IONIZATION ENTHALPY , Class 11

IONIZATION ENTHALPY , ΔᵢH

A) Definition :-

The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom so as to convert it into a gaseous cation is called its ionization enthalpy or ionization energy.
M(g) + ΔᵢH → M⁺(g) + e^-(g)

B) Unit :-

electron volt/atom (eV/atom), kJ/mol, kcal/mol
1 electron volt/atom (eV/atom) = 23.06 kcal/mol = 96.49 kJ/mol

C) Successive ionization enthalpies :-

The ionization enthalpies required to remove first, second, third etc. electrons from an isolated gaseous atom are called successive ionization enthapies.
M(g) + ΔᵢH₁ → M⁺(g) + e^-(g)
M⁺(g) + ΔᵢH₂ → M⁺⁺(g) + e^-(g)
M⁺⁺(g) + ΔᵢH₃ → M⁺⁺⁺(g) + e^-(g)
Here,
ΔᵢH₁ → First ionization enthalpy
ΔᵢH₂ → Second ionization enthalpy
ΔᵢH₃ → Third ionization enthalpy

Relation among successive ionization enthalpies

ΔᵢH₃ > ΔᵢH₂ > ΔᵢH₁

Explanation :-

When one electron has been removed from the neutral gaseous atom, the +vely charged ion formed has one electron less than the number of protons in the nucleus. As a result, the electrostatic attraction between the nucleus and the remaining electrons in the cation increases, i.e., effective nuclear charge increases. The +ve ion holds its remaining electrons more firmly. Therefore, the energy required to remove another electron from this +vely ion or second electron from the neutral atom must be higher than the first i.e., ΔᵢH₂ > ΔᵢH₁
Further the removal of two electrons from the neutral atom gives a doubly +vely charged ion which will hold its remaining electrons even more tightly. As a result, the energy required to remove third electron from the gaseous atom should be even more than that required for the second electron, i.e., ΔᵢH₃ > ΔᵢH₂. Thus,
ΔᵢH₃ > ΔᵢH₂ > ΔᵢH₁

D) Factors governing the Ionization Enthalpy

(a) Nuclear charge :-

The ionization enthalpy increases with increase in nuclear charge.

(b) Atomic size or Radius :-

Ionization enthalpy decreases as the atomic size increases.

(c) Penetration effect of the electrons :-

Ionization enthalpy increases as the penetration effect of the electrons increases.Within the same shell, the penetration effect decreases in th order :
s > p > d > f

(d) Shielding or Screening effect of the inner shell electrons :-

As the shielding or the screening effect of the inner electrons increases, the ionization enthalpy decreases.

(e) Electronic configuration, i.e., Effect of exactly half-filled or completely filled orbitals:-

if an atom contains exactly half-filled or completely filled orbitals, then such an arrangement has extra stability. Therefore, the removal of an electron from such an atom requires more energy than expected.More stable the electronic configuration, greater is the ionization enthalpy.

E) Variation of Ionization Enthalpy in the periodic table

(a)Variation along a period :-

In general, as we move from left to right in a period, the ionization enthalpy increases with increasing atomic numbers.

Explanation :-

As we move across a period from left to right , the nuclear charge increases and the atomic radius decreases though the principle quantum number of the valence shell remains the same. As a result of increased nuclear charge and simultaneous decrease in atomic radius , the valence electrons are more and more tightly held by the nucleus as we move from left to right in a period. Consequently, more and more energy is needed to remove the electron and hence ionization enthalpies keep on increasing .
However, some elements show irregularities in this general trends due to
(i)type of electrons to be removed and
(ii) the extra stability of the exactly half filled and completely filled electronic configurations.