What is formal charge in Lewis structures?

Short Answer

Formal charge in Lewis structures is a number assigned to an atom that helps show how electrons are distributed in a molecule. It is calculated by comparing the number of valence electrons an atom has in its free state with the number of electrons it appears to have in a Lewis structure. Formal charge helps chemists identify the most stable or correct structure of a molecule.

Atoms in a molecule usually prefer formal charges close to zero because this indicates stability. The concept is useful for choosing between multiple possible Lewis structures and understanding charge distribution within a molecule.

Detailed Explanation :

Formal Charge in Lewis Structures

Formal charge is an important concept used in drawing and analysing Lewis structures. It helps chemists determine how electrons are shared or distributed within a molecule or polyatomic ion. The formal charge is not the real charge on an atom but a bookkeeping tool used to decide which Lewis structure is the most likely and stable.

In simple terms, formal charge tells us whether an atom in a Lewis structure has “gained,” “lost,” or kept the same number of electrons compared to its neutral state. The most stable Lewis structures are usually those in which atoms have formal charges close to zero and where negative charges appear on more electronegative atoms.

Definition of Formal Charge

Formal charge is defined as the difference between the number of valence electrons an atom has in its free state and the number of electrons assigned to it in a Lewis structure.

It helps determine:

  • Stability of a molecule
  • Correctness of alternative Lewis structures
  • Charge distribution
  • The most probable bonding arrangement

Formula for Calculating Formal Charge

The formula for formal charge is:

Formal Charge = (Valence electrons) – (Non-bonding electrons) – (Bonding electrons ÷ 2)

Where:

  • Valence electrons = electrons in the outer shell of the free atom
  • Non-bonding electrons = lone pair electrons on that atom
  • Bonding electrons ÷ 2 = electrons shared in bonds, counted half for each atom

This formula helps assign electrons logically rather than assuming actual charge transfer.

How to Calculate: Step-by-Step Example

Consider the nitrogen atom in ammonia (NH₃).

  1. Nitrogen has 5 valence electrons.
  2. In NH₃, nitrogen has 2 lone-pair electrons.
  3. It forms 3 single bonds, contributing 3 bonding pairs = 6 electrons, so nitrogen gets half → 3 electrons.

Formal charge = 5 – 2 – 3 = 0

This means nitrogen has a formal charge of zero, indicating stability.

Why Formal Charge Is Important

Formal charge helps in multiple ways:

  1. Choosing the Best Lewis Structure

Some molecules have more than one possible Lewis structure. The best structure is usually the one in which:

  • Most atoms have formal charge = 0
  • Negative charges are on electronegative atoms
  • Positive charges are on electropositive atoms

For example, CO₂ has two possible structures:

  • O═C═O (formal charges = 0, 0, 0 → stable)
  • O–C≡O⁺ or O≡C–O⁺ (higher charges → less stable)

Formal charge helps identify the correct structure.

  1. Understanding Charge Distribution

Formal charge shows where the electron density is located. Even if the molecule is neutral overall, some atoms may have partial charges.

For example:
In NO₂⁻ (nitrite ion), formal charges explain:

  • One oxygen with a −1 charge
  • One oxygen with 0 formal charge
  • Nitrogen with +1 charge

This clarifies why the ion carries an overall charge of −1.

  1. Predicting Reactivity

Atoms with positive formal charge tend to attract electrons, while those with negative formal charge tend to donate or share electrons. This helps predict how molecules interact in reactions.

  1. Explaining Resonance Structures

Formal charge helps compare the stability of resonance forms.
For example, in ozone (O₃), different resonance structures show charges shifting between oxygen atoms. Formal charge helps show that the true structure is a hybrid.

Examples of Formal Charge Calculation

Carbon dioxide (CO₂)
Structure: O═C═O

Oxygen: valence = 6

  • Non-bonding electrons = 4
  • Bonding = 4 electrons (2 pairs) → 2 electrons

Formal charge = 6 – 4 – 2 = 0

Carbon: valence = 4

  • Non-bonding = 0
  • Bonding = 8 electrons → 4 electrons

Formal charge = 4 – 0 – 4 = 0

All atoms have zero formal charge → most stable structure.

General Rules for Stability Based on Formal Charge

A Lewis structure is considered more stable when:

  • Formal charges are minimal
  • Negative formal charge is on the more electronegative atom
  • Positive formal charge is on the less electronegative atom
  • The sum of all formal charges equals the overall charge of the molecule or ion

These rules guide chemists in checking the correctness of Lewis structures.

Conclusion

Formal charge in Lewis structures is a method of assigning electrons to atoms to understand how charge is distributed within a molecule. It helps identify the most stable Lewis structure, predict reactivity, and compare resonance forms. While formal charge does not represent the real charge on an atom, it is a powerful tool for analysing bonding and ensuring the accuracy of molecular structures.