What is Oxidation Number

An oxidation number is a number that is assigned to an atom to indicate its state of oxidation or reduction during a chemical reaction. Each atom in a redox reaction is assigned an oxidation number to understand its ability to donate, accept, or share electrons. It shows the total number of electrons that have been removed from or added to an element to get to its present state. For example, in Fe₂O₃, the oxidation number of Fe is +3, and in FeO, it is +2. A loss of electrons corresponds to an increase in oxidation number. On the other hand, a gain of electrons corresponds to a decrease in oxidation number ^[1-4].

Oxidation Number Rules ^[1-6]

In order to assign oxidation numbers to atoms, we need to follow a set of rules.

1. The oxidation number of an element in its free state is zero.

Example: The oxidation number of Zn, Al, H₂, O₂, and Cl₂ is zero

2. The oxidation number of a monatomic ion is the same as the charge on the ion.

Example: The oxidation number of Na⁺ is +1, Mg²⁺ is +2, Al³⁺ is +3, Cl^-1 is -1, and O^2- is -2.

3. The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbers in a polyatomic ion is equal to the charge on the ion.

Example: In Fe₂O₃, the oxidation number of Fe is +3, and that of O is -2. The sum of all oxidation numbers is: +3 x 2 + (-2) x 3 = 0. The result is expected since Fe₂O₃ is neutral.

4. The oxidation number of an alkali metal in a compound is +1, and the oxidation number of an alkaline earth metal in a compound is +2.

Example: In NaCl, Na is an alkali metal, and its oxidation number is +1. In MgO, Mg is an alkaline earth metal, and its oxidation number is +2.

5. The oxidation number of oxygen in a compound is usually -2. However, if the oxygen is in a category of compounds called peroxides, its oxidation number is -1. If the oxygen is bonded to fluorine, the number is +1 or +2, depending upon the compound.

Example: The oxidation number of O in H₂O is -2, in H₂O₂ is -1, in OF₂ is +2, and in O₂F₂ is +1

6. The oxidation number of hydrogen in a compound is usually +1. In the case of a binary metal hydride, the oxidation number is -1.

Example: The oxidation number of H is +1 in H₂O and -1 in NaH.

7. The oxidation number of fluorine is always -1.

Example: The oxidation number of F in NaF is -1.

8. Chlorine, bromine, and iodine usually have an oxidation number of -1 unless bonded to oxygen or fluorine.

Example: The oxidation number of Cl in NaCl is -1 and in ClO₂ is +4, and in FCl is +1.

How to Find Oxidation Number

The oxidation number of an atom in an ion or compound can be determined using the above rules. Let us look at a few examples ^[1-6].

1. Sulfuric Acid (H₂SO₄)

The oxidation number of hydrogen (H) and oxygen (O) are +1 and -2, respectively. Sulfuric acid is a neutral compound. Let x be the oxidation number of sulfur (S). Therefore,

(+1) x 2 + x + (-2) x 4 = 0

Or, 2 + x – 8 = 0

Or, x = +6

2. Nitric Acid (HNO₃)

The oxidation numbers of hydrogen (H) and oxygen (O) are +1 and -2, respectively. Nitric acid is a neutral compound. Let x be the oxidation number of nitrogen (N). Therefore,

+1 + x + (-2) x 3 = 0

Or, +1 + x – 6 = 0

Or, x = +5

3. Potassium Permanganate (KMnO₄)

The oxidation numbers of potassium (K) is +1 and oxygen (O) is -2. KMnO₄ is a neutral compound. Let x be the oxidation number of magnesium (Mn). Therefore,

+1 + x + (-2) x 4 = 0

Or, +1 + x – 8 = 0

Or, x = +7

4. Dichromate Ion (Cr₂O₇^2-)

Dichromate is a complex ion. The oxidation number of oxygen (O) is -2. The charge of Cr₂O₇^2- is -2. Let x be the oxidation number of chromium (Cr).

2x + (-2) x 7 = -2

Or, 2x -14 = -2

Or, x = +6

5. Carbonate (CO₃^2-)

The oxidation number of oxygen (O) is -2 and the charge on CO₃^2- is -2. Let x be the oxidation number of carbon (O). Therefore,

x + (-2) x 3 = -2

Or, x – 6 = -2

Or, x = +4

6.  Phosphite (PO₃^3-)

The oxidation number of oxygen (O) is -2 and the charge of PO₃^3- is -3. Let x be the oxidation number of phosphorous (P). Therefore,

x + (-2) x 3 = -3

Or, x – 6 = -3

Or, x = +3

7. Potassium Perchlorate (KClO₄)

The oxidation numbers of potassium (K) and oxygen (O) are +1 and -2, respectively. Let x be the oxidation number of chlorine (Cl). KClO₄ is a neutral compound. Therefore,

+1 + x + (-2) x 4 = 0

Or, 1 + x – 8 = 0

Or, x = +7

8. Potassium Nitrate (KNO₃)

The oxidation number of potassium (K) and oxygen (O) are +1 and -2, respectively. KNO₃ is a neutral compound. Let x be the oxidation number of nitrogen (N). Therefore,

+1 + x + (-2) x 3 = 0

Or, 1 + x – 6 = 0

Or, x = +5

The following image shows a chart consisting of the oxidation numbers of the periodic table elements [7].

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