Formal Charge

Formal charge is a chemical measurement used to understand the way electrons are allocated throughout the molecule or the ion. Each atom in the molecule or ion has a specific charge assigned to it, helping us understand how electrons are distributed around the atom.

In this article, we will explore what is a formal charge, how to calculate a formal charge, importance of formal charge along with a few examples based on it.

What is Formal Charge?

Formal charge is the charge that an atom would have regardless of electronegativity if all bonding electrons were distributed evenly amongst them. It refer to the charge that the particular atom has in any molecule. It is the parameter that helps in calculating the locations of electrons in a molecule and to define the Lewis structure correctly.

The formal charge helps in finding out the most stable Lewis structure by minimizing the charges on the atoms, thus, the structure with stable atoms is selected. Formal charge can be positive, negative, or zero.

Note: Formal charges are hypothetical charges and do not represent the actual charge distribution in a molecule. They just provide valuable insights into the electronic structure and stability of molecules.

Formal Charge Definition

Formal charge is defined as the difference between the number of electrons assigned to that atom in Lewis structure and its valence electron in its elemental state.

Formal charge also determines whether an atom has more protons (positive charge) or electrons (negative charge).

Formal Charge Formula

The formula for calculating formal charge is:

FC = V – (N + B/2)

Where:

• V is the valence electrons of the neutral atom,
• N is the non-bonding valence electrons, and
• B is the total shared electrons.

How to Calculate Formal Charge?

Formal charge calculation is a method used to determine the charge distribution in a molecule. To calculate the formal charge of a molecule follow these simple steps:

Step 1: Determine the Number of Valence Electrons

For each atom in the molecule, count the number of valence electrons. This is the number of electrons in the outermost shell of the atom.

Step 2: Assign Electrons to Atoms in the Molecule

Assign all non-bonding electrons (lone pairs) to the atom to which they belong. For each covalent bond, divide the electrons in the bond equally between the two atoms involved.

Step 3: Calculate the Formal Charge

Use the formula for formal charge i.e.

FC = V – (N + B/2)

Subtract the sum of number of lone pair electrons and half the number of bonding electrons from the total number of valence electrons to calculate the formal charge of the given atom.

Step 4: Repeat for Each Atom

Repeat steps 1-3 for each atom in the molecule to determine the formal charge for each individual atom.

Step 5: Analyze the Results

After calculating the formal charges for each atom, assess the overall charge distribution in the molecule. The sum of the formal charges should be equal to the overall charge of the molecule or ion. If the molecule is neutral, the sum of the formal charges should be equal to zero.

Here are two examples of formal charge calculation:

Formal Charge Calculation in Ammonia (NH₃)

In the composition of ammonia the one nitrogen atom is grouped with three hydrogen atoms. To calculate the formal charge on the nitrogen atom, follow these steps:

• Determine the Number of Valence Electrons of each atom: Nitrogen (N) contributes 5 valence electrons, while each hydrogen (H) contributes 1 valence electron.

• Assign Electrons to Atoms in the Molecule: In ammonia, each hydrogens is bound with nitrogen by 2 electrons Hence for hydrogen B=2, for nitrogen, there will be three bonds with hydrogen. Hence B = 3 × 2 = 6 electrons.

• Calculate the Formal Charge of each atom:
  • For Nitrogen: FC = 5 – (2 + 6 × 1/2) = 0
  • For Hydrogen: FC = 1 – (0 + 2 × 1/2) = 0

Therefore, the formal charge in ammonia is Formal charge on Nitrogen + Formal charge on Hydrogen = 0 + 0 = 0.

Formal Charge Calculation in Cabon dioxide(CO₂)

In carbon dioxide, we have two oxygen atoms bonded with one carbon atom. To calculate the formal charge on carbon dioxide, follow these steps:

• Determine the Number of Valence Electrons of each atom: Carbon has 4 valence electrons, while each oxygen has 6 valence electron.
• Assign Electrons to Atoms in the Molecule: In carbon dioxide, carbon has 0 non-bonding electrons (lone pairs) and 8 bonding electrons. For each oxygen, there are 4 non-bonding electrons (two lone pairs) and 4 bonding electrons (from the double bond with carbon).
• Calculate the Formal Charge of each atom:
  • For Carbon: FC = 4 – (0 + 8 × 1/2) = 0
  • For Oxygen: FC = 6 – (4 + 4 × 1/2) = 0

Therefore, the formal charge on carbon dioxide is Formal charge on carbon + Formal charge on oxygen = 0 + 0 = 0.

Significance of Formal Charge

The significance of formal charge is as follows:

• Molecular Structure: In terms of a molecule, the formal charges of zero on an atom imply that the molecule is in the most stable state with the lowest energy. This implies that there is no free electron pair that is rotating around the molecular orbitals. If there are several possible molecule structures, the one with the lowest formal charge and the most stabilization is the most stable.

• Resonance Structures: The structure having the lowest formal charge for each atom is the most favorable when considering the preferred Lewis structure of (or the predominant resonance structure of) a molecule.

• Stability: The formal charge system tracks all the valence electrons each atom brings with it when forming a molecule. A molecule with a formal charge closer to zero is more stable.

Hence we can conclude that the importance of formal charge lies in its ability to:

• Determine if a Lewis structure is correctly drawn.

• Identify the most valid Lewis structure among multiple possibilities.

• Indicate the most likely site of chemical reactions within a molecule.

• Estimate the distribution of electric charge within a molecule.

Read More,

• Hybridization
• Hybridization of NO₂
• Hybridization of NO₃

FAQs on Formal Charge

What is formal charge and resonance?

Formal charge, which is a method to distribute electrons in a molecule to check if it is stable or not, is seen as an approach used commonly. The interchangeability in a molecule’s way of drawing its structure without changing its overall molecular formula is referred to as resonance.

How do you identify a formal charge?

To calculate formal charge, deduct a number of electrons of lone pairs and half of the number of electrons in the bonds from the valence electrons of an atom.

What is the difference between formal charge and actual charge?

Formal charge is an imaginary charge that is used in the molecular stability analysis; actual charge is the real charge of an atom when interacting in a molecule or tie bond formation.

Does formal charge show electronegativity?

The formal charge, although directly doesn’t show the electronegativity, but it can help to understand the nature of the bonds, where the electrons are shared, between various atoms based on their electronegativity.

What if the formal charge is negative?

Negative formal charge implies the atom has captured additional electrons compared to those required by its valence, implying that it is more reactive or unstable.

What is the formal charge of OH?

The formal charge of OH is -1

How to calculate the formal charge of O₃?

To calculate the formal charge of O₃ (ozone), you can use the formula:

Formal Charge = [Number of valence electrons] – [non-bonding valence electrons] – [bonding electrons / 2]

For the first oxygen atom with a double bond, you have: 6 (valence electrons) – 4 (nonbonding valence electrons) – 4 (bonding electrons) / 2 = 0

For the central oxygen atom with two bonds, you have: 6 (valence electrons) – 2 (nonbonding valence electrons) – 6 (bonding electrons) / 2 = +1

For the final oxygen atom with a single bond, you have: 6 (valence electrons) – 2 (nonbonding valence electrons) – 1 (bonding electron) / 2 = -1

Adding up the formal charges for each atom, you get:0 (first oxygen atom) + 1 (central oxygen atom) – 1 (final oxygen atom) = 0 So, the formal charge of O₃ is 0