Resonance Structures (Simplified): Videos & Practice Problems

Resonance structures are essential for understanding the behavior of polyatomic species that contain at least one pi bond. These structures consist of two or more valid Lewis dot representations that illustrate how electrons can be redistributed within a molecule. In resonance, the movement of electrons occurs from pi bonds, which are present in double and triple bonds, or from lone pairs of electrons.

For example, consider the nitrite anion, \( \text{NO}_2^- \). This ion can be represented by two resonance structures, where either the left or the right oxygen atom is double bonded to the nitrogen atom. Since both configurations are valid, they are depicted using double-headed arrows to indicate that these resonance structures are equivalent. The actual structure of the nitrite ion is not accurately represented by either resonance structure alone; instead, it is best described as a resonance hybrid, which is the average of all major resonance structures.

To illustrate the resonance hybrid, we use dotted lines to represent the locations of the pi bonds. In the case of the nitrite ion, we would draw a dotted line between the nitrogen and each oxygen atom to indicate that the double bonds are delocalized. This hybrid structure can be enclosed in brackets, with the overall charge indicated outside the brackets, effectively capturing the true nature of the nitrite anion.

The carbonate ion, represented as \( \text{CO}_3^{2-} \), exhibits resonance, which means it can be represented by multiple structures that differ only in the placement of electrons, particularly pi bonds. In total, there are three significant resonance structures for the carbonate ion.

In the first resonance structure, the oxygen atom at the top is double bonded to the carbon atom. This double bond means that this oxygen has two lone pairs of electrons, while the other two oxygen atoms, which are single bonded to the carbon, each possess three lone pairs of electrons. The overall charge of the ion is indicated in brackets with the charge \( 2- \) placed in the top right corner.

For the second resonance structure, the double bond shifts to the left oxygen atom. Similar to the first structure, the left oxygen now has two lone pairs, while the other two single-bonded oxygens maintain three lone pairs each. Again, the ion is enclosed in brackets with the charge noted.

The third resonance structure features the double bond with the right oxygen atom. This structure follows the same pattern: the right oxygen has two lone pairs, and the other two oxygens have three lone pairs each, with the charge displayed in brackets.

These three resonance structures illustrate the delocalization of electrons within the carbonate ion. The actual structure of the carbonate ion is a resonance hybrid, which is a composite of all three structures, reflecting the average distribution of electrons rather than any single representation. This concept emphasizes that no one structure is the definitive form; rather, the true nature of the ion is a blend of all possible configurations.