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Homework Help: Draw the Lewis Dot Diagram of a strange compound

  1. Jun 30, 2012 #1
    1. The problem statement, all variables and given/known data

    The following species has been discovered in interstellar space: HNC. Draw the Lewis Dot diagram.

    2. Relevant equations


    3. The attempt at a solution

    I tried several different configurations (see below), but they were all marked incorrect. Here is the basic method of Lewis Dot diagrams from my textbook:

    1) Draw the skeletal structure with all the atoms, the least electronegative in the center. Hydrogens are used as terminal atoms.

    2) Count the total number of valence electrons among all the atoms.

    3) Begin by creating single bonds between the outer atoms and the central atom. Fill the octets of the outer atoms first.

    4) If the central atom doesn't have a full octet, create double or triple bonds with the outer atoms to fill the octet.

    So in total for this compound there are 10 valence electrons (1 from H, 5 from N and 4 from C). I've followed these rules, but still can't seem to get it.

    Again, all the attached configurations were marked wrong. Please excuse the poor contrast (I have no control over that).

    Attached Files:

  2. jcsd
  3. Jun 30, 2012 #2
    There is maximum of three attachments so here is the fourth:

    Attached Files:

  4. Jul 1, 2012 #3


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    Staff: Mentor

    [itex]H\!-C\!\equiv\!N\!:[/itex] is correct if you ask me.
  5. Jul 1, 2012 #4
    That's what I thought too, but apparently that's HCN as opposed to HNC.
  6. Jul 1, 2012 #5
    That's Hydrogen Isocyanide, which is usually written as,

    [tex]H - N^+ \equiv C^-[/tex]
  7. Jul 1, 2012 #6
    How does this use all 10 valence electrons? I only see 4 bonds and no lone pairs.
  8. Jul 1, 2012 #7
    I forgot to type a lone pair on carbon...:rolleyes:

    [tex]H-N^+ \equiv C^-:[/tex]
  9. Jul 1, 2012 #8
    Thank you. If anyone can verify this, that would be great. I've used up 4 of my 5 attempts at this problem and don't want to mess up my final attempt. :)
  10. Jul 1, 2012 #9


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    Staff: Mentor

    Question and the rules you listed are IMHO contradictory:

    If it is HNC, rule is misleading. I assumed you should follow the rules, hence HCN.
  11. Jul 1, 2012 #10
    I got that from my textbook. If you think it is mistaken, I wouldn't be surprised as I've found plenty of other mistakes in the same book. But I see what you mean. If I wanted the least electronegative in the center, it would have to be carbon instead of nitrogen. But they gave the compound specifically with nitrogen in the center of the formula. If it was simply HCN, I wouldn't have made this post because that is commonly known cyanide compound.

    By the way, do you have an opinion on Infinitum's answer above?
  12. Jul 1, 2012 #11
    The rules in the textbook are a bit lacking, IMO. They don't mention formal charges, which is an important aspect in Lewis structures, while determining resonance stability.
  13. Jul 1, 2012 #12
    Sigh . . . we didn't even learn about formal charges or resonance in class which makes me wonder how my professor expected us to do a problem like this in the first place.
  14. Jul 1, 2012 #13
    for future reference:

    Formal Charge= valence e-'s - nonbonding e-'s - (.5*bonding e-'s)

    for HNC:
    -Nitrogen F.C.=5-0-4=+1
    -Carbon F.C.=4-2-(.5*6)= -1

    for NCH:
    -Nitrogen F.C.=5-2-(.5*6)=0
    -Carbon F.C.=4-0-(.5*8)=0

    Borek's answer is most certainly correct. However, as you already tried NCH, I would say HNC-. But I will admit that I hate leaving an open shell on that carbon (not even to mention the unnecessary separation of charges that results).
  15. Jul 1, 2012 #14
    If anyone is curious, Infinitum's answer from post #7 was marked correct by the system when I submitted it. Thanks for your help everyone!
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