Overview
This lecture explains the concept of formal charge, its calculation, and how it helps determine the most accurate Lewis structures for molecules.
Introducing Formal Charge
- Formal charge helps differentiate between possible Lewis (LE) structures for molecules.
- It indicates which atoms in a molecule gain or lose electrons compared to their neutral state.
- The sum of formal charges in a neutral molecule should be zero; in a polyatomic ion, it should equal the ion's charge.
Calculating Formal Charge
- Formal charge = (Valence electrons) - (Number of bonds) - (Number of lone electrons).
- Lone electrons are unshared, whereas a dash represents a pair (shared electrons).
Applying Formal Charge to Lewis Structures
- Assign formal charges to each atom in all possible structures to determine the most likely real structure.
- Minimize the magnitude of formal charges (as close to zero as possible).
- Place negative formal charge on the more electronegative atom.
- Total formal charges should match the overall charge of the molecule or ion.
Worked Examples
- Ammonia (NH₃): All atoms have a formal charge of zero; structure is valid.
- Sulfite (SO₃²⁻): Oxygen atoms each have -1, sulfur has +1. Sum is -2, matching the ion's charge.
- For molecules with multiple possible Lewis structures, compare the sum and distribution of formal charges:
- The structure with all formal charges zero is preferred.
- If zero is not possible, prefer structures with the smallest total magnitude of formal charges.
- If possible, assign negative charges to the most electronegative elements.
Exam Tips & Applications
- On exams, you may be asked for the formal charge of a specific atom in the best Lewis structure.
- Practice by drawing all valid structures, calculating formal charges, and picking the best one based on the discussed rules.
Key Terms & Definitions
- Formal Charge — The calculated charge on an atom in a molecule, determined by (valence electrons) - (bonds) - (lone electrons).
- Lewis Structure — Diagram showing the bonding between atoms and the distribution of electrons.
- Valence Electrons — Electrons in the outer shell of an atom involved in bonding.
- Lone Electrons — Non-bonded electrons shown as dots in Lewis structures.
- Electronegativity — An atom's tendency to attract shared electrons.
Action Items / Next Steps
- Copy down the three guidelines for selecting the best Lewis structure using formal charge.
- Practice calculating formal charges for atoms in various molecules.
- Be prepared to answer exam questions that ask for formal charges of specific atoms in a molecule.