What are the various factors due to which the ionisation enthalpy of the main group elements tends to decrease down a group?
The ionisation enthalpy of the main group elements decreases regularly on moving down the group due to the fallowing two factors
(i) Atomic size On moving down the group, atomic size increases due to addition of new higher energy shell as a result of this forces of attraction of nucleus for valence electrons decreases and ionisation enthalpy also decreases.
(it) Screening effect On moving down the group, screening effect or shielding effect increases, so ionisation enthalpy decreases (because forces of attraction between nucleus and electrons decreases).