Atomic structure

• Democritus


  Greek philosopher in the 5th century BC who thought all matter was made up of identical lumps called "atomos"
• Atom


  Tiny sphere that matter is made up of, according to Democritus
• When the model of the atom was further developed after Democritus


  1800s
• Rutherford


  Replaced the Plum Pudding Model with the Nuclear Model
• John Dalton agreed with Democritus that matter was made up of tiny spheres ("atoms") that couldn't be broken up


  1804
• Dalton's theory


  Each element was made up of a different type of "atom"
• J.J. Thomson discovered particles called electrons that could be removed from atoms


  Nearly 100 years later
• Plum Pudding Model


  Atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding
• Alpha Scattering Experiment


  1. Firing a beam of alpha particles at a thin gold sheet

  2. Most particles went straight through or were slightly deflected

  3. Some were deflected more than expected, a few went backwards
• Rutherford's Model


  Most of the mass of the atom is concentrated in a tiny, positively charged nucleus, with electrons orbiting around it
• The nucleus is tiny but makes up most of the mass of the atom
• Protons


  Positively charged particles in the nucleus
• Neutrons


  Neutral particles in the nucleus
• Electrons


  Negatively charged particles that whizz around the outside of the nucleus
• The number of protons equals the number of electrons, so atoms have no overall charge
• Energy levels


  Electrons can move within or leave the atom, gaining or losing energy
• As new evidence came along, the model of the atom was changed and updated
• Isotopes


  Different forms of the same element, with the same number of protons but different numbers of neutrons
• Radioactive decay


  Unstable isotopes emit radiation to become more stable
• Types of ionising radiation

  • Alpha
  • Beta
  • Gamma
• Alpha particles


  Helium nuclei emitted during radioactive decay
• Beta particles


  High-speed electrons emitted during radioactive decay
• Gamma rays


  Electromagnetic radiation emitted from the nucleus
• Nuclear equations


  Show radioactive decay, with the mass and atomic numbers balancing on both sides
• Alpha decay


  Decreases the charge and mass of the nucleus
• Beta decay


  Increases the charge of the nucleus
• Gamma rays


  Don't change the charge or mass of the nucleus
• Half-life


  The time taken for the amount of radiation emitted by a radioactive source to halve
• Radioactive decay is a totally random process
• Activity


  The rate of radioactive decay, measured in becquerels (Bq)
• As time passes


  The radioactivity of a source decreases
• Radioactive sources


  • Some have short half-lives, decaying quickly
  • Others have long half-lives, decaying slowly
• Measuring half-life


  1. Find the time taken for the activity to halve

  2. Use this to predict the rate of decay
• Half-life


  The time taken for the number of radioactive nuclei in an isotope to halve
• Half-life


  The time taken for the activity, and count-rate, to halve
• Short half-life


  • The activity drops off quickly because the nuclei are very unstable and rapidly decay
  • Sources with a short half-life are dangerous because of the high amount of radiation they emit at first, but they quickly become less dangerous
• Long half-life


  • The activity falls more slowly because most of the nuclei don't decay for a long time
  • The source just sits there, releasing small amounts of radiation for a long time
  • This can be dangerous because nearby people are exposed to radiation for a long time
• Example


  • Initial activity of a sample is 640 Bq
  • Calculate the final activity as a percentage of the initial activity after two half-lives
• Calculating activity after half-lives

  1. Find the activity after each half-life

  2. Divide the final activity by the initial activity

  3. Multiply by 100 to get percentage
• Graph of activity against time

  • Always shaped like the one shown
  • The half-life is found from the graph by finding the time interval on the bottom corresponding to a halving of the activity on the vertical axis