Writing The Lewis Structures For A Molecule With Resonance

Writing Lewis Structures for Molecules with Resonance: A Comprehensive Guide

Resonance structures are a crucial concept in chemistry, particularly when dealing with molecules that defy a single Lewis structure representation. This article provides a comprehensive guide to understanding and drawing Lewis structures for molecules exhibiting resonance, focusing on the steps, common mistakes to avoid, and advanced applications.

Understanding Resonance: Beyond a Single Lewis Structure

A Lewis structure, also known as an electron dot structure, depicts the arrangement of valence electrons in a molecule. It shows how atoms are bonded and accounts for lone pairs. However, some molecules have electron delocalization, meaning the electrons aren't confined to specific bonds or atoms. This leads to resonance, where a molecule can be represented by multiple Lewis structures, none of which accurately depict the actual distribution of electrons. These multiple representations are called resonance structures or canonical forms. The real molecule is a hybrid, a weighted average of all resonance structures, often depicted as a single structure with delocalized electrons represented by dashed lines or electron clouds.

Why Resonance Occurs?

Resonance arises due to the presence of conjugated systems. This generally involves alternating single and multiple bonds, or systems containing lone pairs adjacent to multiple bonds. The electrons involved in these conjugated systems are delocalized, meaning they are not localized to a single bond or atom but spread over several atoms. This delocalization leads to greater stability for the molecule.

Step-by-Step Guide to Drawing Resonance Structures

Let's break down the process of drawing resonance structures for molecules displaying this electron delocalization:

Step 1: Determine the Total Number of Valence Electrons

This is the foundation. Add up the valence electrons contributed by each atom in the molecule. Remember to account for any charge on the molecule (add an electron for each negative charge and subtract an electron for each positive charge).

Step 2: Identify the Central Atom

The central atom is typically the least electronegative atom (except hydrogen, which is always terminal). Draw the skeletal structure placing the central atom in the center and arranging other atoms around it.

Step 3: Connect Atoms with Single Bonds

Connect each atom to the central atom using single bonds (two electrons per bond).

Step 4: Distribute Remaining Electrons as Lone Pairs

Complete the octets (or duets for hydrogen) of all atoms by adding lone pairs of electrons around them. Remember, some atoms like phosphorus and sulfur can have expanded octets, meaning they can accommodate more than eight valence electrons.

Step 5: Identify Potential Resonance Structures

If any atom (other than hydrogen) lacks an octet, or if you can move electrons to create multiple bonds without violating the octet rule, you have potential resonance structures. This usually involves moving pi electrons (electrons in double or triple bonds) or lone pairs that are adjacent to pi bonds.

Step 6: Draw All Possible Resonance Structures

Draw each possible resonance structure by carefully moving electrons. Remember:

• Only electrons move; atom positions remain the same. Only pi electrons (from double or triple bonds) and lone pairs can move.
• Maintain the same overall charge. The total charge of the molecule must remain consistent across all resonance structures.
• Follow the octet rule (where applicable). While some atoms can exceed the octet rule (e.g., phosphorus, sulfur), try to maintain octets whenever possible.
• Use curved arrows to show electron movement. This is crucial for illustrating how you're transitioning from one structure to another. The tail of the arrow starts at the electron pair, and the head points to where the electrons are moving.

Step 7: Evaluate the Resonance Structures

Not all resonance structures contribute equally to the resonance hybrid. Structures with:

• Complete octets on all atoms (where applicable): These are generally more stable and contribute more significantly to the resonance hybrid.
• Minimum formal charges: Formal charges should be minimized, and structures with negative charges on more electronegative atoms are more stable.
• Negative charges on more electronegative atoms and positive charges on less electronegative atoms: This arrangement is energetically favorable.

Step 8: Draw the Resonance Hybrid

The resonance hybrid isn't just one of the individual resonance structures; it's a blend of all of them. You represent the resonance hybrid by drawing a single structure with delocalized electrons shown as dashed lines or an electron cloud over the atoms involved in resonance. This often means showing partial double bond characteristics where electron density is spread out.

