which choice represents a pair of resonance structures

Daniel Parker

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21-02-2025

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Resonance structures are a crucial concept in organic chemistry. They represent the delocalization of electrons within a molecule, leading to a more stable overall structure. Understanding how to identify a pair of resonance structures is vital for predicting molecular properties and reactivity. This article will delve into the concept of resonance and provide a clear method for identifying pairs of resonance structures.

What are Resonance Structures?

Resonance structures, also known as canonical forms, are two or more Lewis structures that contribute to the overall description of a molecule's bonding. They differ only in the placement of electrons – the atoms themselves remain in the same positions. Importantly, resonance structures are not real; they are simply models used to represent the actual, hybrid structure. The true structure is a hybrid, or weighted average, of all the contributing resonance structures.

Think of it like this: a mule is a hybrid of a horse and a donkey. The mule isn't a horse or a donkey; it's a blend of both, possessing characteristics of each parent. Similarly, a molecule's actual structure is a blend of its resonance structures.

Key Characteristics of Resonance Structures

To determine if a pair of structures represents resonance, consider these key characteristics:

• Same Atom Connectivity: The atoms must be connected in the same way in all resonance structures. Only the placement of electrons (bonds and lone pairs) differs.
• Same Number of Valence Electrons: The total number of valence electrons must remain constant across all structures.
• Difference in Electron Placement: The only difference between resonance structures lies in the placement of electrons, including pi electrons and lone pairs.
• Formal Charges May Change: Formal charges on atoms may differ between structures, but the overall charge of the molecule must remain the same.

Identifying a Pair of Resonance Structures: A Step-by-Step Approach

Let's outline a systematic approach to determine if two structures are resonance forms:

1. Examine Atom Connectivity: Ensure the atoms are connected in precisely the same arrangement in both structures. If the atom arrangement changes, they are not resonance structures.

2. Count Valence Electrons: Verify that the total number of valence electrons is identical in both structures. This includes lone pairs and electrons involved in bonding.

3. Compare Electron Placement: Observe if the only differences between the structures are the locations of pi electrons or lone pairs. Sigma bonds should remain unchanged.

4. Assess Formal Charges (if present): Check if the sum of the formal charges in both structures is equal. Remember that formal charges help in assessing the contribution each resonance structure makes to the overall hybrid.

Examples and Non-Examples

Example 1: Benzene

The classic example is benzene (C₆H₆). Its resonance structures show alternating single and double bonds. The actual structure is a hybrid with delocalized electrons, resulting in a molecule more stable than predicted by any single Lewis structure.

(Image: Show two benzene resonance structures. Clearly label them as Resonance Structures A and Resonance Structures B. In the caption, state that both structures contribute equally to the overall structure of benzene.)

Example 2: Nitrate Ion (NO₃⁻)

The nitrate ion also exhibits resonance. One nitrogen atom is centrally located, with three oxygen atoms bonded to it. The negative charge is delocalized across the three oxygen atoms in the various resonance structures.

(Image: Show the three resonance structures of the nitrate ion. Label them appropriately. In the caption, note that the actual structure is a hybrid of these three structures, with the negative charge equally distributed.)

Non-Example: Isomers

Isomers are molecules with the same molecular formula but different atom connectivity. They are not resonance structures. For instance, ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃) have the same formula (C₂H₆O) but different structures. Therefore, they are not resonance structures.

(Image: Show the structures of ethanol and dimethyl ether, clearly indicating the difference in connectivity.)

Conclusion

Recognizing resonance structures is a fundamental skill in chemistry. By systematically checking atom connectivity, valence electron count, electron placement, and formal charges, you can confidently identify which pairs of structures represent resonance and which do not. Remember, resonance structures are just tools to help us understand the actual, hybrid structure of a molecule. Mastering this concept is key to comprehending the stability and reactivity of many organic molecules.