Chemical property of S- block element... …

1) All the alkali metals are highly reactive elements since have a strong tendency to lose the single valence s-electron to form uni positive ions having inert gas configuration.

2) This reactivity arises due to their low ionisation enthalpies and high negative values of their standard electrode potential.

3) Due to their strong tendency to lose the single valence electron ,alkali metals also act as a strong reducing agents. Reactivity of an element is measured in terms of its reducing character.

4) Ionization enthalpy is a measure of the tendency of an atom to lose electrons in the gaseous state.Thus, lower the ionization enthalpy, greater is the tendency of an element to lose electron and hence stronger is the reducing character or higher is the reactivity of the element.

5)The ionization enthalpies of alkali metal decreases down the group, therefore, their reducing character or reactivity in the gaseous state increases from Li to Cs , i.e. Li< Na< K< Rb<Cs.