Short Answer

The importance of formal charge in determining stability lies in its ability to show how electrons are distributed within a molecule. Molecules are usually more stable when atoms have formal charges close to zero because this means electrons are arranged in a balanced and natural way. Structures with large positive or negative formal charges tend to be less stable.

Formal charge also helps identify which resonance structure or Lewis structure is most accurate. A structure is considered more stable when negative charges are on more electronegative atoms and positive charges are on less electronegative atoms. Thus, formal charge is a key tool for comparing and choosing the most stable molecular structure.

Detailed Explanation :

Importance of Formal Charge in Determining Stability

Formal charge is a simple calculation used in Lewis structures to assign electrons to atoms and understand how charge is distributed within a molecule. Although formal charge does not represent the actual charge on an atom, it is extremely important for identifying the most stable structure among several possible arrangements. Stability in chemical structures depends on how naturally electrons are shared or distributed, and formal charge gives a clear picture of this.

When chemists draw Lewis structures, they often find more than one possible structure for the same molecule. These are called resonance structures or alternative structures. To decide which structure is most likely to exist in nature, we examine the formal charge on each atom. The structure with the most favourable distribution of charges is considered the most stable.

Formal charge helps us recognise electron imbalances. Atoms prefer to have the number of electrons that keeps their charge close to zero. When atoms carry unnecessary positive or negative charges, the structure becomes less stable because the electrons are not evenly or naturally arranged.

1. Stability Increases When Formal Charges Are Zero

A key rule is that molecules are most stable when:

• Each atom has a formal charge of zero, or
• The least number of atoms carry a charge.

A zero formal charge indicates that the atom has neither gained nor lost electrons compared to its natural state. Such atoms are in a comfortable electronic arrangement, contributing to the overall stability of the molecule.

For example, in carbon dioxide (CO₂), the most stable structure is O═C═O because all atoms have a formal charge of zero. This makes the molecule symmetric and energetically favourable.

2. Large Formal Charges Reduce Stability

A structure with large positive or negative formal charges is usually less stable.
Examples of instability include:

• A +2 or −2 formal charge
• Charge separation over adjacent atoms
• Multiple atoms carrying unnecessary charges

When atoms have large charges, they tend to either lose or attract electrons strongly, making the molecule reactive and unstable.

Thus, structures with minimized charges are always preferred.

3. Negative Charge Should Be on More Electronegative Atoms

Electronegativity is the ability of an atom to attract electrons.
Atoms with high electronegativity (like oxygen, fluorine, chlorine) prefer to hold extra electrons.

Therefore:

• A negative formal charge should be placed on a more electronegative atom.
• A positive formal charge should be placed on a less electronegative atom.

This arrangement makes the structure more natural and stable.

For example:
Between these two possible structures for the nitrite ion (NO₂⁻):

• One has a −1 charge on nitrogen (less electronegative).
• One has a −1 charge on oxygen (more electronegative).

The structure with the negative charge on oxygen is more stable.

4. Helps Choose the Best Resonance Structure

Many molecules have multiple resonance structures. Formal charge helps identify which resonance forms contribute more to the actual hybrid structure.

For example, ozone (O₃) has two resonance structures.
Both have:

• One oxygen with a +1 formal charge
• One oxygen with a −1 formal charge
• One oxygen with zero charge

Because both structures distribute charges similarly, they contribute equally to the resonance hybrid.

Thus, formal charge explains why resonance is needed and how much each form contributes.

5. Determines the Correct Arrangement of Atoms

Sometimes Lewis structures with different atom arrangements are possible. Formal charge helps choose the correct arrangement.

For example:
For the molecule HCN, you could write:

• H–C≡N
• H–N≡C

By calculating formal charges:

• H–C≡N gives all atoms a formal charge of 0 → stable
• H–N≡C gives large charges (+1 on N, −1 on C) → unstable

Therefore, formal charge helps identify the correct structural formula.

6. Helps Predict Reactivity

Atoms with non-zero formal charges often act as:

• Reaction centres
• Electron donors or acceptors

A negatively charged atom is more likely to donate electrons, while a positively charged atom is more likely to accept electrons. This helps explain how molecules behave during chemical reactions.

7. Ensures the Structure Matches the Actual Charge of the Molecule

The sum of formal charges must equal the total charge of the molecule or ion.
This helps verify that the Lewis structure is correct and chemically sensible.

Example:

• For SO₄²⁻, the sum of formal charges must equal −2.

Conclusion

Formal charge is important in determining stability because it helps identify whether electrons are distributed in the most natural and balanced way within a molecule. Structures with minimal formal charges, proper placement of negative charges on electronegative atoms, and correct resonance forms are always more stable. Therefore, formal charge is a key tool in selecting the best and most realistic Lewis structure for any molecule or ion.