Formal Charge Calculation Rules

Overview

This lecture explains how to calculate formal charges in Lewis structures, focusing on the rules, calculation procedure, and several worked examples.

Formal charge is the positive or negative charge assigned to atoms in a Lewis structure.
Formula: Formal charge = group number - [number of unshared electrons + ½(number of shared electrons)].
When calculating octet satisfaction, all shared electrons are counted for both atoms.
For formal charges, only unshared electrons and half of the shared electrons are counted for each atom.
Lewis structures must include formal charges for completeness.

In ammonium ion (NH₄⁺), hydrogen has a formal charge of 0, and nitrogen has a formal charge of +1.
For carbon species:
- Carbon with three hydrogens: formal charge of +1 (carbocation).
- Carbon with three hydrogens and a lone pair: formal charge of -1 (carbanion).
- Carbon with two hydrogens and a lone pair: formal charge of 0 (neutral species).
Correctly assigning formal charges helps identify carbocations, carbanions, and neutral species.

Calculate formal charges for three oxygen-containing species with various bonds and lone pairs.
Calculate formal charges for nitrogen-containing species, including ammonium ion and others with different numbers of hydrogens and lone pairs.

Formal Charge — The charge assigned to an atom in a Lewis structure based on electron ownership.
Lewis Structure — Diagram showing bonding between atoms and lone pairs in a molecule.
Group Number — The column number of an element in the periodic table, indicating valence electrons.
Unshared Electrons — Electrons not involved in bonds (lone pairs).
Shared Electrons — Electrons involved in covalent bonds between atoms.
Carbocation — Carbon atom with a positive charge.
Carbanion — Carbon atom with a negative charge.

Practice calculating formal charges for the provided oxygen and nitrogen species.
Be prepared to verify answers in the next class session.