Fractional Oxidation States

In chemistry, fractional oxidation states (FOS) are used to describe the distribution of electrons in a molecule. This can help understand how molecules interact with one another. In these simple notes, we will discuss the basics of FOS and how to calculate them. We will also take a look at some examples of molecules that have fractional oxidation states.

Fractional Oxidation Number

A fractional oxidation number is a real or imaginary number that results when the total charge on an atom is divided by the valence of that atom. The term “fractional” refers to the fact that these numbers are not always whole numbers. For example, if an atom has a charge of +0.25, its fractional oxidation number would be +0.25.

One of the most common uses for fractional oxidation numbers is in coordination chemistry, where they are used to describe the way that atoms share electrons in a molecule or complex ion. In this context, fractional oxidation numbers are often called “oxidation states.”

Example: In the molecule HNO₃, the nitrogen atom has an oxidation number of +0. This is because the molecule as a whole is neutral, and so the sum of the oxidation numbers of all the atoms must be 0. Therefore, since the hydrogen atoms each have an oxidation number of +0.25 (remember, this is a fractional oxidation number, so it’s not always going to be a whole number), the nitrogen atom must have an oxidation number of +0.25 for the sum of all the oxidation numbers to add up to 0.

Calculating Fractional Oxidation Numbers

There are a few rules that can be followed to calculate the fractional oxidation numbers of atoms in a molecule:

• The oxidation number of an element in its pure form is 0.
• The oxidation number of a monatomic ion is equal to its charge.
• The sum of the oxidation numbers of all the atoms in a neutral molecule is 0.
• The sum of the oxidation numbers of all the atoms in a polyatomic ion is equal to the charge on the ion.
• In a compound, the element with the more negative oxidation number is the one that was oxidized and the element with the more positive oxidation number is the one that was reduced.
• The oxidation number of oxygen is -0.25 in HNO₃.
• The oxidation number of hydrogen is +0.25 in HNO₃.
• The oxidation number of nitrogen is +0 in HNO₃.

Some Examples of Fractional Oxidation States

There are many examples of molecules that have fractional oxidation states. Here are a few:

Sulphur dioxide (SO₂): The oxidation number of sulphur is +0.25 and the oxidation number of oxygen is -0.25.

Nitric acid (HNO₃): The oxidation number of hydrogen is +0.25 and the oxidation number of nitrogen is +0.

Carbon monoxide (CO): The oxidation number of carbon is +0.50 and the oxidation number of oxygen is -0.50.

Dichlorine monoxide (ClO₂): The oxidation number of chlorine is +0.50 and the oxidation number of oxygen is -0.50.

The paradox of Fractional Oxidation Number: 

It is impossible to have a fractional oxidation number for an atom in a molecule or ion. It is possible, however, to have fractional oxidation numbers for atoms in complex compounds, such as transition metal complexes. The reason for this is that the rules for assigning oxidation numbers are different for complex compounds than for simple molecules and ions. In a complex compound, the oxidation number of an atom is determined by the charge on the atom when all the ligands are removed.

One of the most important rules for assigning oxidation numbers is that the sum of the oxidation numbers of all atoms in a molecule or ion must equal the overall charge on the molecule or ion. This rule is called the “rule of electro neutrality.” For example, consider the molecule HCl. The oxidation number of H is +0 and the oxidation number of Cl is -0, so the sum of the oxidation numbers of H and Cl is 0, which equals the overall charge on the molecule.

Conclusion

Fractional oxidation states are nothing new and have been known since the early days of chemistry. They were first observed in 1808 by Sir Humphry Davy. However, it wasn’t until the late 19th century that their true nature was understood. Overall, fractional oxidation states are a simple but important concept in Chemistry. By understanding how to calculate and identify them, we can better understand the reactivity of molecules and the stability of compounds. With that said, there are still some nuances and exceptions to the rules that we’ve covered today. But with a little practice, you’ll be fractional oxidation state experts in no time! Thanks for reading.