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What does this notation mean?

  1. Jul 6, 2012 #1
    I see this often and am not sure what it means. Suppose X and M are any two elements:


  2. jcsd
  3. Jul 6, 2012 #2


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    Homework Helper

    Do you mean like [itex]C_nH_{2n+2}[/itex]? That is a notation for a class of compounds that contain two elements (carbon and hydrogen) in a particular ratio - in this case, the class is the alkanes. n can take on any integer value starting from 1. There will be n carbon atoms and (2n+2) H atoms. So you can have [itex]CH_4, C_2H_6, C_3H_8,...[/itex] for methane, ethane and propane, respectively.

    I'm guessing your notation is similar, and the subscripts denote the proportion in which X and M make up the molecular formula of each of the compounds in the class you're considering.
  4. Jul 6, 2012 #3
    Yes that makes sense, but the notation I posted doesn't because it would produce negative indices. For example, Bi2Sb3 is one compound which has that formula, but the indices should then be BixSbx+1, so I'm wondering it it doesn't have to do with the stochiometric ratio and perhaps something to do with electron configuration or something.
  5. Jul 6, 2012 #4


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    Staff: Mentor

    Do you know what molar fraction is? Molar fractions sum to 1, my guess is that notation you show means a mixture that contains x moles of M and (1-x) moles of X in each mole of the mixture. Whether it is still a mixture, or a compound, or some combination of both is another question.

    Take an alloy of gold and caesium. When they are in exactly equimolar ratio they create caesium auride, when the mixture is not equimolar, it contains some auride and some pure metal (it is obvious when your observe specific resistance of the mixture - when it is equimolar, resistance goes up by orders of magnitude, as auride replaces pure metals). Using your notation pure caesium auride would be Cs0.5Au0.5.
  6. Jul 6, 2012 #5
    I believe Borek has it as these are the ways that superconductors and ceramics are often often described in "notation". Ceramics will be describing oxides with the noted molar composition of the XM, and superconductors will often use this , but have to describe the anionic components separately.

    X is a metalloid, and M is a metal.
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