

  Atoms of the same element have the same number of protons. Neutral atoms have the same number of electrons and protons. Isotopes are atoms of the same element but with different masses, having the same number of protons but different number of neutrons
• Isotopes

  • Carbon-12
  • Carbon-13
  • Carbon-14
• Atoms and EM Radiation


  When electrons change orbit, the atom can absorb or emit EM radiation. If an electron gains enough energy, it can leave the atom to form an ion
• Decay occurs in a random process
• Forms of decay

  • Alpha (a helium nucleus)
  • Beta Minus (electron)
  • Beta Plus (positron)
  • Gamma (radiation)
  • Neutrons
• Background Radiation is weak radiation that can be detected from natural/external sources
• Examples of background radiation

  • Cosmic rays
  • Radiation from underground rocks
  • Nuclear fallout
  • Medical rays
• Methods of Measuring Radioactivity

  Photographic film: Film goes darker when it absorbs radiation. Geiger-Muller Tube: Detects radiation and produces a clicking sound based on the frequency of clicks
• Dalton said everything was made of atoms


  1800
• JJ Thomson discovered the electron


  1897
• The Plum Pudding Model was formed
• Rutherford realised most of the atom was empty space


  1911
• The Gold Foil Experiment was carried out by Geiger and Marsden
• Rutherford Model: The overall charge of an atom is neutral, so the negative electrons were dispersed through the positive "pudding" to cancel out the charges. Most particles went straight through, indicating most of the atom is empty space. Some alpha particles were slightly deflected


  1913
• Atom

  A positively charged nucleus made of positive protons and neutral neutrons, surrounded by negatively charged electrons that orbit the nucleus at different fixed distances
• Subatomic Particle

  • Proton: 1, +1
  • Neutron: 1, 0
  • Electron: 0.0005, -1
  • Positron: 0.0005, +1
• Size of atom: ~0.1 nanometers, 10^-10
• Isotopes and Elements

  • Atoms of the same element have the same number of protons
  • Neutral atoms have the same number of electrons and protons
  • Isotopes are atoms of the same element but with different masses, they have the same number of protons but different number of neutrons
• Atoms and EM Radiation


  When electrons change orbit, the atom can absorb or emit EM radiation. If an electron gains enough energy, it can leave the atom to form an ion
• Radiation Decay

  Decay occurs in a random process. Forms of decay include Alpha, Beta Minus, Beta Plus, Gamma, and Neutrons
• Background Radiation

  • Weak radiation that can be detected from natural/external sources
  • Examples include cosmic rays, radiation from underground rocks, nuclear fallout, medical rays
• Methods of Measuring Radioactivity

  Photographic film goes darker when it absorbs radiation, Geiger-Muller Tube detects radiation and produces a clicking sound
• How and why the atomic model has changed over time: 1800 - Dalton said everything was made of atoms, 1897 - JJ Thomson discovered the electron and formed the Plum Pudding Model, 1911 - Rutherford realised most of the atom was empty space through the Gold Foil Experiment, 1913 - Rutherford Model
• 1913 - Rutherford Model
• Overall charge of an atom

  Neutral, negative electrons dispersed through positive "pudding" to cancel out charges
• Most particles went straight through, indicating most of the atom is empty space
• Some 𝛼 particles were slightly deflected, indicating the nucleus must be positive, repelling positive 𝛼
• Few 𝛼 particles were deflected by >90°, indicating the nucleus contained most of the mass
• 1913 – Bohr produced the final model of the atom
• Decay Processes

  1. Beta-Minus Decay: Neutron becomes a proton and releases an electron

  2. Beta-Plus Decay: Proton becomes a neutron and releases a positron
• Alpha

  • An alpha particle is equivalent to a helium nucleus
  • Beta: A beta particle is an electron emitted from the nucleus
  • Gamma: A gamma ray is electromagnetic radiation
• Nuclei after decay

  Often have excess energy, which they release as gamma when the atom undergoes nuclear arrangement
• Activity

  • Number of decays in a sample per second
  • Activity decreases exponentially over time
  • Units of Activity are Becquerel, Bq
• Half Life

  • Time taken for half the nuclei in a sample to decay
  • Or the time taken for the activity of a sample to decay by half
• If Rutherford was right, the electrons in the cloud close to the nucleus would get attracted and cause the atom to collapse, therefore, he concluded that the electrons exist in fixed 'orbitals'