Oxidation State

A Brief Introduction to Oxidation State

The chemical process of oxidation-reduction, better known as redox reaction is a common phenomenon around the world. It is also an essential contributor to the metabolic process, whereby nutrient oxidation leads to energy release and enables life forms to thrive. Exposure of various elements, as well as compounds, causes combustion and release of water, carbon dioxide and energy. 

Thus, to gain a better understanding of reactions like redox and combustion, one must be aware of oxidation state or OS, a chemical characteristic exhibited by various elements.

What Does Oxidation State Mean?

OS refers to a specific number assigned to elements in different chemical combinations. These numbers are a representation of electron quantity lost or gained by an element’s atom to result in a chemical bond with another element.

Also interchangeably used with oxidation number, it is used for the determination of changes which are taking place in a redox reaction. It has a similar numerical representation as valence electrons, but is typically differentiated from formal charge.

To better understand the oxidation state definition, you must also know that elements must act as a reducing agent on oxidation, resulting in the release of electrons. 

Origin and Discovery of Oxidation State

The popular French chemist, Antoine Lavoisier first used oxidation as a means to describe the reaction of oxygen with any substance. Later experiments remarkably identified oxidation to result in electron loss.

Use of the term oxidation was thus expanded to other reactions that identified electron loss, whether or not it involved oxygen, et. Thus, it increased its scope of usage. Such electron losses were thus identified with values known as oxidation state.

One could thus define oxidation number or state by putting a value to such electron losses during a reaction, which usually stood as integers. At times, the OS can also be represented as a fraction. For instance, the OS of iron in Fe3O4 is valued at 8/3.

Before moving on to more about oxidation number or state, take a brief look at the process of oxidation.

What Does Oxidation Entail?

Oxidation refers to a chemical reaction that involves electron movement between the elements of any compound. The process’s character is exhibited when an element donates electrons. It is also denoted by an increased oxidation state.

A common example of oxidation is the reaction of iron (Fe) with oxygen (O2). The reaction of these two elements results in the formation of rust, whereby the electrons lost by iron are gained by oxygen.

Oxidation States – The Highest and the Lowest

While oxidation involves the increase in oxidation state, its decrease is denoted by reduction. However, there have been quantity limits identified for the state, whereby the highest OS has been marked at +9 for tetroxoiridium and the lowest in the case of carbon in methane or CH4 at -4.

Another example is presented in the following figure.

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Figure 1: Example of Oxidation States

Rules to Identify and Assign Oxidation States

Identifying of oxidation states in elements or compounds is based on the following few rules one must take care of. You must also follow these rules in the right order and consider the one appearing first in order in the case of conflict.

Order of Rules



Rule 1

An individual atom in a non-combined state maintains an OS of zero (0).

O2 or oxygen in its non-combined state maintains an OS of ‘0’.

Rule 2

The net charge of any species equals the sum total of OS for all atoms in it.

Here, the following rules apply too.

  1. For neutral species, the total OS sum for all atoms equals ‘0’.

  2. In the case of ions, the total OS sum equals the total charge for that ion.

  1. In NaCl, the total OS sum equals ‘0’, whereby, the OS of Na is +1 and that of Cl is -1.

  2. In a calcium ion (Ca2+), the OS equals its charge, i.e., +2. 

Rule 3

For any compound where the OS assigned for any Group 1 (1A) metal is +1 and for Group 2 (2A) metal is -1, then the OS is positive (+).

In MgCl2, the OS of Mg, a Group 2 Alkaline metal, is +2. For Cl, the OS is -1 as it has 2 atoms, which makes the overall charge zero as per rule 2.

Rule 4

Fluorine’s OS in any compound is -1.

OS of fluorine or ‘F’ in both SF6 and HF are -1.

Rule 5

Hydrogen’s OS in any compound is +1.

OS of hydrogen of ‘H’ in CH4, HI, and NH4+ is +1.

Rule 6

The oxidation state of O2 or oxygen in all compounds is -2.

OS of oxygen in H2O, OH-, and CO32- is -2.

Rule 7

In metal compounds with two elements, the elements of Group 15 (3A) have an OS of -3. For Group 16 (6A) and 17 (17A), it is -2 and -1 respectively.

The Group 17 element Br or Bromine in MgBr2 has an OS of -1.

Some other examples that satisfy the OS rules are mentioned below.

  1. In HN4+, the oxidation state of nitrogen is -3 and that of hydrogen in -1.

  2. In SO42-, oxygen’s OS is -2 and sulphur’s, +6.

  3. For OH-, OS for oxygen stands at -2 and that for hydrogen at -1.

Also, while the oxidation states are mostly represented by integers, some can also have fractional values. Nevertheless, the rules still apply when assigning the OS to elements in a given compound.

