Oxidation Number and Its Calculation for JEE

The oxidation number of an atom in a compound helps to find whether the compound is undergoing oxidation or reduction. It helps in determining the nomenclature of chemical compounds and the study of redox reactions. Oxidation number denotes the hypothetical charge of an atom when all the bonds it forms with different atoms are fully ionic. 

Antoine Lavoisier introduced the term oxidation to denote the reactions of substance with oxygen. Later, it was known that when a substance loses electrons, it gets oxidised, regardless of the presence of oxygen as a reactant. To remember the oxidation state of each element in a compound or ion is difficult, so a set of rules are generated to find the oxidation number.

What is the Oxidation Number?

Oxidation number is determined based on a set of rules formulated on the basis that the electron pair in a covalent bond belongs entirely to a more electronegative element.

In other words, oxidation number gives the total number of electrons lost or gained by an atom when it forms a chemical bond with another atom. In a redox reaction, each atom gets an oxidation number which determines its ability to donate, gain or share electrons. E.g., the oxygen ion has an oxidation number (-2) because it can donate 2 electrons. What is an oxidation state? Oxidation state is the term which is often used interchangeably with the oxidation number. In other words, the oxidation state, or oxidation number, determines the degree of oxidation or loss of electrons of an atom in a chemical compound. The oxidation state can be positive, negative or zero or sometimes a fraction. Consider a compound CO₂, here the oxidation state or oxidation number of carbon is +4, and similarly the oxidation state of oxygen is – 2.

How to Find the Oxidation Number?

We cannot always remember in a compound/ion, which element is more electronegative than the other. How to assign oxidation numbers to compounds or ions was solved by developing a set of rules.

The following are the oxidation number rules:

1. In the elements, whether in their free or the uncombined state, each atom always has an oxidation number of zero. For example, each atom in O₂, Cl₂, Na₂, Na, Mg, Al has the oxidation number zero. 

2. The oxidation number for ions composed of only one atom is equal to the charge on the ion. Hence, Na⁺ ion has an oxidation number of +1, Ca²⁺ ion has an oxidation number of +2, Fe³⁺ ion has +3 oxidation state. etc.

3. All alkali metals in compound form have an oxidation number of +1 and all alkaline earth metals in their compounds have an oxidation number of +2. The oxidation number of Aluminium is +3 in all its compounds. 

4. In most compounds oxygen has the oxidation number of –2. Some exceptions occur in the case of peroxides and superoxides, the compounds of oxygen in which oxygen atoms are directly linked to each other. 

1. In peroxides (e.g., H₂O₂, Na₂O₂), each oxygen atom has an oxidation number of –1, and in superoxides (e.g., KO₂) each oxygen atom has an oxidation number of $-\left(\dfrac{1}{2}\right)$.

2.  When oxygen is bonded to fluorine, like in the case of oxygen difluoride (OF₂) the oxygen is assigned an oxidation number of +2.

5. Most of the time, the oxidation number of hydrogen is +1. When it is bonded to metals in binary compounds like in LiH, NaH etc., its oxidation number is –1. 

6. Fluorine has an oxidation number of –1 in all its compounds. Halogen like Cl, Br, and I also have an oxidation number of –1, when they occur as halide ions in their compounds. But when combined with oxygen, they have positive oxidation numbers. 

7. In a compound, the algebraic sum of the oxidation number of all the atoms must be zero. For a polyatomic ion, the algebraic sum of all the oxidation numbers of atoms of the ion is equal to the charge on the ion. For example, the sum of oxidation number of all the atoms in the carbonate ion, (CO₃) ^2- must equal –2. 

8. Transition elements atoms generally have several positive oxidation states. They have incompletely filled (partly filled) d-subshells in their ground state. The existence of the transition elements in different oxidation states is because of the participation of inner (n-1) d-electrons in addition to outer ns-electrons in bond formation. 

The oxidation number of an element which can exist in more than one oxidation state is represented  by a roman numeral in parentheses after the name of the element. For example, iron (II) chloride (FeCl₂) iron is in +2 oxidation state and iron (III) chloride (FeCl₃) iron is in +3 oxidation state.

Thus, we can see that the metallic elements have positive oxidation numbers and nonmetallic elements have positive or negative oxidation numbers. The highest value of oxidation number exhibited by an atom of an element, usually increases across the period in the periodic table.

Definitions Based on Oxidation Number 

• Oxidation: It means an increase in the oxidation number of the element in the given compound. (Loss of electron(s) by species)

• Reduction: Reduction means decrease in the oxidation number of the element in the given compound. (Gain of electron(s))

• Oxidising Agent: It is an agent that can increase the oxidation number of an element in a compound. (Acceptor of electron(s))

• Reducing Agent: It is a reagent that decreases the oxidation number of an element in a compound. (Donor of electron(s))

• Redox Reactions: In redox reactions, there will be change in oxidation number of the interacting species.

Oxidation Number Examples

Consider a reaction:

$CH_{4}(g)+2 O_{2}(g) \rightarrow CO_{2}(g)+2 H_{2} O$

Here, the carbon atom has -4 oxidation number and each hydrogen atom has +1 oxidation number. Hence, methane has an oxidation number of 0. Since oxygen is an element, its oxidation number is zero. Carbon atoms in carbon dioxide have +4 oxidation number and each oxygen atom has -2 oxidation number. In water molecules, hydrogen has +1 oxidation number and the oxygen atom has -2 oxidation number. So, we can conclude that carbon in methane undergoes oxidation.

Conclusion

Redox reactions are the reactions in which oxidation and reduction occur simultaneously. During oxidation-reduction reaction, each atom is assigned a number which shows its ability to acquire, donate, or share electrons. This number is called oxidation number or oxidation state. 

We can also say that the oxidation number of an atom is the hypothetical charge assumed on that atom if the compound was composed of ions. An increase in oxidation state means the element has undergone oxidation reaction. Similarly, a decrease in oxidation number indicates reduction. Oxidation numbers are assigned in accordance with a consistent set of rules.