Oxidation Number

The oxidation number of an atom is the charge it seems to have while forming ionic interactions with other heteroatoms. An element with a higher electronegativity is ascribed a negative oxidation state (even if it forms a covalent connection). The definition assigns an atom’s oxidation state based on the conditions that the atom meets.

i)  Heteroatoms form bonds.

ii) Ionic bonding is always formed by either receiving or losing electrons, regardless of the nature of the bonding.

Because an atom can have numerous valence electrons and make multiple bonds, they will all be considered ionic and given an oxidation state equal to the number o f electrons engaged in the bonding. As a result, oxidation number or state is a hypothetical situation of atoms creating an ionic bond.

Platinum has an oxidation state of 10 in the compound cation tetrox platinum (PtO4)2+. Any atom can have a maximum oxidation state of ten. To produce the ion, it looks to have lost ten electrons. However, the ionisation energy required to remove an electron from a positively charged species rises dramatically.

The harder it is to remove an electron with a greater charge, the higher the ionisation energy. As a result, taking away ten electrons is totally hypothetical. Similarly, adding an electron becomes more difficult as the negative charge increases.

Positive or negative oxidation states greater than three are nearly impossible. Regardless of the assumption, it aids in comprehending the changes that occur when an atom undergoes a chemical transformation.

How Do We Calculate An Atom’s Oxidation Number?

The amount of electrons an atom/ion has gained or lost in comparison to the neutral atom is known as its oxidation number or state. Group I, 2 and 3 electropositive metal atoms lose a particular number of electrons and have a constant positive oxidation number.

Negative oxidation states occur when more electronegative atoms in a molecule receive electrons from a less electronegative atom. The oxidation state’s numerical value is equal to the number of electrons lost or gained.

The oxidation state is determined by:

I Calculating the continuous oxidation state of additional atoms/molecules/ions linked to it, and

ii) Calculating the overall charge of a molecule or ion by multiplying its total oxidation state.

Atoms, molecules, and ions that have a constant oxidation number.

a) Neutral atoms or molecules have no net charge. As a result, their total oxidation state is zero.

The oxidation state of elemental atoms like sodium, magnesium, and iron, for example, is zero. Similarly, neutral molecules such as oxygen, chlorine, water, ammonia, methane, and potassium permanganate have zero net oxidation state.

Atoms in homo-polar compounds have no oxidation state. An atom in an oxygen molecule has no oxidation number.

b) The oxidation state of charged ions is the same as the ion’s net charge. So,

All alkali metal ions have an oxidation number of +1

All alkaline earth metal ions have an oxidation number of +2

All boron family metal ions have an oxidation number of +3

The oxidation number of hydrogen in a proton (H+) is +1, while it is -1 in a hydride.

The oxygen oxidation number in oxide ions (O2-) is -2, while it is -1 in peroxide ions (O-O2-).

Calculation of an Atom’s Oxidation Number in a Molecule/Ion

Potassium permanganate (KMnO4) oxidation number = sum of oxidation numbers (K + Mn + 4O) = 0

Permanganate ion (MnO4) –  oxidation number= Sum of oxidation number of (Mn + 4O)= -1

Calculation of the Number of Atoms That Only occur Once in a Molecule

Example 1: Chlorine oxidation state in KCl

Because KCl is neutral, its net charge is zero.

KCl oxidation state = potassium oxidation state + chlorine oxidation state = 0.

Potassium oxidation state = +1

+1 + x = 0: x = -1 Oxidation states

K Cl are atoms in the species

Chlorine oxidation state in KCl = -1

Number of Atoms Occurring More Than Once in a Molecule and Having Identical Bonding Calculation

Atoms that occur more than in a molecule may or may not be bound in the same way. There will be no difference between them if they are identically bound, and all of the atoms will have the same oxidation values. By using the standard method, the oxidation status of such an atom in a molecule can be computed.

The average oxidation number will be the same as the total number determined individually.

Example 1: The molecules Cl2O, Cl2O5, and Cl2O7 all have two chlorine atoms. However, as their structures reveal, the environment of both chlorine atoms is the same. The oxidation number of the atoms is the same whether measured individually or as a whole molecule.

i) Cl2O:

Cl2O is neutral, hence its net charge is zero.

Cl2O net oxidation state = 2 x chlorine oxidation state + 1 x oxygen oxidation state = 0.

The oxygen oxidation state is -2.

2 x + (-2) = 0: x = +1 Oxidation states

2ClO atoms in the species

Chlorine oxidation state in Cl2O= +1

ii) Cl2O5:

Because Cl2O5 is neutral, it has no net charge.

Cl2O5 oxidation state = 2 x chlorine oxidation state + 5 x oxygen oxidation state = 0.

⸪ The oxygen oxidation state is -2.

