Question

What are the factors due to which the ionization enthalpy of the main group elements tends to decrease down the group?

Answer 1

Ionization enthalpy is a minimum amount of energy required to remove an electron from the outermost orbital. Ionization enthalpy of the main group elements tends to decrease down the group due to the following factors such as atomic size, shielding or screening effect and nuclear charge.

Complete step by step answer:
From your chemistry lessons you have learned about the ionization enthalpy of ionization energy and their trends.
Ionization enthalpy or energy is defined as the minimum amount of energy required by an isolated atom to remove an outermost electron from the valence shell in its gaseous state or neutral state.
As we know that on moving down the group ionization enthalpy of main group elements decreases. There are various factors which are responsible for the decrease in ionization enthalpy down the group and the following factors are:

(1) Atomic size:-
As we know that atomic size of the elements increases down the group because the number of main energy shells increases (n). Due to the increase in size the distance of the electron from the nucleus increases and force of attraction between the nucleus and the valence electron decreases. So, the energy required to remove valence electrons will be less and they can be easily removed. Thus the ionization energy decreases down the group.

(2) Shielding effect:-
On moving down the group the no. of inner shells of electrons also increases and thus the shielding effect on the valence or outermost electron from the nucleus due to the inner core electron will increase. So, the hold of the nucleus on the outermost electron will decrease, they will not be held tightly and as a result the energy required(ionization enthalpy) to remove valence electron will decrease down the group.

(3) Nuclear charge:-
Nuclear charge is the charge that is occupied by the electrons from the nucleus. As we move from top to bottom in a group the force of attraction between the nucleus and valence electron decreases and thus the nuclear charge occupied by the valence electron decreases. Thus the ionization energy will be low.

Note: Oxygen and Boron family are the exceptions because they do not follow the trends of decrease in ionization energy down the group. Ionization enthalpy increases as we move across the period because the atomic size of the elements decreases across the period and the force of attraction felt by the electron from the nucleus will be high and thus it requires more energy to remove an electron from the valence shell.