Periodic Properties And Variation Of Properties Physical And Chemical — Chemistry STD 10 — Question

The particular with lowest ionization potential in a group will ionize most easily.
For example among elements of group $1$ Value of ionization potential is:
 

Element	Li	Na	K	Rb	Cs
Ionization energy $KJ mol^{-1}$	$520$	$496$	$419$	$403$	$375$

Cesium (Cs) has lowest ionization potential value which is $375 KJmol^{-1}$ So it will ionize most easily.
Concept Insight :
Lesser is the ionization potential value; more easily is the tendency to form a cation.