If the ions of an element in a given compound can be separated, the OS is determined by the process of fragmenting, which can result in fractional values for the element.

Oxidation Number Calculation

You can calculate an atom’s oxidation number in a given compound by taking care of the following few computation rules.

  1. Remember rule 1 that suggests every atom that is free or non-combined carry an OS of ‘0’. As per rule 2, ions with only one atom carry a charge that equals the ion’s OS.

  2. While the OS in the case of oxygen stands at -2, some exceptions to this rule include – 

  • Each oxygen atom carries an OS of -1 in the case of peroxides.

  • Oxygen atoms carry an OS of –(1/2) in the case of super oxides.

  • When forming a bond with fluorine, the OS of oxygen can vary.

  1. For hydrogen bonding with metals carrying two elements, OS is -1.

  2. OS is positive for halogens like chlorine, iodine and bromine when combined with oxygen.

  3. The sum of oxidation numbers added for different atoms in a compound should equal zero. 

  4. The OS of an ionic compound equals the ion’s charge when calculating it for polyatomic ions.

Quick Practice Questions

Assign oxidation number to the elements of the following compounds using the rules and the calculation method given above.

  • H2SO4

  • CuSO4

  • FeCl3

  • S2O32-

  • NaH

Oxidation States and Numbers: The Difference

Although oxidation number and oxidation state are terms with interchangeable use, they are not exactly the same. The following points state the difference between oxidation state and oxidation number.

Points of Difference

Oxidation States

Oxidation Numbers

  1. Meaning

It refers to the total number of electrons a given atom can accept, donate or share with any other atom in a chemical reaction.

It represents the central atom’s charge in a coordination complex only when the surrounding binds are ionic in nature.

  1. Representation

Its representation is in Arabic numbers, i.e., 1,2,3, etc.

It is represented in a formula by Roman numbers, like I, II, III, etc.

  1. Application

Its application is prevalent to all elements and compounds.

Its application is limited to the use in coordination complex.

  1. Charge indication

Along with a numeric representation, oxidation state also indicates the charge of a given atom via a positive (+) or a negative (-) sign.

It indicates only the numeric value of the central atom and not the charge.

Now, take a look at some other essential details on oxidation state and numbers that can help in calculation.

When expressed in terms of periodic table, the oxidation numbers of various elements can be identified based on the following properties.

  • Alkali metals of Group 1A carry an OS of -1.

  • Alkali earth metals of Group 2A carry an OS of -2.

  • Elements in Group 3A often exhibit an OS of +3. They can also have an OS of +2 or +1.

  • OS for elements in Group 3A remains in the range of -4 and +4.

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Figure 2: Oxidation State Representation on Periodic Table

  • In the case of non-metals, the OS can vary on a wide range. However, when calculated, subtracting the minimum OS from the maximum results in +8.

  • The maximum OS of an element can go up to 7 subtracted by its group number.

  • Elements of Group 1 have an OS of +1.

  • For Group 2, it is +2 and for Group 3, +3.

  • Group 4 elements range between -4 and +4 while Group 5 elements between -3 and +5.

  • Group 6 and 7 elements range from -2 to +6 and -1 to +7.

  • For inert gases, the OS is always zero.

While this was all about oxidation state, its exhibition and properties for different elements, you can carry on with your exam preparation by referring to chemistry solutions as well. Vedantu hosts a range of study material online to assist students with a thorough preparation. Download our Vedantu app today for a streamlined approach to exam preparation.

FAQ (Frequently Asked Questions)

1. How Similar Are The Oxidation States and Numbers?

Although often used interchangeably, oxidation states differ from oxidation numbers in meaning, utility, representation and charge indication. While the former represents the total electrons lost, gained or shared by an atom, the latter indicates the charge of a central atom. Also, OS is represented by Arabic number and is used for all elements and compounds. 

Contrarily, oxidation numbers are represented in Roman numbers and are used in coordination complexes. Plus, they do not exhibit the central atom’s charge. Yet, OS is represented either by a (+) or a (-) symbol.

2. What is Maximum Positive OS Possible For An Element?

The maximum OS that can be assigned to an element can go up to +9 and not beyond.

3. What Uses Can OS be Put to?

Alongside helping determine the charge of an atom in a chemical reaction, OS is also useful in the determination of the strength of acids and bases. While an increase in OS indicates the increasing strength of an acid, it also means a decrease in the strength of a base.

4. What is The OS for Nitrogen?

As nitrogen is a Group 5 element, the oxidation state for nitrogen can range anywhere between -3 and +5 based on the compound it has reacted to form. The lower side of OS represents ammonia and amines while the higher side indicates the presence of nitric acid.