Which is the strongest reducing agent in Periodic Table?

Reducing agent is a substance that causes something to be reduced. In other words reducing agent is a chemical substance that donates electrons to atoms of other elements. All the alkali metals are strong reducing agents as they have strong tendency of losing electrons and the reducing power increases.

Lithium (Li) is the strongest reducing agent in the periodic table. The E^o value (reduction potential) depends on the three factors that is sublimation, ionization and hydration enthalpies. With the small size of its ion lithium has the lightest hydration enthalpy which accounts for its high negative E^o value and its reducing power.

Lithium is the lightest metal known. It is least fusible, least dense and least soft of all alkali metals. Though lithium has the highest ionization energy, yet lithium is strongest reducing agent among alkali metals, because of its largest heat of hydration which sufficiently exceeds the ionization energy. Lithium has the lowest value of standard reduction potential among all the alkali metals, and hence it is best reducing agent.

In general oxidizing agent acts by acquiring electrons; a reducing agent by giving up electrons. Hence elements with high EA values and high electronegativities are oxidizing agents and elements of low ionization energies are reducing agents. The reducing strength of elements becomes progressively lower across the periods and higher down the groups. Thus fluorine should be the strongest oxidizing agent and lithium is the strongest reducing agent of all the elements.