Textbook Question
Use formal charges to identify the better Lewis structure.
Ch.10 - Chemical Bonding I: The Lewis Model
Chapter 10, Problem 69
Draw the Lewis structure (including resonance structures) for the acetate ion (CH3COO-). For each resonance structure, assign formal charges to all atoms that have formal charge.
Verified step by step guidance1
insert step 1> Start by counting the total number of valence electrons. Carbon has 4 valence electrons, hydrogen has 1, and oxygen has 6. The acetate ion (CH3COO-) has a negative charge, which means you add one extra electron. Calculate the total number of valence electrons: 2 Carbons (2x4) + 3 Hydrogens (3x1) + 2 Oxygens (2x6) + 1 extra electron = 24 valence electrons.
insert step 2> Draw the skeletal structure of the acetate ion. Place the two carbon atoms in the center, with one carbon bonded to three hydrogen atoms (CH3) and the other carbon bonded to two oxygen atoms (COO). Connect the two carbon atoms with a single bond.
insert step 3> Distribute the remaining valence electrons to satisfy the octet rule. Start by placing lone pairs on the oxygen atoms to complete their octets. Then, place any remaining electrons on the central carbon atoms.
insert step 4> Consider resonance structures. In the acetate ion, the negative charge can be delocalized between the two oxygen atoms. Draw a double bond between one of the oxygen atoms and the central carbon, and a single bond with the other oxygen. Then, switch the positions of the double and single bonds to show the resonance structure.
insert step 5> Assign formal charges to each atom in both resonance structures. The formal charge is calculated as: Formal Charge = (Valence electrons) - (Non-bonding electrons) - 0.5*(Bonding electrons). Calculate the formal charges for each atom in both resonance structures to ensure the overall charge of the ion is -1.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Lewis Structures
Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They use dots to represent valence electrons and lines to represent bonds. Understanding how to draw Lewis structures is essential for visualizing molecular geometry and predicting reactivity.
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Lewis Dot Structures: Ions
Resonance Structures
Resonance structures are different ways of drawing the same molecule that illustrate the delocalization of electrons. In molecules like the acetate ion, resonance structures help depict how electrons are shared among atoms, leading to more accurate representations of the molecule's actual electron distribution. Each resonance structure contributes to the overall hybrid structure.
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Formal Charge
Formal charge is a theoretical charge assigned to an atom in a molecule, calculated based on the number of valence electrons, the number of bonds, and the number of lone electrons. It helps in determining the most stable Lewis structure by indicating how electron distribution varies among atoms. Assigning formal charges is crucial for evaluating resonance structures and ensuring charge neutrality in the overall molecule.
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Related Practice
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In N2O, nitrogen is the central atom and the oxygen atom is terminal. In OF2, however, oxygen is the central atom. Use formal charges to explain why.
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Draw the Lewis structure (including resonance structures) for methyl azide (CH3N3). For each resonance structure, assign formal charges to all atoms that have formal charge.
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What are the formal charges of the atoms shown in red?
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What are the formal charges of the atoms shown in red?
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