Oxidation Number

The oxidation number of an atom changes during oxidation or reduction. So, to identify the loss of electrons (oxidation) and gain of electrons (reduction), we must be aware of oxidation number or oxidation state.

What is oxidation number?

Oxidation number: It is the number that appears on an element in a chemical combination or molecule. It shows the count of electrons that are lost, gained or shared by an atom when it reacts with other atoms to form molecules or other chemical species. So, the numerical value of the oxidation state is equal to the number of electrons lost or gained by the atoms. It is also known as the oxidation state.

We can say that it is a charge that an atom gains after forming an ionic bond with other atoms. This charge is positive if electrons are lost and negative if electrons are gained, and is equal to the number of electrons an atom has lost or gained as compared to the neutral atom.

Let us understand it with the following example;

Let us find the oxidation number of magnesium (Mg) in a molecule of magnesium oxide (MgO), which contains one atom of magnesium and one atom of oxygen. The formation of magnesium oxide is shown below;

2Mg + O₂ → 2MgO

The atomic number of oxygen is 8. Its electronic configuration is 2, 6, so, It needs 2 electrons to complete its octet, so it will tend to gain two electrons, so its valency is 2. After gaining two electrons, it becomes O^2- as one electron has a -1 charge. It takes two electrons from Magnesium which gains 2⁺ charge on losing two electrons.

So, whenever, oxygen forms an oxide with an element, the oxygen in that oxide will have -2 charge. So, the oxidation number of -2 tells gain of electrons by oxygen and loss of two electrons by magnesium, so there is the reduction of oxygen and oxidation of magnesium.

Magnesium oxide is electrically neutral. So, magnesium must have + 2 charge as oxygen has - 2 charge. So, the oxidation number of magnesium is + 2 as it has lost two electrons. So, the oxidation number changes in a chemical reaction. If oxidation number increases there is oxidation (loss of electrons) and if it decreases, it shows reduction (gain of electrons).

So, in a neutral molecule, if you know the charge on one atom, another atom in this molecule will have an equal and opposite charge as shown below;

Mg + O₂ → Mg⁺²O^-2

In this reaction, Mg changes to Mg2+. A positively charged ion that shows it has lost electrons. So, Mg is oxidised in this reaction and its oxidation number of Mg is +2. In this reaction, neutral O₂ changes to O ^-2. A negatively charged ion that shows it has gained electrons. So, reduction of oxygen takes place here.

Besides this, oxidation number can be positive or negative as described below;

• The atoms of electropositive metals located in groups 1, 2 and 3 tend to lose a specific number of electrons to get a stable state. So, they always have a positive oxidation number.
• When a chemical bond is formed between different atoms, the more electronegative atom gains electrons from the less electronegative atom and thus have a negative oxidation number or state. The atom which donates or loses electrons has a positive oxidation number or state.

Atoms or ions with a constant oxidation state

i) Some atoms and ions have a constant oxidation state. For example, the net charge on neutral atoms or molecules is zero. So, their oxidation state or number is zero (0). For example, the oxidation state of neutral atoms like sodium, iron and magnesium is zero. Similarly, the oxidation number of neutral molecules like water, ammonia, chlorine, oxygen is zero.

ii) The atoms in homo-polar molecules (made of the same atoms) also have a zero oxidation sate. For example, the oxidation number of oxygen atoms in an oxygen molecule (O₂) is zero.

iii) The oxidation state of charged ions is equal to the net charge on these ions. For example;

• Ions of alkali metals such as sodium ion (Na⁺) always have constant oxidation number equal to +1.
• Alkaline earth metal ions always have a +2 oxidation number.
• Metal ions of the boron family have a constant +3 oxidation number.

Calculation of Oxidation Number of an Atom in a molecule or ion

Calculation of oxidation no. of permanganate ion (MnO₄) in potassium permanganate (KMnO₄);

Oxidation no. of potassium permanganate (KMnO₄) = Sum of oxidation numbers of different atoms present in it (K+Mn+4O) = 0, as it is a neutral molecule.

Let the oxidation no. of permanganate ion = X

In this molecule, potassium ion (K⁺) has +1 oxidation state, so we have;

Oxidation no. of K + oxidation no. of MnO₄ = 0

1 + X = 0

X = -1

Let us see how to calculate of oxidation number of an atom that occurs once in a molecule;

i) Find the oxidation number of chlorine in KCl?

KCl is a neutral molecule, so, the net charge on this molecule is 0 as the oxidation state of potassium + oxidation state of chlorine = 0

Let the oxidation state of Cl = x

Oxidation state of K = +1

So, we have +1 + x = 0

X = -1

So, Chlorine's oxidation state in KCl is -1.

ii) Find oxidation no. of Manganese in permanganate ion MnO₄^-

The charge of this ion is -1.

Oxidation no. of Oxygen = -2

Let the oxidation state of Mn = x

Now, we have x+ (4x-2) = -1

X - 8 = -1

X = -1 + 8

= + 7

iii) Let us find the oxidation no. of nickel (Ni) in Ni (CO) ₄

i) Ni (CO) ₄

The net charge on this molecule is zero.

CO is also a neutral molecule, so it also has 0 charge.

Let the charge on nickel is x.

So, we have;

X + (4x0) = 0

X = 0

So, the oxidation state of nickel in this molecule is 0.

iv) Oxidation state of Cr₂ in the formula K₂Cr₂O₇?

Let the oxidation state of Cr₂ is Y.

Now, let us write the oxidation state of different atoms in this molecule.

K₂=+2; Cr₂=2xY; O₇= (-2x7)

As it is neutral molecule so net charge on it is 0.

So, we have the following equation;

2 + 2Y - 14 = 0

2Y = 14 - 2

2Y =12

Y = +6

So, in this molecule, the oxidation state or number of chromium = 6

Characteristics of oxidation number

• It can be positive or negative and is assigned to an atom.
• It shows the degree of oxidation and reduction.
• The oxidation no. of the monoatomic ion (made of one atom) is the same as the charge on that ion, e.g. Na⁺
• The oxidation number of oxygen in oxides is always -2. For example, in MgO and CaO, it is -2.
• Fluorine always has a -1 oxidation number or state in all molecules.
• In a neutral molecule, when we add oxidation numbers of all atoms then it will be zero.
• In a polyatomic ion, the sum of oxidation no. of all atoms is equal to the net charge on that ion.
• It shows the number of electrons lost or gained by atoms while forming compounds.
• The oxidation no. of an atom in its pure or elemental form is zero. In pure elemental form, an atom is not combined with atoms of different elements. Such as, Cu, Cl₂, O₂, He, etc.
• The oxidation number of hydrogen can be +1 or -1. When it combines with non-metals, its oxidation no. is +1, e.g., HCl. Whereas, when it combines with metals, its oxidation number is -1, e.g. sodium hydride.