Oxidation State

What is Oxidation State?

Oxidation state and oxidation number are terms frequently used interchangeably. They are the quantities which describe the number of electrons lost in an atom. Ideally, the oxidation state is either zero, positive, or negative.

Discovery and Representation

Antoine Lavoisier was the first to use the term oxidation to denote the reaction between a substance and oxygen. Later, it was noticed that when the substance is oxidized it loses electrons. Therefore, the elucidation diversified to include other reactions as well where the electrons are lost, irrespective of the involvement of oxygen atom.

The value of oxidation state usually constitutes integers. Sometimes the oxidation state is a fraction. For example,

  • The element iron in Fe3O4 (magnetite) is 8/3.

What is Oxidation?

A chemical reaction which includes the movement of electrons is called oxidation. A substance which donates electrons is said to be oxidized. For example,

  • Reaction of iron and oxygen – When iron (Fe) reacts with oxygen it forms rust because iron loses electrons and oxygen gains electrons.

Lowest and Highest Oxidation States

The oxidation state of an atom is not regarded as the real charge of the atom. In a chemical reaction if there is an increase in oxidation state then it is known as oxidation whereas if there is a decrease in oxidation state, it is known as reduction. The lowest known oxidation state is −4, for carbon in CH4 (methane). The highest known oxidation state is +9 in the tetroxoiridium (IX).

How to calculate Oxidation Number?

To calculate oxidation number we need to understand and follow certain rules. There are six rules:

  • Each atom in an element either in its free or uncombined state holds up an oxidation number of zero. Clearly, each atom in H2, Cl2, P4, Na, Al, O2, O3, S8, and Mg, has an oxidation number zero.
  • The oxidation number of ions which comprise of only one atom is equal to the actual charge on the ion.
  • In most of the compounds, the oxidation number of oxygen is –2. There are two exceptions here.
    • Peroxides- Every oxygen atom is allocated an oxidation number of –1. Example, Na2O2
    • Superoxides- Every oxygen atom is allocated an oxidation number of –(1/2). Example, KO2
    • Oxygen is bonded to fluorine- Example, dioxygen difluoride where the oxygen atom is allocated an oxidation number of +1.
  • Hydrogen’s oxidation number is +1, excluding when it is bonded to metals containing two elements. For example, CaH2, its oxidation number equals to –1.
  • Fluorine and other halogens have an oxidation number –1 when they appear as halide ions in their compounds. When iodine, chlorine, and bromine are combined with oxygen, their oxidation number is positive.
  • When the oxidation number of the atoms of a compound are added together the algebraic sum must be equal to zero. In case of polyatomic ion, when the oxidation number of the atoms of an ion are added together the algebraic sum must be equal to the charge on the ion. Consider (CO3)2– the algebraic sum of oxidation number of one carbon atom and three oxygen atom is equal to -2.

Learn more about how to calculate oxidation number along with the steps.

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