How to Calculate Formal Charge?

Formal charge is calculated to help determine the most likely or predominant Lewis structure for a molecule. It's a theoretical charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity.

Here's how to calculate formal charge:

The Formula

The formula for calculating formal charge (FC) is:

FC = VE - [LPE + ½(BE)]

Where:

• FC = Formal Charge
• VE = Number of valence electrons in the free (neutral) atom
• LPE = Number of lone pair electrons (unshared electrons) on the atom in the molecule
• BE = Number of bonding electrons (shared electrons) around the atom in the molecule

Steps to Calculate Formal Charge:

1. Draw the Lewis structure: Start by drawing the Lewis structure of the molecule or ion you are analyzing. Make sure to include all atoms and bonds.

2. Determine the valence electrons (VE): Find the number of valence electrons for the atom in question. This corresponds to its group number in the periodic table. For example:

   • Oxygen (Group 16/6A) has 6 valence electrons.
   • Nitrogen (Group 15/5A) has 5 valence electrons.
   • Carbon (Group 14/4A) has 4 valence electrons.
   • Hydrogen (Group 1) has 1 valence electron.

3. Count lone pair electrons (LPE): Count the number of lone pair electrons associated with the atom in the Lewis structure. Remember that lone pairs are electrons that are not involved in bonding.

4. Count bonding electrons (BE): Count the total number of electrons in the bonds around the atom. Remember that each bond contains two electrons.

5. Apply the formula: Plug the values for VE, LPE, and BE into the formal charge formula and calculate the result.

Example: Carbon Dioxide (CO2)

Let's calculate the formal charge on each atom in carbon dioxide, assuming the structure O=C=O

• Carbon (C):

  • VE = 4
  • LPE = 0
  • BE = 8 (4 bonds x 2 electrons/bond)
  • FC = 4 - [0 + ½(8)] = 4 - 4 = 0

• Oxygen (O):

  • VE = 6
  • LPE = 4
  • BE = 4 (2 bonds x 2 electrons/bond)
  • FC = 6 - [4 + ½(4)] = 6 - 6 = 0

In the O=C=O structure, all atoms have a formal charge of zero, which generally indicates a more stable and likely Lewis structure.

Why is Formal Charge Important?

• Determining the Best Lewis Structure: When multiple Lewis structures are possible, the one with the smallest formal charges (closest to zero) on all atoms is generally the most stable and preferred structure.

• Predicting Molecular Properties: Although formal charge is a theoretical concept, it can provide insights into the charge distribution within a molecule and help predict its reactivity and other properties.

• Assessing Stability: Structures with large formal charges, especially positive charges on highly electronegative atoms (like oxygen or fluorine) and negative charges on electropositive atoms, are typically less stable.

Key Considerations:

• Formal charge is a theoretical charge and doesn't represent the actual charge distribution in a molecule. Actual charge distribution is better described by partial charges, which reflect differences in electronegativity.

• The sum of the formal charges in a molecule should equal the overall charge of the molecule (or ion). For neutral molecules, the sum should be zero.