(2351) S2.2.14 Formal charge (HL)

Formal Charge Overview

  • Definition of Formal Charge:

    • The charge an atom would have if all atoms in a molecule had the same electronegativity.

    • Used to determine the preferred Lewis structure when multiple structures are possible.

    • Preferred structure has individual atoms with formal charges closest to zero.

Calculating Formal Charge

  • Formula:

    • Formal Charge = V - N - 1/2B

      • V = Number of valence electrons in the atom

      • N = Number of non-bonding electrons

      • B = Number of bonding electrons

Example 1: Sulfur Dioxide (SO2)

  • Possible Lewis Structures:

    • Structure 1: One single bond, one double bond

    • Structure 2: Two double bonds

Formal Charge Calculations

  • Left Structure:

    • Oxygen (Left):

      • Formal Charge = 6 (V) - 6 (N) - 1/2(2) (B) = -1

    • Sulfur:

      • Formal Charge = 6 (V) - 2 (N) - 1/2(6) (B) = +1

    • Oxygen (Right):

      • Formal Charge = 6 - 4 - 1/2(4) = 0

  • Middle Structure:

    • Formal Charges: 0 (Oxygen) +1 (Sulfur) -1 (Oxygen)

  • Right Structure:

    • Oxygen (Left): 0

    • Sulfur: 0

    • Oxygen (Right): 0

Preferred Lewis Structure for SO2

  • The structure on the right is preferred as all atoms have a formal charge of zero.

Example 2: Dinitrogen Monoxide (N2O)

  • Possible Lewis Structures:

    • Structure 1: One triple bond, one single bond

    • Structure 2: Two double bonds

Formal Charge Calculations

  • Left Structure:

    • Left Nitrogen: 0

    • Middle Nitrogen: +1

    • Oxygen: -1

  • Right Structure:

    • Left Nitrogen: -1

    • Middle Nitrogen: +1

    • Oxygen: 0

Preferred Lewis Structure for N2O

  • The left structure is preferred as the negative charge is on the more electronegative oxygen atom.

Example 3: Nitrate Ion (NO3-)

  • Possible Lewis Structures:

    • Structure 1: Nitrogen with single and double bonds

    • Structures yield the same formal charges: +1 for Nitrogen, 0 for double-bonded Oxygen, -1 for each single-bonded Oxygen.

Formal Charge Calculations

  • Left Structure:

    • Nitrogen: +1

    • Oxygen (Double Bond): 0

    • Oxygens (Single Bond): -1 each

  • Overall Charge:

    • The sum of the formal charges equals the overall charge of -1 for the nitrate ion, confirming the validity of the structures.