Difference Between Formal Charge and Oxidation State

The key difference between formal charge and oxidation state is that formal charge is the charge of an atom in a molecule we calculate assuming that electrons in chemical bonds are shared equally between atoms whereas oxidation state is the number of electrons an atom loses or gains or shares with another atom.

Formal charge and oxidation state are different terms though we usually assume they are the same. The formal charge determines the number of electrons that occur around an atom of a molecule while oxidation state determines the number of electrons exchanged between atoms during the formation of a molecule.

CONTENTS

1. Overview and Key Difference
2. What is Formal Charge 
3. What is Oxidation State
4. Side by Side Comparison – Formal Charge vs Oxidation State in Tabular Form
5. Summary

What is Formal Charge?

The formal charge is the charge of an atom in a molecule we calculate assuming that electrons in chemical bonds are shared equally between atoms. Therefore, when determining the formal charge, we are comparing the number of electrons around a neutral atom and the number of electrons around that atom when it is in a molecule. In this determination of the formal charge, we have to assign the electrons of the molecule to individual atoms. Here, we need to consider the following requirements as well;

1. We have to assign nonbonding electrons to the atom in which the non-bonding electrons occur
2. We have to divide the bonding electrons equally between the shared atoms

The mathematical relationship for this calculation is as follows:

Formal charge = (number of valence electrons in neutral atom)- (number of lone pair electrons) – ({1/2}*bond pair electrons)

Let us consider an example to understand this phenomenon. For ammonia molecule, there are three N-H bonds and a lone electron pair on the nitrogen atom. Then when can calculate the followings;

Formal charge of N = 5 – 2 – {1/2}*6 = 0

Formal charge of H = 1 – 0 – {1/2}*2 = 0

What is Oxidation State?

Oxidation state is the number of electrons a particular atom can lose, gain or share with another atom. This term is applied to any chemical element in molecules (the term oxidation number is applied mainly to central metal atoms of coordination complexes though we use these terms interchangeably). Oxidation state actually gives the degree of oxidation of an atom in a compound. We should always give the oxidation state as a whole number, and it is represented in Hindu-Arabic numbers, including the charge of the atom. For example, the oxidation state of iron in FeO is +2.

Tips for the Determination of Oxidation State:

1. Oxidation state of a single element is zero (this includes the molecules made of a single element as well).
2. The total charge of a molecule or ion is the sum of the charges of each atom.
3. The oxidation state of alkali metals is always +1, and for alkali earth metals it is +2.
4. Meanwhile, the oxidation state of fluorine is always -1.
5. Further, the oxidation state of hydrogen is usually +1. But sometimes it is -1 (when bound to alkali or alkali earth metals.)
6. Also, generally, the oxidation state of oxygen is -2 (but in peroxides and super-oxides it may vary).
7. The most electronegative atom in a molecule gets the negative charge, and the other one gets the positive charge.

Oxidation state is very useful for the determination of products in redox reactions. Redox reactions are chemical reactions that include electron exchange between atoms. In redox reactions, two half-reactions occur parallel simultaneously. One is the oxidation reaction, and the other one is the reduction reaction. The oxidation reaction involves the increase of oxidation state of an atom, while reduction reaction involves the decrease of the oxidation state of an atom.

What is the Difference Between Formal Charge and Oxidation State?

The key difference between formal charge and oxidation state is that the formal charge is the charge of an atom in a molecule that we calculate assuming that electrons in chemical bonds are shared equally between atoms whereas oxidation state is the number of electrons an atom loses or gains or shares with another atom. For example, the formal charge of the nitrogen atom in the ammonia molecule is 0, while the oxidation state is +3.

Below infographic summarizes the difference between formal charge and oxidation state.

Summary – Formal Charge vs Oxidation

Formal charge and oxidation state are different terms though some people assume they are the same. The key difference between formal charge and oxidation state is that the formal charge is the charge of an atom in a molecule that we calculate assuming that electrons in chemical bonds are shared equally between atoms whereas oxidation state is the number of electrons an atom loses or gains or shares with another atom.

Reference:

1. “2.3: Formal Charges.” Chemistry LibreTexts, Libretexts, 25 July 2019, Available here.

Image Courtesy:

1. “Formal charge Principle V.1” By Jü – Own work (Public Domain) via Commons Wikimedia
2. “Oxidation State Assignments” By SARANPHONG YIMKLAN – Own work (Public Domain) via Commons Wikimedia