Atomic Theory of Mass

• Isotopes


  Elements with the same number of protons but a different number of neutrons. Consequently they have the same chemical properties but different physical properties.
• Relative Atomic Mass (A r) or Atomic Weight


  1/12 of the mass of a C-12 isotope (the most common isotope of carbon) is considered to be one unit. Is calculated using experimental values for the isotopic abundance of a given atom multiplied by it's relative isotopic mass.
• Mass Spectroscopy


  Is use to identify the presence and relative abundance of isotopes in a sample of element. It can also be used to measure the mass of molecules and hence help determine their formula.
• Mass Spectroscopy


  1. Ionization - The sample is vaporized into a gas and then ionized by the ion source, usually by losing an electron to become a cation.

  2. Acceleration - the ions are then accelerated through a potential difference.

  3. Deflection - The ion beam passes through a magnetic field which bends the charged stream. Lighter components or components with more ionic charge will deflect in the field more than heavier or less charged components.

  4. Detection - A detector counts the number of ions at different deflections. The data is plotted as a graph or spectrum of different masses.