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 the double bond can be positioned either on the left or the right oxygen atom. Since both configurations are valid, they are depicted using double-headed arrows to indicate that these resonance structures are equivalent. This notation signifies that the actual structure of the nitrite ion is not solely represented by either resonance structure but is instead a resonance hybrid, which is an average of the two.

The resonance hybrid reflects the true electronic structure of the molecule, incorporating characteristics from all major resonance forms. To illustrate the resonance hybrid, dotted lines are used in place of pi bonds, indicating the delocalization of electrons. In the case of the nitrite ion, dotted lines would be drawn between the nitrogen and both oxygen atoms, representing the shared nature of the pi bonds. The overall structure can be enclosed in brackets, with the charge indicated outside, effectively summarizing the resonance hybrid 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 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 entire structure is enclosed in brackets with the charge \( 2- \) indicated 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, this structure is also enclosed in brackets with the charge noted.

The third resonance structure features the double bond with the right oxygen atom. This follows the same pattern: the right oxygen has two lone pairs, and the two single-bonded oxygens each have three lone pairs. This structure is also represented in brackets with the charge indicated.

Overall, the resonance hybrid of the carbonate ion is a composite of these three structures, reflecting the delocalization of electrons across the molecule. It is important to note that none of the individual resonance structures accurately represent the true nature of the ion; rather, the actual structure is a blend of all three, illustrating the concept of resonance in molecular chemistry.