Formal Charge

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Core Concept

Formal charge is a theoretical charge assigned to atoms in a molecule, calculated based on the assumption that electrons in all chemical bonds are shared equally between atoms.

Purpose: The formal charge helps chemists determine the most plausible Lewis structure and to predict the distribution of charges within a molecule.

Formal charge = (Valence electrons) - (Non-bonding electrons) - (1/2 of bonding electrons).

Practice Tips

  • Neutral Molecules: The sum of all formal charges in a neutral molecule must equal zero.

  • Ions: The sum of all formal charges in a polyatomic ion must equal the ion's overall charge.

  • Minimization: Structures with formal charges closest to zero are typically the most stable.

  • Electronegativity: Negative formal charges are more stable on more electronegative atoms, and positive charges are better on less electronegative atoms.

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How to Calculate Formal Charge

The formula to calculate the formal charge (FC) of an atom in a molecule is:

Formal Charge=Valence Electrons−(Nonbonding Electrons+12×Bonding Electrons)\text{Formal Charge} = \text{Valence Electrons} - (\text{Nonbonding Electrons} + \frac{1}{2} \times \text{Bonding Electrons})Formal Charge=Valence Electrons−(Nonbonding Electrons+21​×Bonding Electrons)

  • Valence Electrons: The number of electrons in the atom’s outer shell when isolated.

  • Nonbonding Electrons: Electrons that are not shared (lone pairs).

  • Bonding Electrons: Electrons involved in bonds (shared between atoms).

Step-by-Step Process to Assign Formal Charges

  • Step 1: Draw the Lewis structure for the molecule or ion.

  • Step 2: Identify the number of valence electrons for each atom.

  • Step 3: Count the number of lone pair (nonbonding) electrons on each atom.

  • Step 4: Count the number of electrons in bonds and divide by 2 to find the bonding electrons for each atom.

  • Step 5: Use the formal charge formula to calculate the formal charge on each atom.

Why is Formal Charge Important?

  • Molecular Stability: The best or most stable Lewis structure usually has formal charges as close to zero as possible.

  • Charge Distribution: Formal charges can reveal the location of electron density, which is important for understanding molecular reactivity.

  • Choosing the Correct Structure: When multiple Lewis structures are possible (resonance), the one with the lowest formal charge or more negative charges on more electronegative atoms is generally the correct one.

Rules for Evaluating Formal Charge

  • Rule 1: The sum of formal charges in a neutral molecule must be zero.

  • Rule 2: The sum of formal charges in an ion must equal the ion’s overall charge.

  • Rule 3: Smaller formal charges (closer to zero) are more favorable.

  • Rule 4: Negative formal charges should reside on more electronegative atoms, and positive formal charges should be on less electronegative atoms.

Choosing the Most Stable Structure

  • Best Lewis Structure: The most stable Lewis structure is the one where formal charges are minimized or where negative charges are on the most electronegative atoms.

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