Why is cyanide writted as CN- when the negative charge is on carbon?

  1. The Title pretty much sums it up.
    http://en.wikipedia.org/wiki/Cyanide

    Why is it written like that when it is clear that the carbon must be the one with the negative charge?
     
  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,261
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    Convenience. It's actually [CN]-
     
  4. Borek

    Staff: Mentor

    Charge on polyatomic molecules is almost never localized on a single atom, it is typically spread over part of the molecule. Doesn't mean there are equal parts of the charge on all atoms either, CN- has a "more negative end".
     
  5. I guess this then implies to OH- as well. But how should interpret a molecular formula like that when reading it somewhere. As an overall negative molecule, or "cause I know" interpret it as a negative charge on carbon. The same goes with http://en.wikipedia.org/wiki/Hydroxide I guess then. But this is so frustrating to me now because if the negative charge is not on Oxygen why do they not just simply write it like that. The same goes with the cyanide example why do they have to write it like that when they could just have written it C-N
     
  6. Borek

    Staff: Mentor

    It is not an obvious thing to someone just starting with chemistry that charge is not a point like thing, but a cloud spread around some volume, with different densities in different places. We start with simplified models (think Lewis structures) which require electrons to be point like, electron cloud is introduced much later.
     
  7. dextercioby

    dextercioby 12,317
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    The minus in the exponent is shorhand for -1, which is the net charge of the ion (due to the extra electron to the normal shell configuration 'melted' into the triple bond b/w C and N). It's an exponent and nothing more. I believe that the convention to write CN- and not NC- is due to another convention: namely that if you can decompose a molecule into ionic parts, the positive ion is written on the LHS, the negative on the right. That's why you don't write (OH)2 Ca but the reverse. And of course to this convention of positioning the ions as a whole you add the convention to place the element which needs electrons to obtain the octet to the left and the 'neutral' elements to the right of the ion. So it's Ca (OH)2 and not Ca (HO)2. This of course, if you picture chemistry in terms of GN Lewis's model of 1916.

    Elementary (i.e. non-quantum) chemistry contains conventions which may not seem obvious.
     
    Last edited: Oct 16, 2013
  8. Oh well, I guess I`ll just deal with it, I mean I can still handle myself around it and I can still do some of the tasks and questions in the book. I understand what they mean, and I don`t really wanna go to deep into this. Thanks though anyway people :)
     
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