What is how to calculate formal charge?

Formal Charge: A Guide

The Formal Charge is 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. It's a tool that helps in determining the most likely Lewis structure for a molecule. It does not represent the actual charge distribution on the molecule.

Formula:

The formula to calculate formal charge is:

Formal Charge = (Valence Electrons) - (Non-bonding Electrons) - (1/2 Bonding Electrons)

Where:

• Valence Electrons: The number of electrons in the outermost shell of the neutral, isolated atom. You can determine this from the atom's group number on the periodic table.
• Non-bonding Electrons: The number of electrons that are not involved in bonding. These are the lone pair electrons.
• Bonding Electrons: The number of electrons shared in bonds with other atoms. Each single bond represents 2 electrons.

Steps to Calculate Formal Charge:

1. Draw the Lewis Structure: Start by drawing the correct Lewis structure for the molecule or ion. This is crucial because the structure determines the number of bonding and non-bonding electrons around each atom.
2. Identify Valence Electrons: Determine the number of valence electrons for each atom in the molecule or ion from its position in the Periodic Table.
3. Count Non-bonding Electrons: Count the number of non-bonding electrons (lone pairs) associated with each atom in the Lewis structure.
4. Count Bonding Electrons: Count the number of electrons in the bonds around each atom. Remember to only take half of this number because the other half is attributed to the other atom in the bond.
5. Apply the Formula: Plug the values into the formal charge formula and calculate the formal charge for each atom.

Using Formal Charge to Evaluate Lewis Structures:

Formal charge is used to determine the most stable Lewis Structures when multiple structures are possible. The best Lewis structure will have:

• Formal charges as close to zero as possible.
• Negative formal charges on the more electronegative atoms.
• Avoidance of large formal charges (+2, -2, etc.)

Important Notes:

• The sum of the formal charges of all atoms in a neutral molecule must equal zero.
• The sum of the formal charges of all atoms in an ion must equal the charge of the ion.
• Formal charge is a theoretical concept, not an actual charge.
• Resonance Structures can be evaluated using formal charge.