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• Atom

  Positively charged nucleus (which contains neutrons and protons) surrounded by negatively charged electrons
• Electron

  Relative Mass: 0 (0.0005), Relative Charge: -1
• Typical radius of an atom: 1 × 10ିଵ଴ metres
• Radius of the nucleus is 10 000 times smaller than 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
• 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 proposed the Rutherford Model with a positive nucleus at the centre and negative electrons existing in a cloud around the nucleus
• In 1913, Bohr produced the final model of the atom
• 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.

  
• 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 Radioactive Decay

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

  Causes both the mass and charge of the nucleus to decrease
• Beta Decay

  Does not cause the mass of the nucleus to change but does cause the charge of the nucleus to increase
• Gamma Decay does not cause the mass or charge to change
• Half-Life

  The time taken for half the nuclei in a sample to decay or the time taken for the activity or count rate of a sample to decay by half
• Net Decline

  The ratio of net decline of radioactive nuclei after X half-lives, calculated by: (initial number - number after X half-lives) / initial number
• Contamination

  Radioactive atoms transferred to an object, lasting for a long period of time

  
• Irradiation

  Exposing an object to nuclear radiation, but does not make it radioactive, lasting for a short period of time

  
• Scientific reports on the effects of radiation on humans need to be peer reviewed to ensure accuracy
• Background Radiation

  Weak radiation that can be detected from natural / external sources such as cosmic rays, radiation from underground rocks, nuclear fallout, and medical rays
• Radiation Dose

  Measured in Sieverts (Sv)
• Uses of Radioactive Isotopes


  • Technetium as a medical tracer
  • Gamma emitters used in chemotherapy

  
• Nuclear Fission

  The splitting of a large and unstable nucleus (e.g. uranium or plutonium), releasing energy and neutrons that can cause a chain reaction
• Nuclear Fusion

  Two small nuclei fusing to form a heavier nucleus, releasing energy
• Electrical Charge

  • Circuit must be closed (no open switches)
  • There must be a source of potential difference (battery/cell)
• Electrical Current

  • Flow of electrical charge
  • Greater the rate of flow of charge, greater current
  • Q = It (Charge = Current x Time)
• In a single closed loop, the current has the same value at any point
• Current (I)

  Depends on both the resistance (R) of the component and the potential difference (V) across the component
• Resistors

  • If resistance is constant (ohmic conductor), current is directly proportional to potential difference (linear graph)
  • If resistance is not constant (e.g. lamps, diodes, thermistors, LDRs), graph is non-linear

  
• How resistance changes with current

  1. As current increases, electrons have more energy

  2. Electrons collide with atoms, causing them to vibrate more

  3. This makes it more difficult for electrons to flow, increasing resistance and decreasing current