Formal Charge Formula from Lewis Structure is an important concept in Class 11 Chemistry that helps determine the most stable Lewis structure of a molecule or ion. The Class 11 Chemistry notes of Anand Classes, written by Neeraj Anand, explain the formal charge formula with clear theory, step-by-step calculations, solved examples, and exam-oriented practice questions. Designed for CBSE, JEE Main, JEE Advanced, and NEET preparation, these study materials help students build strong conceptual understanding and improve problem-solving accuracy in chemical bonding.
What is a Formal Charge?
A Formal charge is also known as a Fake Charge. It’s a theoretical charge over an individual atom of an ion as the real charge over a polyatomic molecule or ion is distributed on an ion as a whole and not over a single atom.
How to calculate Formal Charge from Lewis structure ?
In case of polyatomic ions, the net charge is possessed by the ion as a whole and not by a particular atom. It is possible, however, to assign a formal charge on each atom.
The formal charge of an atom in a polyatomic ion or molecule is defined as the difference between the number of valence electrons in an isolated (or free) atom and the number of electrons assigned to that atom in a Lewis structure.
It may be expressed as :
Formal charge on an atom in a Lewis structure = [total number of valence electrons in free atom] – [total number of nonbonding (lone pair) electrons] – ½ [total number of bonding (shared) electrons]
FC = V – L – ½ S
where,
FC = Formal charge on an atom
V = Total number of valence electrons in the free atom
L = Total number of electrons present as non-bonding (lone pair)
S = Total number of bonding (shared) electrons
The counting is based on the assumption that each atom owns one electron of each bonding pair and both the electrons of a lone pair.
We can count the number of electrons assigned to an atom on this assumption and then compare the number with the number of electrons in the free atom.
If the atom has more electrons in the molecule than in the free or neutral atom, then the atom has a negative formal charge. On the other hand, if the atom has less electrons in the molecule than in the free or neutral atom, then it has a positive formal charge.
Understand related topics like Rules for writing Lewis formula structures of molecules and ions
How to calculate formal charge on oxygen atoms in ozone (O3) molecule from Lewis structure ?
Let us consider ozone molecule. The ozone molecule may be represented as :
The central O atom marked 1 has 6 valence electrons, one lone pair (2 electrons) and three bonds (or 6 bonding electrons). Therefore, its formal charge is
FC on O (1) = 6 – 2 – ½ × 6 = + 1
The end O atom marked 2 has 6 valence electrons, two lone pairs (4 electrons) and two bonds (4 bonding electrons). Therefore, its formal charge is
FC on O(2) = 6 – 4 – ½ × 4 = 0
The end O atom marked 3 has 6 valence electrons, three lone pairs (6 electrons) and one bond (2 bonding electrons). Therefore, its formal charge is
FC on O (3) = 6 – 6 – ½ × 2 = –1
Therefore, the formal charges on the oxygen atoms in the Lewis structures of ozone molecule are written as :
It may be noted that formal charges do not indicate real charge separation within the molecules. These only help in keeping track of the valence electrons in the molecule. The formal charges help in the selection of the lowest energy structure from a number of possible Lewis structures for a given molecule or ion.
In general, the lowest energy structure is the one, which has lowest formal charges on the atoms.
You may also like to study Lewis Structures of multiple covalent bonds molecules (O₂, CO₂, N₂, C₂H₄, C₂H₂)
How to calculate formal charge on carbon, oxygen and chlorine atoms in phosgene (COCl2) molecule from Lewis structure ?
Let us consider another example of phosgene (COCl2) which is a poisonous gas.
Important related topics are Covalent Bond, Formation of Hydrogen (H₂), Chlorine (Cl₂), Hydrogen Chloride (HCl), Water (H₂O), Ammonia (NH₃) Molecules
How to calculate formal charge on oxygen atoms in sulphate (SO42-) ion from Lewis structure ?
The Lewis structure of SO42- is as follows:
1,2,3 and 4 are for the indexing of oxygen while calculating the formal charge.
| Atom | Valence e– in free state | No. of non-bonding e– in Lewis structure | No. of bonding pairs in Lewis structure | Formal Charge |
| Sulphur (S) | 6 | 0 | 12 | = 6 – 0 – 12/2=6-6=0 |
| Oxygen (O) – 1 | 6 | 4 | 4 | = 6 – 4 – 4/2= 6 -6= 0 |
| Oxygen (O) – 2 | 6 | 6 | 2 | = 6 -6 -2/2= 6 -7= -1 |
| Oxygen(O) – 3 | 6 | 4 | 4 | = 6 – 4 – 4/2= 6 -6= 0 |
| Oxygen(O) – 4 | 6 | 6 | 2 | = 6 -6 -2/2= 6 -7= -1 |
Important exam-related topics include JEE Main PYQs Previous Year Questions MCQs : Classification of Elements and Periodicity in Properties
What is the Importance of Formal charge ?
- The formal charge being a theoretical charge doesn’t indicate any real charge separation in the molecule.
- Formal charges help in the selection of the lowest energy structure from a number of possible Lewis structures for a given species.
- Knowledge of the lowest energy structure helps in predicting the major product of a reaction and also describes a lot of phenomena.
- Generally, the lowest energy structure is the one with the smallest formal charges on the atoms and the most distributed charge.
Important Classification of Elements and Periodicity in Properties Chapter Interlinks
This section provides a complete and interconnected study of Classification of Elements and Periodicity in Properties, starting with detailed theory and notes for Class 11 Chemistry to build a strong conceptual foundation. You can explore atomic radius and its types including covalent, van der Waals, metallic, and ionic radii to understand periodic trends in atomic size. It also includes Screening Effect (Shielding Effect) : Calculation of Effective or Reduced Nuclear Charge (Slater’s Rules), which explains how inner electrons reduce the nuclear attraction on outer electrons and influence periodic trends. In addition, topics like Radius of Cation is Less and Anion is More Than Its Parent Atom, Size Variation in Isoelectronic Series help explain how ionic size changes due to gain or loss of electrons and how nuclear charge affects size in species with the same number of electrons. The causes of periodicity explain why elements show repeating properties based on electronic configuration, which is further supported by the modern periodic law and structure of the modern periodic table including groups, periods, and blocks for elements even beyond atomic number 100. The historical development is covered through Mendeleev’s periodic law and table, leading to the modern classification of elements into s, p, d, and f blocks with prediction of period, group, and block. To strengthen exam preparation, you can practice JEE Main PYQs, IMU CET PYQs and Merchant Navy sponsorship exam MCQs, and other previous year questions with solutions, along with solved examples, conceptual questions, and practice problems on the modern periodic table. Learn more in this section also to radius of cation is less and anion is more than its parent atom and size variation in Isoelectronic Series. Additionally, complete study material, mock tests, and guidance are provided under Anand Classes Chemistry notes, along with expert support from Er Neeraj Anand, making this section a comprehensive resource for competitive exam preparation. This section also includes detailed study of What is Ionization Enthalpy? Definition, Units, Factors and Successive IE and Ionization Enthalpy Trends Along a Period and Down a Group for better understanding of periodic properties and reactivity of elements.