[MJF]

This was a problem we worked out in lecture. 2 Nitrogen atoms equals to 10 valence electrons and 1 Oxygen atom has 6 valence electrons, for a total of 16. After adding 2 single bonds (4 electrons), we have 12 electrons left. I just noticed that our professor placed 6 electrons on the peripheral atoms of O and N.

I can understand placing 6 dots around the O atom but why place 6 dots around the peripheral N atom? The peripheral atom should have 5 valence electrons so why do we place 6 around it? Shouldn't we take that last remaining electron and place around the central N atom?

MJF

[Illuminatus]

Hello MJF,

N20 has three valid lewis structures:
1) a single bond between nitrogens and a triple bond from the center nitrogen to the oxygen
2) double bonds between nitrogens and center nitrogen and oxygen
3) a triple bond between nitrogens and a single bond from the center nitrogen to the oxygen

These three configurations satisfy the octet rule. Your reasoning for placing 5 electrons around the peripheral nitrogen is very reminiscent to formal-charge predictions.
If you calculate the formal charges for the above three structures, you will find that it is inevitable for the center nitrogen to have a positive formal charge (it will always have 4 electrons around it, its stable state is 5 valence electrons = a +1 formal charge).
Then you have to make sure to avoid formal charges >1, so 1) is the least stable (all of the atoms have non-neutral formal charges, with the peripheral nitrogen having a -2).
This leads us to 2) in which you will find that the peripheral oxygen has a neutral charge, and the nitrogens have a +1 (center) and a -1 (peripheral); this is much more stable than 1).
Finally, you take a look at 3) and notice that the peripheral nitrogen has a neutral formal charge, the center has a +1, and the peripheral oxygen has a -1 formal charge. This is the most stable state, because oxygen's greater electronegativity (compared to nitrogen) stabilizes this formal charge.
You are likely to see a resonance between 2) and 3) with 3) being relatively more common; 1) will occasionally occur but is a lot less likely.

Now on to your actual question: I am sure you can gather from the above explanation that nitrogen does not necessarily need to have 5 electrons around it to create a working Lewis structure; it just so happens that it creates the most stable one. The central nitrogen needs to always have 4 bonds around it in order to uphold octet conditions for all of the atoms in the molecule.

Enjoy