Question

Question: Three resonance structures can be drawn for CO₂ . Which resonance structure is best from a formal charge standpoint?

Short answer

Answer

The best resonance structure is

Step-by-step solution

Definition of formal charge:

Formal charge = (number of valence electrons on a free atom) - (number of valence electrons assigned to the atom in the molecule).

The concept of formal charge requires that we compare

1. the number of valence electrons present on the free neutral atom having a charge of zero as the number of electrons is equal to the number of protons, and

2. the number of valence electrons that belong to a given atom in a molecule.

Explanation:

The simplest approach involves considering the formal charges in the two possible structures. Note that in the Lewis structure given previously for carbon dioxide, all atoms have formal charges of 0.However, if the atoms are arranged as follows, all the Lewis structures give unreasonable formal charges. The possible formal charges are listed below:

Conclusion:

None of these Lewis structures (with their resulting formal charges) agrees with our observation that oxygen has a significantly greater electronegativity than carbon. That is, it doesn’t make sense that a compound would contain a negatively charged carbon atom next to a positively charged oxygen atom.