the first question is about low energy in Lewis structure, so in the picture you can see the Lewis structure of 2 CN2 and 3 AsO4.
I mark in red numbers every Formal charge of any atoms.
now I need to decide between the 2 CN2 who have the low energy and between the 3 AsO4 who have the low energy. How can I know it?
http://p1cture.me/images/17133936970776968378.jpg
the second question I need to say if this Quantum numbers are possible: n=0,L=1,m1=0
here I think is not possible because what is n=0 is really exist ?

thanks.

I never did that sort of problem, but I think that using the bond energies will give you the answer. You work out the sum of the bond energy in a given molecule. The higher it is, the lower energy the molecule has.

This is because the bond is more difficult to break, the molecules will be on a lower energy level.



I'm not sure what's this about, but is an ion with no electrons at all, though it's very reactive and tends to bind with water to form

While I don't know what L and m1 are (if I guessed n correctly)

I did mistake is not m1 is ml and you can see what is mean here:http://en.wikipedia.org/wiki/Quantum_number
and the range of values of n is must be n>1 or n=1 so I think is not correct :)
what do you say?

What is the CN2(-2)ion? Am I getting senile, or is this a purely intellctual exercise?
The two structures you show (there is a third one with the N C triple bond on the other nitrogen atom) are RESONANCE structures. None of them exist by themselves, rather the ion is all of them simultaneously. The most realistic one(s) have the charges spread out as much as possible. . i.e. -N=C=N-
By the rules of Quantum Mechanics, the principal quantum number, n, must be a positive integer. So 0 is not allowed.

Yes, from what I read,

Good! :)

well I think is just a purely intellctual exercise.
I just asked to decide about the energy, I read this in some website: http://www.buzzle.com/articles/calculating-formal-charge.html "Nature prefers low energy states which lead to stable molecular structures" so maybe is can help me, if I will know which one has the most stable structure

this is how it work: The most stable structure is the one with:
a) the lowest sum when the absolute values of the formal charges are added
b) the largest negative formal charge on the most electronegative element (when 2 or more
structures have the same lowest sum)
truth ?

thanks.

This sounds very reasonable

Still thank you for your trying to help :)