Ionization enthalpy is the minimal quantity of energy which is demanded to get rid of the most loosely bound electron from a neutral isolated gaseous atom to form a cation.

The factors responsible for the decrease in ionization enthalpy in moving down the group are given as follows:

[i] Increase in the atomic size of elements: On going down the group the number of shells in the atom also increases. As a result, the size of the atom also increases gradually on going down the group. As a result, the valence electrons get farther away from the nucleus and thus weakening the effective nuclear charge of the atom and hence electrons can be easily removed from the atoms. Hence, ongoing down the group, ionization energy decreases.

[ii] Increase in the shielding effect: The number of inner shells of electrons increases on moving down a group. The inner shells or orbits of the atom acts as a shield for the outer or valence shell electrons. The inner shell shields the nuclear charge and as a result only a small amount of nuclear charge is able to attract the valence shell electrons. Hence the energy required to remove the electron reduces on moving down the group.