Reduction and Oxidation Definition and Examples in Periodic Table (Chemistry)

Reduction: A reducing substance (atom, ion or molecule) is that which loses one or more electrons and is thereby oxidized to a higher valency state.

M(g)                     →                M⁺(g)       +          e^–
Reducing agent                (Higher valency state= +1)
(Lower valency state)

The reducing power of an element in its gaseous state is determined by its tendency to lose the electrons. Greater its tendency to lose the electrons, greater its reducing power. Again it is known that lower the value of ionization energy of an element in the gaseous state, greater its tendency to lose the electrons. Thus the elements (in the gaseous state) which have low values of ionization energy are strong reducing agents.

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Oxidation and Reduction in Periodic Table

Since ionization energy of the elements decreases on descending a group and increases from left to right in a period, the reducing power of the elements increases in going down a group and decreases as we move from left to right in a period.

Thus the elements lying at the extreme left of the periodic table (i.e., Group IA) which have low values of ionization energy are strong reducing agent while those lying at the extreme right of the periodic table (i.e. halogens) which have high values of ionization energy are weak reducing agents.

An oxidizing substance (atom, ion or molecule) is that which gains one or more electrons and is thereby reduced to a lower valency state.

X(g)     +      e^–              →          x^–(g)       (Oxidizing agent)·

(Higher valency state = 0)      (Lower valency state = -1)

The oxidizing power of an element in, its gaseous state is determined by its tendency to accept the electrons. Greater its tendency to accept the electrons, greater it’s oxidizing power. Again it is known that higher the value of electron affinity of an element in the gaseous state, greater its tendency to accept the electrons. Thus the elements (in the gaseous state) which have high electron affinity values are strong oxidizing agents.

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Oxidation and Reduction in Term of Electron Affinity

Since electron affinity of the elements decreases on descending a group and increases from left to right in a period, the oxidizing power of the elements decreases on descending a group and increases from left to right in a period.

Thus the elements lying at the extreme left of the periodic table (i.e. Group IA) which have low values of electron affinity are poor oxidizing agents while those lying at the extreme right of the periodic table (e.g. halogens) which have high values of electron affinity are strong oxidizing agents.

It should be noted that we have considered the reducing and oxidizing powers of the elements in the gaseous state. If the ·chemical reaction is carried out in solution using water as the main solvent, reducing and oxidizing powers are better understood in terms of standard reduction potential value of the elements.