The total number of electrons that an atom gets or loses in order to create a chemical connection with another atom is referred to as the oxidation number, also known as the oxidation state of the atom. Each atom that participates in an oxidation-reduction reaction is assigned an oxidation number that reflects its ability to acquire, donate, or share electrons. The oxidation number of an atom is the number of electrons it can acquire, contribute, or share. In the case of the iron ion Fe³⁺, for example, the oxidation number is +3 because it can acquire three electrons to establish a chemical bond; in contrast, the oxygen ion O₂ has an oxidation number of -2 because it may donate two electrons to make a chemical bond. It is impossible to have an electronically neutral substance without having the sum of the oxidation numbers equal to zero; for example, in hematite (Fe₂O₃), the total sum of the oxidation numbers of the two iron and three oxygen atoms equals zero because the total sum of the oxidation numbers of the two iron and three oxygen atoms equals zero.

Oxidation Number 

It is necessary to know the oxidation number, also known as the oxidation state, in order to determine how many electrons an atom contains. Oxidation numbers, on the other hand, may not always correspond to actual charges on molecules. As a result, we can determine the oxidation numbers of atoms of any element, regardless of whether or not they are bound together by covalent or ionic bonds.

Despite the fact that it is a rare reaction, we may observe that the H atom in the neutral state (zero) in H₂ transforms to a positive state following the production of H₂O, despite the fact that it is a rare reaction. Additionally, the oxygen atom in O₂ that is present in its zero state transforms to the di-negative state in H₂O, as shown in the graph below. So we can clearly observe that there is an electron transfer taking on between hydrogen and oxygen during the reaction’s first stage. As a result of this reaction, H₂ and O₂ are oxidized and reduced in equal amounts.

Oxidation states make it easier to figure out what is being oxidized and what is being reduced in redox reactions since they simplify the entire process. However, it would be beneficial if you were familiar with the following topics for the purposes of this introduction:

In terms of electron transfer, oxidation, and reduction, electron-half-equations

Rules For Assigning Oxidation Number 

The oxidation number of a given element in a compound is determined by applying a series of principles to determine the oxidation state of that element. In order to establish the guidelines, an electron pair of a covalent molecule and the electronegativity of an element were taken into consideration. While it is possible to determine which element is more electronegative than the other in any particular molecule or ion, it is more difficult to distinguish between them.

The development of a specific set of rules for determining the presence of an element in a given molecule or ion was the result of this discovery. Furthermore, if two or more elements are present in a molecule, compound, or ion, the average of all the atoms of the particular element is taken into consideration when determining the oxidation number of the given element. We’ll get a better understanding of this later on by looking at an example.

1. When writing a formula, the cation is written first, followed by the anion, which follows the convention. For example, the hydrogen atom in NaH is H^–, whereas the hydrogen atom in HCl is H⁺
2. In the case of a free element, the oxidation number is always zero. As an example, the atoms in He and N₂ have oxidation numbers equal to zero.
3. The charge of a monatomic ion is equal to the number of oxidation electrons in the ion. For example, the oxidation number of Na⁺ is one, whereas the oxidation number of N₃^– is a negative three.
4. When it comes to hydrogen, the oxidation number is usually one. In compounds containing elements that are less electronegative than hydrogen, such as CaH₂, the oxidation number of hydrogen is -1, indicating that the element is oxidized.
5. In most cases, the oxidation number of oxygen in compounds is a negative number. OF₂ is an exception because F is more electronegative than O, and BaO₂ is an exception because the peroxide ion has a [O-O]2-structure, which makes it more electronegative than O.
6. When a Group IA element is present in a compound, its oxidation number is one.
7. In a compound, the oxidation number of a Group IIA element is equal to two.
8. Except when paired with an element with a greater electronegativity, an element belonging to Group VIIA has an oxidation number of -1 in a combination. In HCl, the oxidation number of Cl is -1, but in HOCl, the oxidation number of Cl is +1.
9. It is equal to zero when all of the atoms in a neutral molecule have equal oxidation numbers.
10. The charge of a polyatomic ion can be calculated by adding the total of oxidation numbers in each nucleus. If we consider SO₄^2- as an example, the sum of the oxidation numbers is -2.

Conclusion 

The total number of electrons that an atom gets or loses in order to create a chemical connection with another atom is referred to as the oxidation number. Each atom that participates in an oxidation-reduction reaction is assigned an oxidation number that reflects its ability to acquire, donate, or share electrons.  The oxidation number (or charge) of a monatomic ion is equal to the number of oxidation electrons in the ion. In compounds containing elements that are less electronegative than hydrogen, such as CaH₂, the oxidation number of hydrogen is -1, indicating the element is oxidized.