Resonance & Lewis Structures

Some molecules can be represented by more than one Lewis structure
Resonance structures are different variations of the same molecule, but with different arrangement of electrons
The resonance hybrid is the actual structure of a molecule, and it is a mixture of all its resonance structures
Resonance occurs because electrons are often delocalized
Delocalized electrons are not fixed to a single atom or covalent bond, therefore they can be spread through the molecule
The resonance hybrid is the most stable form of a molecule because the delocalization of electrons stabilize the structure

To draw the resonance structure of the nitrate(V) ion, the first step is to count the valence electrons that are present in the ion
There is one nitrogen atom, three oxygen atoms and a negative charge
- Nitrogen in group 15 contributes with 5 valence electrons
- Oxygen in group 16 contributes with 6 valence electrons
- The negative charge means that an electron has been gained

$Valence electrons = N + (3 \times O) + 1 e^{-}$

$Valence electrons = 5 + (3 \times 6) + 1 e^{-}$

$Valence electrons = 24 e^{-}$

After drawing an skeletal structure (N as the central atom) and placing the bonding and nonbonding electrons, three Lewis structures are possible
These Lewis structures consist of a double bond and two single bonds. However, a pair of electrons from the double bond and lone pairs of electrons can oscillate between three different positions
These oscillation allows the double bond to migrate from one part of the molecule to another

Resonance structures in the nitrate ion

One the resonance structures are identified, the resonance hybrid can be drawn using dotted lines to show the position of delocalised electrons
The three resonance structures have more energy than the resonance hybrid of nitrate(V) ion
- Therefore, the resonance hybrid is is the most stable and the actual structure of the nitrate(V) ion