Discuss in brief the factors which influence the magnitude of ionsiation enthalpy.

The main factors are:
(i) Size of the atom: As the size of the atom increases, the distance between the nucleus and outermost electrons increases. So the attraction between the nucleus and the outermost electron decreases and the enthalpy required to remove the electron also decreases. Thus, with an increase in atomic size, the ionisation enthalpy decreases.

(ii) Nuclear charge: With the increase in nuclear charge, the force of attraction between the nucleus and the valence electrons increases. Consequently, more enthalpy is required to remove a valence electron. Hence ionisation enthalpy increases with the increase in nuclear charge.

(iii) Screening effect of inner shells: The electron shells present in between the outermost shell and the nucleus act as screens. These shells reduce the attraction of nuclear charge over the outermost electrons. This reduction in force of attraction by the shells present in between the nucleus and valence electrons is called screening or shielding effect. As the number of inner shells increases, the screening effect also increases. As the screening effect increases, the ionisation enthalpy decreases.

(iv) Penetration effect of electrons: It is well known that s-electrons are more penetrating towards the nucleus than p-electrons and the penetration power decreases in a given shell (same value of n) in the order s >p >d>f.
Now if the penetration of the electron is more, it will be closer to the nucleus and held firmly. Hence ionisation enthalpy will be high. Thus, for the same shell, it is easier to remove p-electron in comparison to s-electron. Therefore, for the same value of n, ionisation enthalpy decreases in the order s > p > d > f

(v) Stability of the electronic configuration: More enthalpy is required to remove an electron from an element having configuration in which all the orbitals of the same sublevel are exactly half filled (p³ or d⁵ : stable configuration) or completely filled (p⁶or d¹⁰: stable configuration). Thus, more stable the electronic arrangement, greater is the ionisation enthalpy.