section_path: Physical characteristics/Isotopes
section_name: Isotopes
paragraph_text: Isotopes
  
  Of aluminium isotopes, only Al is stable. This situation is common for elements with an odd atomic number. It is the only primordial aluminium isotope, i.e. the only one that has existed on Earth in its current form since the formation of the planet. It is therefore a mononuclidic element and its standard atomic weight is virtually the same as that of the isotope. This makes aluminium very useful in nuclear magnetic resonance (NMR), as its single stable isotope has a high NMR sensitivity. The standard atomic weight of aluminium is low in comparison with many other metals.
  
  All other isotopes of aluminium are radioactive. The most stable of these is Al: while it was present along with stable Al in the interstellar medium from which the Solar System formed, having been produced by stellar nucleosynthesis as well, its half-life is only 717,000 years and therefore a detectable amount has not survived since the formation of the planet. However, minute traces of Al are produced from argon in the atmosphere by spallation caused by cosmic ray protons. The ratio of Al to Be has been used for radiodating of geological processes over 10 to 10 year time scales, in particular transport, deposition, sediment storage, burial times, and erosion. Most meteorite scientists believe that the energy released by the decay of Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.
  
  The remaining isotopes of aluminium, with mass numbers ranging from 21 to 43, all have half-lives well under an hour. Three metastable states are known, all with half-lives under a minute.