    2x + (5 x -2) = 0 oxidation states: x = +5

  Cl2O5  atoms in the species 

Chlorine oxidation state in Cl2O5= +5

iii) Cl2O7

Because Cl2O7 is neutral, it has no net charge.

Cl2O7 oxidation state = 2 x chlorine oxidation state + 7 x oxygen oxidation state = 0.

⸪ The oxygen oxidation state is -2.

     2x + (7x -2) = 0 oxidation states: x = +7

Cl2O7 has the following atoms:

Chlorine oxidation state in  Cl2O7 = +7

Note that, with the exception of the atoms/molecules/ions listed as having a constant oxidation state, the oxidation state of other atoms/molecules and ions varies depending on the molecule in which they are present.

The oxidation state of chlorine is changeable (+1, +5, and +7) in the instances provided.

Number of Atoms Occurring More Than Once in a Molecule and Having a Bonding Difference Calculation of Oxidation

The oxidation state of atoms with different bond structures will differ. As a result, their oxidation status must be assessed separately based on their molecular structure. By assuming that they are equivalent, the average oxidation state may be computed. In this scenario, rather than being a whole integer, the average oxidation could be fractional.

i) Cl2O4 as an example

Cl2O4 is neutral, hence its net charge is zero.

Cl2O4 oxidation state = 2 x chlorine oxidation state + 4 x oxygen oxidation state = 0. ⸪

The oxygen oxidation state is -2.

2x + (4x -2) = 0 oxidation states: x = +4

 Cl2O4     atoms in the species

Chlorine oxidation state in Cl2O4 = +4

Oxidation Number of Atoms in a Diatomic Molecule

• Diatomic homonuclear molecule:

The concept of oxidation number only applies to heteroatoms that make up a molecule. As a result, the oxidation number of the atoms in a homonuclear diatomic molecule is zero. In each molecule, the oxidation number of hydrogen, oxygen, nitrogen, and chlorine is zero.

•  Diatomic heteronuclear molecule:

All bonds produced between the atoms in hetero diatomic compounds are termed ionic.

The bonding electrons from the less electronegative atom are supposed to be taken away by more electronegative atoms. As a result, the electronegative atom has a negative oxidation state with a magnitude equal to the amount of electrons it has taken.

The more electronegative atom is said to have gained an electron from the less electronegative atom. As a result, the atom with the least electronegative charge will have a positive oxidation state equal to the number of electrons it has lost.

Example 1: HCl 

Hydrogen is more electronegative than chlorine. As a result, chlorine is thought to remove one electron from hydrogen. Chlorine has an oxidation number of -1 because it receives one electron, whereas hydrogen has an oxidation state of +1 because it loses one electron.

Example 2: Water

Hydrogen is less electronegative than oxygen. As a result, each of the two hydrogen atoms gives one electron to the oxygen atom, giving it an oxidation number of -2. Both hydrogens will have an oxidation number of +1 after losing one electron.

State of Fractional Oxidation

The number of electrons presumed to have been lost or taken by heteroatoms during their bonding is known as the oxidation state. Because electron numbers are whole numbers, the oxidation number of individual atoms must be a whole number as well.

However, some compounds contain one atom many times, each bound differently. Such atoms will have different oxidation states at different places, thus each one must be computed separately, taking into account the atoms with which it is bonded.

The conventional method of calculating an atom’s oxidation state assumes that all of the same atoms are equal, yielding merely an average of the different oxidation states of the same atom in the molecule. Instead of a whole number, the average oxidation state is typically a fraction.

As a result, the fractional oxidation state is always the average oxidation number of the identical atoms in a molecule and does not represent the genuine oxidation state of atoms.

Redox Reactions, Oxidation and Reduction

Products are formed when atoms and molecules react. The nature of the change on the reactants to generate products is used to classify reactions into several categories. Reactant and product atoms/ions in a reaction may have the same or different numbers of valence electrons, depending on the reaction type.

Redox reactions are defined as those in which the number of valence electrons in the reactant atom/ion differs from the number on the product side. During the reaction, the atom/ion may have received or lost electrons. As a result, an atom/ion is either oxidised or reduced.

The atomic symbol with a superscript is used to denote the atoms/ions in the processes. The superscript denotes the number of electrons that the atom/ion has in comparison to the neutral atom. In comparison to the neutral atom, the superscript also has a positive sign if the electron is lost and a negative sign if the electron is acquired.

The atom’s oxidation state is indicated by the superscript combined with the sign. Depending on how many electrons are obtained or lost, the atom might have different oxidation states. The oxidation state of neutral atoms is zero.

Conclusion

The charge that an atom would have in a molecule if all of the bonding electrons were assigned to the more electronegative element is known as its oxidation number. The oxidation number of an atom increases as a result of oxidation. The oxidation number of an atom is reduced during reduction.