(P4) 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
• Atomic Notation


  𝑍𝑍𝑋��±𝑛𝑛

  𝐴𝐴

  , where X is the letter of the element, A is the mass number, Z is the proton number, and N is the charge
• 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
• In 1800, Dalton said everything was made of tiny spheres (atoms) that could not be divided
• In 1897, JJ Thomson discovered the electron and the Plum Pudding Model was formed
• In 1911, Rutherford realised most of the atom was empty space
• In 1913, Rutherford produced the final model of the atom with a positive nucleus at the centre and negative electrons existing in a cloud around the nucleus
• Later, the positive charge of the nucleus was 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 Radiation


  • Alpha (a helium nucleus)
  • Beta Minus (electron)
  • Gamma (radiation)
  • Neutrons
• Alpha Radiation


  Highly ionising, weakly penetrating (~5cm of air)
• Beta Radiation


  Medium ionising, medium penetration (~50cm of air, sheet of paper)
• Gamma Radiation


  Low ionising, highly penetration (very far in air, few cm of lead)