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
• 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.

  
• 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
• 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
• 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

  
• Nuclear Fission


  1. The splitting of a large and unstable nucleus (eg uranium or plutonium)

  2. Spontaneous fission is rare, usually the unstable nucleus must first absorb a neutron

  3. When the unstable nuclei absorbs a neutron it splits into two smaller nuclei, roughly equal in size, and it then emits two or three neutrons and gamma rays

  4. This is a chain reaction, as energy is being released and one 'split' causes another to occur
• Nuclear Fusion


  When two small nuclei fuse to form a heavier nucleus, releasing energy
• Isotopes are Different Forms of the Same Element

  1. All elements have different isotopes - usually one or two stable ones
  2. The other unstable isotopes decay into other elements and give out radiation as they try to become more stable (they try to balance number of protons and neutrons in their nucleus or get rid of any excess energy) - Radioactive decay
  3. Radioactive substances spit out one or more types of ionising radiation from their nucleus - alpha, beta, gamma radiation.
  4. Can also release neutrons when they decay to balance number of protons + neutrons
• • Ionising radiation is radiation that knocks electrons off atoms, creative positive ions
  • The ionising power of a radiation source is how easily it can do this
• In the medical industry, radiation is directed at medical equipment sealed in packaging. The radiation sterilises the equipment. Explain whether alpha radiation would be suitable for this use (2)

  It would not be suitable because it would be stopped by a few cm of air and would not be able to pass through the packagings to sterilise the equipment.
• Alpha radiation is used in smoke detectors, as they ionise air particles causing a current to flow. If there is smoke in the air, it binds to the ions and the current stops and alarm sounds