Oxidation Number - NEET Important Topic

In simple terms, we can describe oxidation number as the number that is allocated to elements in a chemical combination. Basically, the oxidation number is the count of electrons that atoms in a molecule can share, lose, or gain while forming chemical bonds with other atoms of a different element. It is also referred to as the oxidation state. In some cases, these two terms can have a different meaning depending on whether we are considering the electronegativity of the atoms or not. Let us know more about the oxidation number.

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What is the Oxidation Number?

The oxidation number is a measure of the degree of oxidation or reduction for an atom in a chemical compound. The higher the oxidation number, the more oxidised the atom will be. The lower the oxidation number, the more reduced the atom will be.

How to Find Oxidation Number?

In order to find the oxidation number of an element in a compound, you will need to know the rules for assigning oxidation numbers. Once you know the rules, you can use them to determine the oxidation number of any element in any compound.

The rules to determine oxidation number are as follows:

• The oxidation number of an element in its elemental form is always 0. For example, the oxidation number of sodium (Na) is 0, and the oxidation number of chlorine (Cl2) is also 0.

• The oxidation number of a monatomic ion is equal to its charge. For example, the sodium ion Na⁺ has an oxidation number of +1, and the chloride ion Cl^- has an oxidation number of -1.

• The sum of all the oxidation numbers in a polyatomic ion is equal to the charge on that ion. For example, consider the nitrate ion NO₃^- It contains one nitrogen atom with an oxidation number of +5 and three oxygen atoms each with an oxidation number of -2. 

• The sum of these four numbers is -1, which is equal to the charge on this particular polyatomic ion (-). Therefore, we can say that 5 + (-2) + (-2) + (-2) = -1 or simply put 5 - 6 = -1 . 

• In general, then we can say that Sum of Oxidation Numbers = Charge On Ion. Applying this rule to our previous examples, we get, Na⁺ : 0 + 1 = 1; Cl^- : 0 + (-1)= -1 ; NO₃^- : 5+(-6) = -1.

What is an Oxidation State?

The oxidation state of an atom (sometimes referred to as the oxidation number) in a chemical compound provides insight into the number of electrons lost and, therefore, describes the extent of oxidation of the atom. The oxidation state of an atom can be defined as the hypothetical charge that would be held by that atom if all of its bonds to other atoms were completely ionic in nature. +4 is the sulphur oxidation number.

Oxidation Number Rules

In Chemistry, the oxidation number is a measure of the degree of oxidation of an atom in a chemical compound. The rules for assigning oxidation numbers to atoms are as follows:

• The oxidation number of any free element is always 0.

• The charge of the monatomic ion is nothing but the charge of the ion.

• The hydrogen has an oxidation of ‘+1’, and it will be ‘-1’ when it will react with an element which has less electronegative value than the hydrogen.

• The oxidation number of oxygen in the compound will be ‘-2’ and it will be ‘-1’ in peroxides.

• The oxidation number of all group I elements in a compound is ‘+1’.

• The oxidation number of group II elements in a compound is ‘+2’.

• The oxidation number of group XVII elements in all binary compounds is ‘-1’.

• In a neutral compound, the sum of the oxidation state of all the atoms is zero.

• The sum of the oxidation numbers of all the atoms in a polyatomic ion is equal to the total charge of the ion.

• Nitrogen has five electrons in its outermost orbital. It requires three more electrons to complete its valence shell and attain the noble gas configuration. That means nitrogen has five valence electrons and has a valency of 3. So, nitrogen can either accept 3 electrons or donate 5 electrons. Therefore, the oxidation state of nitrogen ranges from -3 to +5. 

• This is how we can calculate the oxidation number of an element in a compound.

Examples of Finding the Oxidising Number

1. Calculate the oxidation number of nitrogen (N) in NO.

Solution: The oxidation state of oxygen(O) = -2

Consider the oxidation state of nitrogen(N) = x

Now,

x-2 = 0

x = +2

Hence, the oxidation number of  N in NO is +2.

2. Calculate the oxidation number of sulphur (S) in sulphuric acid:H2SO4

Solution: The oxidation state of oxygen(O) = -2

The oxidation state of hydrogen(H) = +1

Consider the oxidation state of sulphur(S) = X

Now,

$2(+1)+X+(-2 \times 4)$

$-2+X+8=0$

$X=+6$

Hence, the oxidation number for sulphur in sulphuric acid is +6.

Average Oxidation Number

The oxidation state is the number of electrons assumed to have either been lost or taken by heteroatoms during their bonding. Since the numbers of electrons are whole numbers, the oxidation number of individual atoms also has to be a whole integer.

However, there are molecules that contain an atom, more than once and each bonded differently. Such atoms shall have different oxidation states at different positions and hence has to be calculated individually, taking into consideration the atoms it bonds.

Calculation of the oxidation state of the atom using the normal method assumes that all the same atoms are equal and will give only an average of the different oxidation states of the same atom in the molecule. This average oxidation state is mostly a fraction, instead of the whole number.

So, the fractional oxidation state is always an average oxidation number of the same atoms in a molecule and does not reflect the true state of the oxidation state of atoms.

Example

Superoxide KO₂, Potassium ions have an oxidation number of +1. Potassium superoxide molecule being neutral, the oxidation state of two oxygen atoms together is -1. So, the average oxidation number of oxygen in superoxide is $\dfrac{1}{2}$.

Periodic Table with Oxidation Numbers

Here, we are going to summarise the oxidation number of all elements in the periodic table into one group. The below table shows the oxidation numbers list of a few elements of the periodic table.

Summary

In this article, we have discussed the oxidation concept. Let us summarise the article with the key concepts that we learned. The oxidation number is a measure of the degree of oxidation or reduction for an atom in a chemical compound. The higher the oxidation number, the more oxidised will be the atom; the lower the oxidation number, the more reduced the atom will be. The rules for assigning oxidation numbers are as follows: Sum of Oxidation Numbers = Charge on Ion. The oxidising number of an element is the number of electrons that it can gain or lose in order to form a stable ion. The most common oxidation states for oxygen are -2, 0, and +2.