Problem 105 - Chemistry: An Atoms-Focused Approach

Question

Describe how formal charges are used to choose between possible molecular structures.

Step-by-Step Solution

Answer

Answer: Formal charges help determine the most stable Lewis structure of a molecule by indicating the distribution of charges among atoms when electrons are shared equally between atoms. The most stable structure will have the smallest total formal charge, the least separation among the formal charges, and negative formal charges on more electronegative atoms. For the ozone molecule, the most stable Lewis structure is O=O-O with a double bond between the first two oxygen atoms and a single bond between the second and the third oxygen atoms.

Step 1: Understanding Formal Charge

Formal charge is the electric charge assigned to an atom in a molecule, assuming that electrons are shared equally between atoms. It helps us determine the most stable Lewis structure for a compound. A good Lewis structure is the one that has formal charges close to zero or with the least-separate formal charges. A high absolute value of formal charges indicates that the structure is less likely to be stable.

Step 2: Formula for Calculating Formal Charge

The formula for calculating the formal charge of an atom in a Lewis structure is given by: Formal Charge = Valence electrons - Nonbonding electrons - 1/2(Bonding electrons) Here, Valence electrons are the number of electrons in the outermost shell of the atom. Nonbonding electrons are the electrons that are not involved in any bond formation. They are the lone pairs. Bonding electrons are the electrons involved in bond formation. Each single bond has two electrons, double bonds have four electrons, and triple bonds have six.

Step 3: Calculating Formal Charges for Different Structures

For each possible Lewis structure of a molecule, calculate the formal charges of all the atoms and sum these charges to get the total formal charge for that structure.

Step 4: Comparing Formal Charges of Different Structures

Compare the calculated formal charges for different structures. The most stable structure will be the one with: 1. The smallest total formal charge (closer to zero). 2. The least separation among the formal charges. 3. Negative formal charges on more electronegative atoms.

Step 5: Example – Picking the Most Stable Structure for O3 (Ozone)

Let's consider two possible Lewis structures for ozone: Structure 1: O=O-O with a double bond between the first two oxygen atoms and a single bond between the second and third oxygen atoms. Structure 2: O-O=O with a double bond between the second and third oxygen atoms and a single bond between the first and second oxygen atoms. Calculating the formal charges for both structures: Structure 1: Central O (Formal Charge) = 6 - 4 - 1/2(4) = 0 Terminal O (Double Bond)(Formal Charge) = 6 - 2 - 1/2(4) = +1 Terminal O (Single Bond)(Formal Charge) = 6 - 6 - 1/2(2) = -1 Total formal charge = 0 + 1 - 1 = 0 Structure 2: Terminal O (Double Bond)(Formal Charge) = 6 - 2 - 1/2(4) = +1 Central O (Formal Charge) = 6 - 4 - 1/2(4) = 0 Terminal O (Single Bond)(Formal Charge) = 6 - 6 - 1/2(2) = -1 Total formal charge = 1 + 0 - 1 = 0 Both structures have the same total formal charge. However, in Structure 1, the negative formal charge is on the terminal oxygen atom with a single bond, which is more electronegative than the central oxygen atom. Thus, Structure 1 is the more stable Lewis structure for the ozone molecule.