What are the factors that decide the ionization potential?
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(i) Atomic radius:
The values of ionisation potential of an element decreases as its atomic radius increases. This is because the elecrostatic force of attraction between the nucleus and the outermost electron decreases as the distance between them increases. So the energy required for the removal of electron will comparatively be less

(ii) Effective nuclear charge:
The greater the effective charge on the nucleus of an atom, the more difficult it would be to remove an electron from the atom because electrostatic force of attraction between the nucleus and the outermost electron increases. So the greater energy will be required to remove the electron.
Ionisation potential ∝ Effective nuclear charge (Zeff)
(iii) Penetration effect of orbitals:
The order of energy required to remove electron from s,p,d-and f-orbitals of a shell is s > p > d > f because the distance of the electron from the nucleus increases.
For example : The value of ionisation potential of Be(Z = 4 , Is2 2s2) and Mg(Z = 12, 1s2 2s2 2p6 3s2) are more than the I.P.s of B (Z = 5 , 1s2 2s2 2p1x) and Al (Z= 13, 1s2 2s2 2p6 3s23p2x) because the penetration power of 2s and 3s electrons is more than 2p and 3p orbitals respectively. More energy will be required to separate the electrons from 2s and 3s orbitals.
(iv) Shielding or screening effect:
The shielding or screening effect increases if the number of electrons in the inner shells between the nucleus and the outermost electrons increases. This results in decrease of force of attraction between the nucleus and the outermost electron and lesser energy is required to separate the electron. Thus the value of I.P. decreases.
Ionisation potential ∝ (1/Shielding or screening effect)
(v) Stability of half−filled and fully-filled orbitals:
According to Hund’s rule the stability of half filled or completely filled degenerate orbitals is comparatively high. So comparatively more energy is required to separate the electron from such atoms.
For example :
(a) Removal of electron is comparatively difficult from the half filled configuration of N (Z = 7 , Is2 2s2).
(b) The ionisation potential of inert gases is very high due to most stable s2p6 electronic configurations.
The values of ionisation potential of an element decreases as its atomic radius increases. This is because the elecrostatic force of attraction between the nucleus and the outermost electron decreases as the distance between them increases. So the energy required for the removal of electron will comparatively be less

(ii) Effective nuclear charge:
The greater the effective charge on the nucleus of an atom, the more difficult it would be to remove an electron from the atom because electrostatic force of attraction between the nucleus and the outermost electron increases. So the greater energy will be required to remove the electron.
Ionisation potential ∝ Effective nuclear charge (Zeff)
(iii) Penetration effect of orbitals:
The order of energy required to remove electron from s,p,d-and f-orbitals of a shell is s > p > d > f because the distance of the electron from the nucleus increases.
For example : The value of ionisation potential of Be(Z = 4 , Is2 2s2) and Mg(Z = 12, 1s2 2s2 2p6 3s2) are more than the I.P.s of B (Z = 5 , 1s2 2s2 2p1x) and Al (Z= 13, 1s2 2s2 2p6 3s23p2x) because the penetration power of 2s and 3s electrons is more than 2p and 3p orbitals respectively. More energy will be required to separate the electrons from 2s and 3s orbitals.
(iv) Shielding or screening effect:
The shielding or screening effect increases if the number of electrons in the inner shells between the nucleus and the outermost electrons increases. This results in decrease of force of attraction between the nucleus and the outermost electron and lesser energy is required to separate the electron. Thus the value of I.P. decreases.
Ionisation potential ∝ (1/Shielding or screening effect)
(v) Stability of half−filled and fully-filled orbitals:
According to Hund’s rule the stability of half filled or completely filled degenerate orbitals is comparatively high. So comparatively more energy is required to separate the electron from such atoms.
For example :
(a) Removal of electron is comparatively difficult from the half filled configuration of N (Z = 7 , Is2 2s2).
(b) The ionisation potential of inert gases is very high due to most stable s2p6 electronic configurations.
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