[mcrios2]

I need to draw 2 structures  for cyanogen (C₂N₂), both of which must satisfy the octet rule.

Then I choose the one that is more stable.

I was able to get this structure that has formal charge 0:
:N:::C:C:::N:

But I cannot get the other one without using more than 18 electrons! Please let me know about the best way to do this problem. thank you.

[Hunter2]

Probably this one |N=C=C=N| 20 Electrons.

[mcrios2]

Probably this one |N=C=C=N| 20 Electrons.

That has 20 electrons!! (C * 2) + (N * 2) = 18 electrons!!

[Hunter2]

|N=C=C=N|

I count in this way | 2, _ 2, = 4, = 4, = 4, _ 2,|2  in sum 20

[curiouscat]

|N=C=C=N|

I count in this way | 2, _ 2, = 4, = 4, = 4, _ 2,|2  in sum 20

How could C2N2 have 20 electrons? 

Shouldn't have only 18?

[Hunter2]

In the form |N=C-C=N| I count | 2, = 6. - 2, = 6,  | 2 = 18

but the other one get 20. Where is the error.

[curiouscat]

In the form |N=C-C=N| I count | 2, = 6. - 2, = 6,  | 2 = 18

but the other one get 20. Where is the error.

The structure with 20 electrons seems the error.

[liebig]

The structure with a CC double bond is in error because of the different oxidation states.  Consider charged cyano resonance forms for this problem.

[Hunter2]

Well, right.

I can built an molecule Butatriene H₂C=C=C=CH₂, if I substitute the outer CH₂ to N. Then I get N=C=C=N. Right it has to carry two negative charges. ^-N=C=C=N^-.
In the first drawing there a missing.

[liebig]

You are still at 20 electrons.  Just do the resonance form of a cyano R-C(+)=N(-)

[Hunter2]

R-C(+)=N(-) is the same like the neutral R-C=N|. The question was is there another molecule with same formula with more then 18 electrons and I think there is.

[liebig]

R-C(+)=N(-) is the same like the neutral R-C=N|. The question was is there another molecule with same formula with more then 18 electrons and I think there is.

I thought the question was a resonance form of cyanogen?  Cyanogen with two more electrons is no longer cyanogen.  All that is being asked for is a resonance form.  And as the question being asked is which is more stable, naturally the non-charged form will be.

[AWK]

Two nitrogen atoms and two carbon atoms possess 18 valence electrons together and nothing more.