Difference Between Oxidation State and Oxidation Number

Oxidation State vs Oxidation Number

Oxidation State

According to the IUPAC definition, oxidation state is “a measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have.” Oxidation state is an integer value, and it can be either positive, negative or zero. Oxidation state of an atom is subjected to change upon chemical reaction. If the oxidation state is increasing, then the atom is said to be oxidized. And if it is decreasing, then the atom has undergone reduction. In the oxidation and reduction reaction, electrons are transferring. In pure elements oxidation state is zero. There are few rules we can use to determine the oxidation state of an atom in a molecule.

• Pure elements have a zero oxidation state.

• For monatomic ions, oxidation state is same as their charge.

• In a polyatomic ion, the charge is equal to the sum of oxidation states in all the atoms. So the oxidation state of an unknown atom can be found if the oxidation state of other atoms is known.

• For a neutral molecule, the sum of all the oxidation states of atoms is zero. 

Other than the above methods, oxidation state can also be calculated using the Lewis structure of a molecule. Oxidation state of an atom is given by the difference between the number of valence electron of the atom, if the atom is neutral, and the number of electrons belongs to the atom in the Lewis structure. For example, the methyl carbon in the acetic acid has a -3 oxidation state. In the Lewis structure, carbon is bonded to three hydrogen atoms. Since the carbon is more electronegative, the six electrons in the bonds belong to carbon. Carbon makes the other bond with another carbon; therefore, they split the two bond electrons equally. So all together, in the Lewis structure, carbon has seven electrons. When carbon is in the neutral state, it has four valence electrons. Therefore, the difference between them makes the oxidation number of carbon to be -3.

Oxidation Number

Oxidation number is a characteristic of the central atom of a coordination compound. Sometimes the charge and the oxidation number are similar, but sometimes it is different. For instance, s block and p block elements have the same oxidation number as their charges. Also polyatomic ions have the same oxidation number as the charge. Same element can have different oxidation numbers, depending on the other atoms it is connected with. In a free element, oxidation number is always zero. Transition metal ions (d block), elements have different oxidation numbers.


What is the difference between Oxidation State and Oxidation Number?

• The term oxidation number is mainly used in coordination chemistry. It has a slightly different meaning than the oxidation state.

• The method of calculating the oxidation number is a bit different from the way the oxidation state is counted.

• When oxidation state is determined, the electronegativity of the atoms in a bond is taken into consideration. But when determining the oxidation number, electronegativity is not taken into consideration. All the electrons in the bond belong to ligands.

• Usually oxidation numbers are represented with Roman numerals whereas oxidation states are represented in Indo-Arabic numerals.