Overview
This lecture explains how to calculate formal charge in molecules using a simple mnemonic (V-B-D) and walks through several common examples.
Formal Charge Calculation Method
- Formal charge helps determine the distribution of electrons in a molecule.
- Use the mnemonic "We Break Down" (V-B-D): V = Valence electrons, B = Number of bonds, D = Number of lone pair electrons (dots).
- Formula: Formal charge = V − B − D.
Valence Electrons for Common Atoms
- Hydrogen (H): 1 valence electron
- Carbon (C): 4 valence electrons
- Nitrogen (N): 5 valence electrons
- Oxygen (O): 6 valence electrons
- Fluorine (F): 7 valence electrons
Example Calculations
HCN Molecule
- Carbon: 4 (valence) − 3 (bonds) − 2 (dots) = −1 formal charge
- Nitrogen: 5 (valence) − 3 (bonds) − 2 (dots) = 0 formal charge
Hydronium Ion (H₃O⁺)
- Oxygen: 6 (valence) − 3 (bonds) − 2 (dots) = +1 formal charge
- Each Hydrogen: 1 (valence) − 1 (bond) − 0 (dots) = 0 formal charge
Carbon Dioxide (CO₂)
- Carbon: 4 (valence) − 4 (bonds) − 0 (dots) = 0 formal charge
- Each Oxygen: 6 (valence) − 2 (bonds) − 4 (dots) = 0 formal charge
Ozone (O₃)
- First Oxygen: 6 (valence) − 3 (bonds) − 2 (dots) = +1 formal charge
- Second Oxygen: 6 (valence) − 2 (bonds) − 4 (dots) = 0 formal charge
- Third Oxygen: 6 (valence) − 1 (bond) − 6 (dots) = −1 formal charge
Key Terms & Definitions
- Formal Charge — The theoretical charge on an atom in a molecule, calculated using V − B − D.
- Valence Electrons (V) — Electrons in the outer shell of an atom.
- Bonds (B) — Total number of covalent bonds formed by the atom.
- Dots (D) — Total number of lone pair electrons on the atom.
Action Items / Next Steps
- Practice calculating formal charge on more complex molecules.
- Memorize common valence electron values for main group elements.