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Offline jjkwest1

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Lewis structure molecule
« on: November 20, 2009, 09:59:54 PM »
1. Complete the Lewis structure for the molecule:

    CH3 O
       :  :
CH3-C-C-C-N   (just think that : is equal to a single bond, i didnt know how to make a straight line between C and CH3 and O)

This molecule has__________ single bond and _________  multiple bonds
A. 4,2
B. 6,3
C. 11,5
D. 11,2
E. 13,0

Is there mistake in this question? i counted that the number of bonds for this molecule was 12 . Can anyone help? Thanks




2. Which of the following is a valid Lewis structure for I3-?

      .. .. .. 
a. [ :I-I-I:]  -
      .. .. ..
      .. . ..
b. [ :I- I- I:]-
      ..  ..  ..

c. none of these

The book says that the answer is none of these but i'm pretty sure that b is correct. Is the book wrong? Thanks

Offline BetaAmyloid

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Re: Lewis structure molecule
« Reply #1 on: November 20, 2009, 11:02:39 PM »
For #1:

I'm not sure which pieces of the molecule they are counting as a whole and which they are counting as seperate. If you counted everything seperate such as the hydrogens there would be 19 bonds; if you counted what is shown it would be 12 bonds; if you counted what is shown with the hydrogen atom bond against the oxygen atom bond then there would be 13; but, if you seperated one hydrogen you should have to seperate all hydrogens. If I were you, I would draw this whole lewis structure out with the hydrogens with the bonds to ensure (through bond math) that there are no multiple bonds, if so, then the answer would most likely have to be E. (There should be no multiple bonds in this molecule)

For #2:

Have you drawn out the lewis structure yet with the valence electrons?
Discovery consists of seeing what everybody has seen and thinking what nobody has thought - Albert Szent-Györgyi

Offline jjkwest1

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Re: Lewis structure molecule
« Reply #2 on: November 20, 2009, 11:09:17 PM »
For #1 they said that the answer was D. 11,2 I still don't see how that's the asnwer

For #2 i did draw the lewis structure (i got a total of 22 valence electrons) and i got the exact same thing as B.

Offline BetaAmyloid

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Re: Lewis structure molecule
« Reply #3 on: November 20, 2009, 11:17:59 PM »
For #1:

Well, all I can see from the molecule drawn is that they may be considering the double hydrogens coming from the lone carbon connected to the nitrogen as having a double bond with one hydrogen (which doesn't make sense) and that the nitrogen is having a double bond with hydrogen (which again doesn't make sense). This would give you 11 single bonds and 2 double bonds, but again, that makes no sense. This is an assumption, wait for someone like Borek to come along and answer this question for sure though.

For #2:

B is correct.
Discovery consists of seeing what everybody has seen and thinking what nobody has thought - Albert Szent-Györgyi

Offline cth

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Re: Lewis structure molecule
« Reply #4 on: November 21, 2009, 11:34:39 AM »
1) I think there is a hydrogen atom missing in the molecule drawing.

If you calculate the degree of unsaturation http://en.wikipedia.org/wiki/Degree_of_unsaturation, you get:
1+[5*(4-2)+1*(3-2)+1*(2-2)+6*(1-2)]/2 = 3.5
It is not an integer number, which indicates you have an unpaired electron. So, either this molecule is a radical, or there is one hydrogen missing somewhere.

Furthermore, the answer given in the book is 11,2. So they say there are 13 bonds in total, considering both single and multiple bonds. But, one can only see 12 bonds in the drawing. One hydrogen is missing.

If you add one hydrogen to the molecule, the degree of unsaturation becomes 3. This means you have 3 unsaturations in total. What do I mean by unsaturation:
- double bonds count for 1 unsaturation each
- triple bonds count for 2 unsaturations each
- cycles count for 1 unsaturation each


* Let's add one hydrogen to the oxygen, so it forms an alcohol group. In order to have the octet rule true for each carbon and nitrogen atom, one must add 1 double bond and 1 triple bond:
       CH3
       | 
CH3-C=C-C=N
          |
          OH
The unsaturation is 3 indeed, as we had calculated.

* Let's add the hydrogen on the carbon next to the CH3:
       CH3
       |
CH3-CH-C-C=N
           ||
            O

In both cases, the answer to your question is 11,2.


2) I3-

It seems to me that structure b is correct.

I3- is constituted from I2 (where each iodine has 3 lone pairs) + I- (which has 4 lone pairs) where I- gives one lone pair to I2 to form a covalent bond. So, all iodine atoms have three lone pairs each. And the molecule is linear http://en.wikipedia.org/wiki/Triiodide.

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