Sometimes, a single Lewis structure does not adequately represent the true structure of a molecule.

Consider the carbonate ion, CO₃^2-

carbon (C) has four valence electrons x 1 carbon = 4 e^-
oxygen (O) has six valence electrons x 3 oxygens = 18 e^-
The ion has an overall negative two charge so we add 2 e^- to give a total of 24 e^- to be placed in the Lewis structure.

Carbon is the central atom, the three oxygens are bound to it and electrons are added to fulfill the octets of the outer atoms.

All the available electrons have been used but carbon is electron deficient - it only has six electrons around it. So, we share a non-bonding electron pair on an oxygen with the carbon to create a double bond and thereby fulfill carbon's octet.

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Three equivalent Lewis structures (formal charges are included) can be drawn for the carbonate ion. The true structure of the carbonate ion is an average of the three resonance structures connected by a double headed arrow and enclosed in brackets.

This can also be represented by a resonance hybrid where a dashed line is used to indicate the delocalized electrons.

This affords a bond order for the carbon - oxygen bond of 1 1/3. (4 bonds averaged over three structures.)

1. Lewis Structures
2. Formal Charge
3. Resonance Structures
4. Valence Shell Electron Pair Repulsion Theory
5. Bond and Molecular Polarity
6. Intermolecular Forces
7. VSEPR Tutorial
8. VSEPR Quiz

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