UNIT 2.6 RESONANCE AND FORMAL CHARGE
Chemistry is the study of matter and interactions. Chemistry overlaps with many other sciences.
Below is the contents for this sub unit.
RESONANCE
Some ions have something called resonance. Using the octet rule, you can come up with more than 1 possible Lewis diagram. See the picture above.
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NO3 -1 is the nitrate ion has 3 oxygen atoms around a central nitrogen and the whole thing has a -1 charge.
So looking at the valence electrons we have 6 x 3 for the oxygen (18) + 5 for the nitrogen (23) +1 as it has a negative charge = 24 electrons. The only way to have meet the octet rule is to have 2 oxygen atoms with single bonds and one with a double bond.
However each oxygen is different and therefore 3 different valid structures is available. These are resonance structures.
FORMAL CHARGE
Formal charge is a way to see how stable a structure is. If you take a molecule (NO3-1) you can assign a formal charge to each atom.
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1: How many electrons it starts with. Nitrogen has 5 and Oxygen has 6. This is from their location on the periodic table.
2: Subtract however many electrons are assigned to that atom (lone pair)
3: Subtract 1/2 of the electrons in each bond (they are sharing these so they only get 1 of the 2 electrons for each bond) - alternate: subtract the number of bonds
The lower the formal charge the more stable it is.
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Example calculation
N: 5 - 0 - 1/2 x 8 = +1
O (single bond): 6 - 6 -1/2 x 2 = -1
O (double bond): 6 - 4 - 1/2 x 4 = 0
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Limitation of this model (Lewis diagram and formal charge) is it doesn't work well with odd number of valence electrons