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

Explanation

(1) Atomic size : With increase in the atomic size, the distance from the nucleus to the outer electrons increases. So the attraction of the nucleus on outer electrons decreases. Hence IE decreases.
(2) Nuclear charge: With increase in nuclear charge i.e., effective nuclear charge, attraction of the nucleus on the outer electrons increases. So IE increases.
(3) Screening effect or Shielding effect: The inner orbits shield the nuclear attraction on the outer electrons. So with increase in the inner electrons shielding effect increases and thus IE decreases.
(4) Extent of penetration of valence shell into inner electron: The penetrating power of the orbitals towards the nucleus is in the order `s > p > d > f`. Nuclear attraction on the electrons in these orbitals also will be in the same order. So to remove an electron from different orbitals of the same orbit the energy required is in the order `s > p > d > f`.
(5) Number of charges on the ion : With increase in the number of positive charges on an ion the nuclear attraction on the electrons increases. So IE increases.
(6) Electronic configuration: Atoms having octet in the outer orbit, or exactly half-filled and completely filled orbitals give stability to the atom. The energy required from the stable electronic configurations will be more.