[happysmiles364]

Draw all significant resonance structures:



I have:
(1 Lone pair on N)
(2 Lone pair on N not H)
(2 Lone pair on N not H)

I am wondering why
(3 lone pair on N not H₂)
Isn't a resonance structure that would be an answer. (Not in answer book)

Thanks.
PS. Simple explanation please

[orgopete]

I suggest you refresh yourself on the rules of writing resonance structures. No atom movements are allowed. You move electrons and this may result in changes to the formal charges.

[zsinger]

3 LP's on Nitrogen?  Yikes! .  As Pete said, resonance structures are an average of the molecule's true structure.  Therefore, atoms must remain sedentary while electron pushing is allowed.
               

[happysmiles364]

Thanks for the answers but I'm still confused. My textbook doesn't go into much detail about resonance structures and gives no other examples with triple bonds, so I have no clue what to do when there is a triple bond, like in this question.

When you say no atom movements are allowed, which ones do you mean? I don't think I moved any atoms. The structure is still CH3CH2OCN for all of the ones i drew i think unless I'm thinking of something else.

My guess atm is that the large formal charges on the nitrogen atom (2-) and on the carbon atom (2+) doesn't make it significant because its not very likely to happen.

Maybe, the images of the structures I posted wasn't clear enough, here is another version of it:
1.
H3C - CH2 - O - C ≡N
 
2.
H3C - CH2 - O - C = N [Formal Charges: 1+ on C, 1- on N]
 
3.
H3C - CH2 - O = C = N [Formal Charges: 1+ on O, 1- on N]
-----------------------
4.
H3C - CH2 - O - C - N [Formal Charges: 2+ on C, 2- on N)
 
5.
H3C - CH2 - O = C - N [Formal Charges: 1+ on O, 1+ on C, 2- on N)

4 and 5 aren't answers, I need an explanation on why, my guess is above.

[happysmiles364]

No one knows?

[orgopete]

Yes, 1, 2, and 3 are major contributors. Structure 2 would be a lesser contributor than the other two as carbon does not have a completed octet. This is similar to a carbonyl group in which one can draw a resonance structure with C(+)-O(-). Again, a lesser contributor as carbon does not have a completed octet.