Examples of Molecules with Resonance

Let's illustrate the process with some examples:

Example 1: Nitrate Ion (NO₃⁻)

1. Valence Electrons: 5 (N) + 3 * 6 (O) + 1 (negative charge) = 24 electrons
2. Central Atom: Nitrogen
3. Connect with Single Bonds: Connect N to each O with single bonds (6 electrons used).
4. Distribute Lone Pairs: Complete octets for O atoms (18 electrons used, total 24 electrons).
5. Identify Resonance: Nitrogen lacks an octet. You can create multiple structures by moving one lone pair from an oxygen atom to form a double bond with nitrogen.
6. Draw Resonance Structures: This results in three equivalent resonance structures.
7. Resonance Hybrid: The resonance hybrid shows a delocalized pi bond across all three N-O bonds, represented by dashed lines or an electron cloud above and below the plane of the molecule.

Example 2: Benzene (C₆H₆)

Benzene is a classic example of resonance. Each carbon atom has one hydrogen atom attached.

1. Valence Electrons: 6 * 4 (C) + 6 * 1 (H) = 30 electrons.
2. Central Atoms: Carbon atoms form a ring.
3. Connect with Single Bonds: Carbon atoms are connected in a ring with single bonds (12 electrons used). Each carbon has a hydrogen atom attached using single bonds (6 electrons used).
4. Distribute Remaining Electrons: The remaining 12 electrons are distributed as three double bonds within the ring. This structure still has resonance. You can draw several canonical structures that differ only in the location of the double bonds.
5. Draw Resonance Structures: There are two major resonance structures (Kekule structures).
6. Resonance Hybrid: The resonance hybrid shows delocalized electrons across all six carbon-carbon bonds in the ring, usually represented by a circle inside the hexagon.

Example 3: Carbonate Ion (CO₃²⁻)

1. Valence Electrons: 4 (C) + 3 * 6 (O) + 2 (negative charge) = 24 electrons.
2. Central Atom: Carbon
3. Connect with Single Bonds: Carbon connects to three oxygen atoms with single bonds (6 electrons used).
4. Distribute Lone Pairs: Complete octets for oxygen atoms (18 electrons used).
5. Identify Resonance: Carbon still lacks an octet. Move a lone pair from an oxygen to form a double bond with carbon to create three resonance structures.
6. Draw Resonance Structures: This leads to three equivalent resonance structures.
7. Resonance Hybrid: The resonance hybrid depicts delocalized electrons across all three C-O bonds.

Common Mistakes to Avoid

• Moving atoms: Only electrons move between resonance structures, not atoms.
• Ignoring formal charges: Always calculate formal charges and minimize them in the most stable structures.
• Incorrect electron counting: Double-check your electron count to ensure it matches the total valence electrons.
• Ignoring the octet rule (where applicable): Remember exceptions to the octet rule (e.g., expanded octets).
• Not using curved arrows: Curved arrows are crucial to showing the movement of electrons.

Advanced Applications of Resonance

Understanding resonance is crucial in several areas:

• Predicting reactivity: Resonance structures can help predict which parts of a molecule are more likely to participate in reactions.
• Explaining molecular properties: Resonance helps explain unusual bond lengths and molecular stability.
• Understanding spectroscopic data: Resonance contributes to the interpretation of NMR and IR spectroscopy data.
• Organic chemistry: Resonance is fundamental in organic chemistry, particularly in understanding aromatic compounds and reaction mechanisms.

Conclusion

Mastering the art of drawing Lewis structures for molecules with resonance is essential for understanding molecular structure and reactivity. By carefully following the steps outlined in this guide and avoiding common mistakes, you'll be able to accurately depict and interpret the behavior of molecules that defy simple Lewis structure representations. Remember, the resonance hybrid is the most accurate representation of the molecule, reflecting the delocalization of electrons. The more you practice, the more confident you'll become in handling this fundamental concept.