WHICH IS THE MOST STABLE LEWIS STRUCTURE?
There are a number of key chemical principles that can help you decide which possible valid Lewis structure is most stable, of which the most important are:
The most electropositive (non-hydrogen) atom usually goes in the middle;
Formal charges should be minimized;
Like charges should not be next to each other;
Negative charge is preferred on the more electronegative atom and positive on the less electronegative atom;
Small rings (3 or 4 atoms) are disfavored over larger rings due to ring strain;
Single bonds between N, O and F atoms are weak if both atoms have lone pairs;
Resonance structures improve stability.
CONSIDER NITRATE AGAIN
When looking at nitrate, for example, we could have drawn many possible bonding arrangements with a "valid" Lewis structure, some of which are shown to the right.
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We can use our chemical principles to explain why these structures are not stable:
Principles 1 and 2 rule out F and G;
Principle 3 rules out D and G;
Principle 4 rules out C and D;
Principle 5 rules out A and B;
Principle 6 rules out all of these (note that the correct structure has N–O single bonds, but the N atom does not have any lone pairs).
Some other useful principles are:
Multiple bonds are favored by small atoms of the first row (B to F), but one second row element (Si to Cl) in a multiple bond is reasonably strong;
Atoms should be able to adopt their preferred geometry based on VSEPR.
If you know of other useful principles that should be listed here, please let me know.
