Explain Formal Charges in the C₂H₆ Structure

Formal charge is a bookkeeping method used to assign "ownership" of valence electrons within a molecule. It helps determine whether a Lewis structure is stable—structures where all atoms have formal charges near zero are generally the most stable.

Formula for Formal Charge

FC = [Valence e⁻] − [Lone-pair e⁻] − ½ × [Bonding e⁻]

1) Formal Charge on Carbon (C)

In ethane, both carbon atoms are equivalent. Each carbon is bonded to one other carbon and three hydrogens (four bonds total).

• Valence e⁻: 4
• Lone-pair e⁻: 0
• Bonding e⁻: 4 bonds × 2 e⁻/bond = 8 e⁻

FC(C) = 4 − 0 − (8 ÷ 2) = 4 − 0 − 4 = 0

2) Formal Charge on Hydrogen (H)

Each of the six hydrogens is identical and bonded to one carbon atom (single bond).

• Valence e⁻: 1
• Lone-pair e⁻: 0
• Bonding e⁻: 1 bond × 2 e⁻/bond = 2 e⁻

FC(H) = 1 − 0 − (2 ÷ 2) = 1 − 0 − 1 = 0

Every atom in the ethane molecule (C₂H₆) has a formal charge of 0. This confirms that the Lewis structure is fully balanced and represents a highly stable, neutral molecule.