3.18 What are the various factors due to which the ionization enthalpy of the main group elements tends to decrease down a group?
Ionization enthalpy of the main group elements tends to decrease down a group because of the following reasons:
(a) Atomic Size: moving down the group there is an increase in the atomic size, increasing number of electron shells, which results in the weaker binding force with the nucleus. Hence the ionization enthalpy decreases.
(b) Screening or shielding effect: moving down the group increases the new shells, the number of inner electron shells which shield the valence electrons increases. As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence the ionization enthalpy decreases.