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How to Calculate Formal Charge?

Published in Chemistry 3 mins read

You can calculate formal charge on an atom within a molecule using a simple formula.

Understanding Formal Charge

Formal charge helps determine the most likely Lewis structure when multiple structures are possible. It assumes electrons in a chemical bond are shared equally between the atoms, regardless of relative electronegativity.

The Formal Charge Formula

The formula to calculate formal charge is:

Formal Charge (FC) = VE - LPE - ½(BE)

Where:

  • VE = Number of valence electrons of the free (neutral) atom. This is found by looking at the element's group number on the periodic table.
  • LPE = Number of lone pair electrons (non-bonding electrons) on the atom in the molecule.
  • BE = Number of bonding electrons (shared electrons) around the atom in the molecule. This is usually determined by the number of bonds multiplied by two since each bond contains two electrons.

Step-by-Step Calculation with Examples

Let's illustrate this with examples:

Example 1: Carbon Dioxide (CO2)

  1. Draw the Lewis Structure: The most common Lewis structure for CO2 has each oxygen double-bonded to the carbon atom: O=C=O

  2. Calculate Formal Charge for Carbon (C):

    • VE (Carbon) = 4
    • LPE (Carbon) = 0
    • BE (Carbon) = 8 (4 bonds x 2 electrons/bond)

    FC (Carbon) = 4 - 0 - ½(8) = 4 - 0 - 4 = 0

  3. Calculate Formal Charge for Oxygen (O):

    • VE (Oxygen) = 6
    • LPE (Oxygen) = 4
    • BE (Oxygen) = 4 (2 bonds x 2 electrons/bond)

    FC (Oxygen) = 6 - 4 - ½(4) = 6 - 4 - 2 = 0

In CO2, with the given Lewis Structure, both Carbon and Oxygen have a formal charge of 0.

Example 2: Ozone (O3)

  1. Draw the Lewis Structure: Ozone has a single bond and a double bond between the oxygen atoms. One oxygen atom will have a formal positive charge and another a formal negative charge. A possible Lewis structure is: O=O-O

  2. Calculate Formal Charge for the Doubly Bonded Oxygen:

    • VE (Oxygen) = 6
    • LPE (Oxygen) = 4
    • BE (Oxygen) = 4 (2 bonds x 2 electrons/bond)

    FC = 6 - 4 - ½(4) = 0

  3. Calculate Formal Charge for the Singly Bonded Oxygen:

    • VE (Oxygen) = 6
    • LPE (Oxygen) = 6
    • BE (Oxygen) = 2 (1 bond x 2 electrons/bond)

    FC = 6 - 6 - ½(2) = -1

  4. Calculate Formal Charge for the Central Oxygen:

    • VE (Oxygen) = 6
    • LPE (Oxygen) = 2
    • BE (Oxygen) = 6 (3 bonds x 2 electrons/bond)

    FC = 6 - 2 - ½(6) = +1

In this Lewis structure of Ozone, the formal charges are 0, -1, and +1, respectively. It is important to remember that formal charge is a tool to evaluate the "best" Lewis structure, and does not represent the actual charge on the atoms. The "best" Lewis structure is usually the one with the fewest formal charges, and where negative formal charges reside on the more electronegative elements.

Key Considerations

  • The sum of all formal charges in a neutral molecule should be zero.
  • The sum of all formal charges in an ion should equal the charge of the ion.
  • Formal charges are not real charges; they are a bookkeeping method to help determine the most plausible Lewis structure.

By following this formula and the steps outlined above, you can effectively calculate formal charges and use them to assess the validity of Lewis structures.

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