pmt notes - 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 Radioactive Decay

  • Alpha (α, a helium nucleus)
  • Beta Minus (β, electron)
  • Gamma (γ, 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
• The half-life of an isotope is a constant that enables the activity of a very large number of nuclei to be predicted during the decay
• Short Half-Life

  The source presents less of a risk, as it does not remain strongly radioactive for a long time
• Long Half-Life


  The source remains weakly radioactive for a long period of time
• Net Decline

  The ratio of the decline of radioactive nuclei after X half-lives, calculated by halving the initial number of nuclei X times

  
• Contamination

  The unwanted presence of radioactive atoms on other materials, where the hazard is the decaying of the contaminated atoms releasing radiation
• Irradiation


  Exposing an object to nuclear radiation, but does not make it radioactive
• Scientific reports on the effects of radiation on humans need to be peer reviewed to ensure accuracy and safety
• 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 to control cancerous cells
• Nuclear Fission


  1. The splitting of a large and unstable nucleus (e.g. uranium or plutonium), where the unstable nucleus absorbs a neutron, splits into two smaller nuclei, and emits two or three neutrons and gamma rays, releasing energy

  2. This can lead to a chain reaction if the neutrons collide with other radioactive nuclei
• Nuclear Fusion

  When two small nuclei fuse to form a heavier nucleus, releasing energy
• Fusion would be a more efficient way of producing energy compared to fission, but no design has been produced that could accomplish positive net energy on Earth