Question

why lithium is a strong reducing agent the cesium

Answer 1

because Lithium have small size high effective nuclear charge

Answer 2

Lithium has a higher ionisation enthalpy than caesium indicates that that caesium should have a better tendency to lose electrons (oxidation)than lithium or caesium is a better reducing agent than Li. And the order should be Li
But the actual order was found to be just reverse i.e., Li>Na>K>Rb>Cs.

This is because standard electrode potential that determines a reducing agent is a summation of

i) Sublimation energy Li(s) to Li(g)

ii) Ionisation energy Li(g) to Li+(g)

And iii) Hydration energy Li+(g) to Li+(aq)

Among these three quantities the first two (i) Enthalpy of sublimation and (ii) Enthalpy of Ionisation are positive in magnitude and (iii) Enthalpy of Hydration is a negative quantity. Although the most important key here is that the -ve value of (iii) is much higher to compensate the +ve values of (i) and(ii) especially in case of Li which is smaller in size and have high charge density, hence will have highest negative magnitude of hydration energy.

Therfore, the above order follows as it makes overall Eo value most negative for Lithium.

The actual order of Eo values of group 1 is;

Li/Li+= -3.05

Na/Na+ = -2.71

K/K+ = -2.93

Rb/Rb+ = -2.92

Cs/Cs+ =-2.92

(Source of values● J.D.Lee..Concise Inorganic Chemistry)