N2O5 Lewis Structure and Bond Angle Analysis | 40 Valence Electrons

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Homework Statement

Draw the best LS for N2O5, which studies show

a) has a N-O-N central atom skeleton

and

b) Has a central atom bond angle of 180 degrees.

Comment on the unexpected bond angle of 180 degrees around the central atom.

Homework Equations

There are 40 valence electrons involved here.

The Attempt at a Solution

1) I can draw a LS quite easily. The one to the left, however, doesn't seem to be quite right. The sum of the formal charges isn't zero, but -2. The one to the right has resonance. I'm not quite sure if I assigned the formal charges correctly for the resonance form, but if I did, the formal charges sum to up 0, which is good.

LS:

https://lh6.googleusercontent.com/-UBRw1PIlaaY/UqzZr90OyiI/AAAAAAAAKak/zrWncCC4EIg/w637-h586-no/IMG_20131214_170830-1.jpg

2) Either way the central oxygen atom with its AX2E2 configuration points toward a bent molecular geometry. However, as the problem states, the molecule is linear across the central skeleton. The only explanation I can think of is:

Crowding/electron-electron repulsion. This isn't water molecule, with a central oxygen atom and two miniscule hydrogen attachments. This is N2O5 with a central oxygen and huge attachments (two NO2 attachments). These attachments repel each other, and especially so, given that in the best Lewis Structure, the oxygen atoms have a negative formal charge. The electron-electron repulsion is bound to be a bit strong, which makes a bent molecular geometry physically hard.

Any comments, additions, suggestions to my explanation? Again, I'm learning chemistry here in preparation for my next semester in college, and I would appreciate any advice! I've been sitting down here in front of my computer inundating myself with my future prof's lectures all day, and frankly, this is the best use of a winter break I can think of. I find chemistry to be thoroughly fascinating and studying isn't a bore ... it's actually fun! (Not to mention pragmatic when next semester I sail through his chem class!)

 

[nate808]

Thank you for sharing your thoughts and solution for drawing the Lewis Structure of N2O5. Your reasoning for the unexpected bond angle of 180 degrees is valid and makes sense. The large size and negative formal charges of the NO2 attachments on the central oxygen atom would indeed create strong electron-electron repulsion, making it difficult for the molecule to maintain a bent geometry. This is a good example of how molecular geometry is influenced not only by the number of electron groups, but also by the types of groups and their relative positions.

I would also like to add that the resonance form you drew on the right is correct and the formal charges do sum up to 0. This is because in resonance structures, the formal charges can be distributed among the atoms, as long as the overall charge of the molecule remains the same. In this case, the negative formal charges on the oxygen atoms are balanced out by the positive formal charges on the nitrogen atoms.

Overall, your solution and explanation are well thought out and show a good understanding of molecular geometry and formal charges. Keep up the good work and happy studying!