Formal Charge and Determining Preferred Lewis Structures
What Is Formal Charge?
Definition
Formal charge
Formal charge is a theoretical concept that assigns a charge to each atom in a molecule or ion. It assumes that bonding electrons are shared equally between the bonded atoms.
The formula for calculating formal charge is:
$$
\text{Formal Charge (FC)} = \text{VE} - (\text{NBE} + \frac{1}{2} \text{BE})
$$ where:
- VE = Number of valence electrons the atom has in its free (unbonded) state.
- NBE = Number of non-bonding (lone pair) electrons assigned to the atom.
- BE = Number of bonding electrons assigned to the atom (count all electrons in bonds directly attached to the atom).
Formal charge is a useful tool to evaluate which Lewis structure is most likely to represent a molecule's actual bonding arrangement.
Steps to Calculate Formal Charge
- Draw the Lewis Structure: Ensure all valence electrons are accounted for, and aim to satisfy the octet rule wherever possible.
- Assign Electrons to Atoms:
- Lone pair electrons belong entirely to the atom they are on.
- Bonding electrons are split equally between the two atoms in the bond.
- Apply the Formula: Use the formal charge formula for each atom in the structure.
