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Question 18

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

Answer

The minimum amount of energy which is needed to remove the most loosely bound electron from a neutral isolated atom to form a cation is called as ionization energy or enthalpy.

The factors responsible for the ionization enthalpy of the main group elements to decrease down a group are listed below:



(i) Increase in the atomic size of elements: As we move down a group, the number of  main energy shells(n)  increases. As a result, the atomic size also increases gradually on moving down a group. As the distance of the valence electrons from the nucleus increases, the electrons are not held very strongly. Thus, they can be removed easily. Hence, on moving down a group, ionization energy decreases.

(ii) Increase in the shielding effect: The number of inner shells of electrons increases on moving down a group. Therefore, the shielding of the valence electrons from the nucleus by the inner core electrons increases down a group. As a result, the valence electrons are not held very tightly by the nucleus. Hence, the energy required to remove a valence electron decreases down a group.

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