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Ch.10 - Chemical Bonding I: The Lewis Model
Tro - Chemistry: A Molecular Approach 6th Edition
Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217Not the one you use?Change textbook
All textbooksTro 6th EditionCh.10 - Chemical Bonding I: The Lewis ModelProblem 70
Chapter 10, Problem 70

Use formal charges to identify the better Lewis structure.

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Step 1: Draw the Lewis structures for the molecule or ion. Remember that Lewis structures are based on the concept that shared or bonding electrons come from the atomic orbitals of the bonded atoms. A Lewis structure also helps to make a prediction on the molecular geometry of the molecule.
Step 2: Calculate the formal charge for each atom in the Lewis structure. The formal charge of an atom in a molecule is the hypothetical charge the atom would have if we could redistribute the electrons in the bonds evenly between the atoms. The formula to calculate the formal charge is: Formal Charge = [# of valence electrons on atom] - [non-bonded electrons + number of bonds].
Step 3: Compare the formal charges with the actual charges on the atoms. The Lewis structure that has the charges closest to zero is typically the most stable. Also, any negative charges should reside on the more electronegative atoms.
Step 4: If the formal charges are not equal, the structure with the smallest formal charges (closest to zero) is usually the correct one. If the formal charges are equal, the structure with the negative formal charges on the most electronegative atom is preferred.
Step 5: Remember that the best Lewis structure is the one that has the minimum formal charges on the atoms, and those charges should match the actual charge of the molecule or ion. If multiple structures are possible, the structure with the least formal charge is the best.

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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 between atoms. Understanding how to draw and interpret Lewis structures is essential for visualizing molecular geometry and predicting reactivity.
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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 non-bonding electrons, and the number of bonds. It helps in determining the most stable Lewis structure by minimizing the formal charges across the molecule. A structure with the lowest formal charges is generally preferred, as it indicates a more stable arrangement of electrons.
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Resonance Structures

Resonance structures are different Lewis structures that represent the same molecule, differing only in the arrangement of electrons. They are used when a single Lewis structure cannot adequately depict the actual distribution of electrons. The true structure is a hybrid of these resonance forms, and understanding resonance is crucial for accurately predicting molecular behavior and stability.
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Related Practice
Textbook Question

How important is the resonance structure shown here to the overall structure of carbon dioxide? Explain.

Textbook Question

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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Textbook Question

Write a Lewis structure that obeys the octet rule for each molecule or ion. Include resonance structures if necessary and assign formal charges to each atom. a. SeO2

Textbook Question

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.

Textbook Question

Write a Lewis structure that obeys the octet rule for each ion. Include resonance structures if necessary and assign formal charges to each atom. a. ClO3- b. ClO4- c. NO3- d. NH4+

Textbook Question

Write the Lewis structure for each molecule or ion. a. H3COCH3