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Homework Help: Lewis Structures

  1. Sep 28, 2004 #1
    i'm having trouble drawing up some lewis structures related to VSPER theory.
    i was given 22 Lewis structures to draw and i had problems with these ones...

    1 - [N(CHsubscript3)subscript4]^+
    2 - IClsubscript2
    3 - [IClsubscript4]^-

    thanks if anyone can help out with these questions.
  2. jcsd
  3. Sep 28, 2004 #2


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    Homework Helper

    Try visiting the Chemistry forum in Other Sciences, i remember doing this exercices, but a long time ago :smile:
  4. Sep 28, 2004 #3
    Were you taught the 5-step method for drawing Lewis structures?
  5. Sep 29, 2004 #4
    Clarke's Method


    Here's something known as the Clarke method for drawing Lewis Structures. I read it from a General Chemistry book by Dr. Hubbard (I forget the complete name). I've found it very effective and thought I should share it with you folks.

    1. Find the TOTAL number of electrons ([tex]n_{e}[/tex]) for a species. For [tex]ICl_{4}^{-}[/tex] this would be 7+4(7)+1 = 36.
    2. Next compute the quantity (6y+2) where y is the total number of atoms minus hydrogen (for [tex]H_{2}O[/tex] this would be 1).
    3. Compare [tex]n_{e}[/tex] and (6y+2):

    (a) if [tex]n_{e}[/tex] < (6y+2) then the species contains a double bond. For ICl4-, 6y+2 = 32 which is less than 36 by 4 so there is either a triple bond or two double bonds (and you can see that there are actually 2 double bonds here).

    (b) if [tex]n_{e}[/tex] = 6y+2, the structure contains only single bonds. As an example consider the water molecule. For it, [tex]n_{e} = 8[/tex] and 6y+2 = 8.

    (c) if [tex]n_{e}[/tex] > (6y+2) the central atom has an expanded octet (which means it cannot belong to the second period as the second period elements do not have d-orbitals). For example, [tex]IF_{7}[/tex] has [tex]n_{e} = 7+7(7) = 56[/tex] and 6y+2 = 6(8)+2 = 50.

    Note that this method will only take you a bit further in making the structure but won't tell you the crucial information: that of connectivities. You must then use formal charges and some experience to rule out incorrect structures if more than one is possible.

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