Inert Pair Effect

The Inert Pair Effect is often considered to be a minor effect on the properties of an element. But this effect is of profound importance not only on the element but also on the trends of the periodic table. 

The inert-pair effect occurs mainly in elements that come after the transition elements in the periodic table. It is the tendency of the outermost s-orbital electrons to not participate in chemical reactions.


The inert pair theory was proposed by Nevil Sidgwick. He originally devised it in order to explain the phenomenon of stable +1, +2, +3 and +4 oxidation states in the heavier elements of the 13th, 14th, 15th, and 16th groups of the periodic table. 

Sidgwick was an English chemist who was fascinated with explaining chemical bonding through atomic structure, and it was through this study that Sidgwick arrived at the inert-pair theory.

Another possible explanation to the inert-pair effect was given by Drago, based on the M-X bond enthalpies and ionization energies for various oxidation states.

Nevil Vincent Sidgwick proposed Inert pair effect.
Nevil Vincent Sidgwick (Source)

Inert Pair Effect Explained

The inert-pair effect is the phenomenon of non-participation of valence shell filled s-orbital electrons in the chemical reactions of the element or the inability of the electrons in the valence shell s-orbital to ionize.

Sidgwick tried to explain this effect through the sudden rise in the ionization enthalpies of the p-block elements at their second ionization enthalpy.

An alternative explanation of the inert pair effect was Drago in 1958 attributed the effect to low M−X bond enthalpies for the heavy p-block elements and the fact that it requires less energy to oxidize an element to a low oxidation state than to a higher oxidation state.

The inert-pair effects were proposed to explain the greater stability of lower oxidation states than the higher oxidation states of heavy p-block elements. This theory fails to explain some of the cases though, which we can consider as limitations.

Ionization Enthalpy
(∆Hⱼ KJ/mol⁻¹)
Table 1: Ionization energies of 13th group elements which are influenced by inert-pair effects and lanthanide contraction. (Source: NCERT XI Chemistry)

In Table 1, the ∆H₂ of the 13th group elements is almost double compared to ∆H₁. This observation was taken as evidence for the inert-pair theory by Sidgwick.

The order of relative stability of +1 oxidation state among 13th group elements: Al< Ga< In< Tl. In the case of Tl, its +1 state is more predominant than the +3 state and Tl⁺³ immediately reduces to form Tl⁺¹. 

Limitations of Inert Pair Effect

There are some limitations in the inert pair theory. If we consider its explanation of ionizability of electrons or in other words their expected high value of ionization enthalpy.

Let us consider the same example of 13th group elements, in order to see some of these limitations.

Ionization Enthalpy
Table 2: Ionisation enthalpy values of s-orbital electrons.

Basically, the Ionization enthalpies have to decrease as we go down the group.

But in Table 2, there are a few anomalies: Increase in ionization enthalpy from Al to Ga and from In to Tl.

These observations cannot be explained using Sidgwick’s explanation of the inert pair theory. Their simple explanations are Ga’s IE(ionization enthalpy) is affected by d-block contraction and Tl’s relatively high IE is due to relativistic effects caused by poor shielding of d- and f- orbitals.

The latest explanation of the inert-pair theory is:

In the p-block elements which come after d-block elements-like 6th row elements- the poor shielding effect of d- and f- orbitals in them leads to an increase in the effect of nuclear charge on the valence shell s-orbital electrons thereby tightly holding them and decreasing their participation in bond formation process.

Applications of Inert Pair Effect

  1. Explains the relatively high stability of low oxidation states of heavy p-block elements.
  2. Explains the high increase IE values of s-orbital electrons in p-block elements.
  3. The highly oxidative nature and high reactivity of Tl⁺³ is also explained by this effect.
  4. Explains the anomaly in the melting and boiling points of elements.

Example Problems

Question 1. The E values of Al⁺¹/Al and Tl⁺¹/Tl are +0.55 and -0.34. Use these and the inert-pair effects to compare the stability of their +1 in solution and their non-metallic nature.

Answer. From the given half cell potentials we can say that Al⁺¹ is not stable at all compared to Tl⁺¹, but we can say that through inert-pair effect also as Tl has d and f-orbitals which have very poor shielding effect. As Al⁺¹ is unstable and reduces to Al⁺³ it has less non-metallic nature, whereas Tl⁺¹ is stable so it has more non-metallic nature.

Question 2. The boiling points of Te and Po are 725K and 520K respectively. Explain this anomaly using the inert-pair theory.

Answer. According to the trends in the periodic table the boiling points of elements must increase going down the group. This anomaly is because according to the inert-pair effects, the s-orbital electrons must be tightly bound due to poor shielding effects. Here, Po is also under the effect f-orbital not only the d-orbital like Te, so, its s-orbital is even tightly bound, this reduces its intermolecular van der waals forces thereby reducing its boiling point.


What is the inert pair effect?

The inability of participation of valence shell s-orbital electrons in the chemical reactions of the element and their inability to ionize is called the inert-pair effect.

Why do B and Al not show the inert-pair effect?

The inert-pair effect is only shown by the elements which have d- and f-orbitals influencing their outermost s-orbital electrons by having poor shielding and increasing the effective nuclear charge on them.

Is the inert-pair effect valid for transition elements also?

No, the inert-pair effect is not valid to transition elements because, this effect only depends upon the s-orbital electrons and they do not have them. Moreover the d-orbital electrons in transition elements are degenerate(i.e, have the same energy), so, the inert-pair effect cannot be valid to them.

Why can Tl for both TlF₃ and TlF, whereas Al can only form AlF₃?

Tl can form both TlF and TlF₃ because of inert-pair effect. Due to inert-pair effect Tl is more stable in its +1 oxidation state so it can TlF. But it can also form TlF₃, because though not stable +3 oxidation state of Tl exists.

What does it mean when one says that electrons are inert?

Inert electron essentially means that it is reluctant to participate in a chemical reaction. This reluctance can have causes, but some important ones are inert-pair effect, poor shielding effect, and pairing between electrons of opposite spin.



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