Inert Pair Effect in 60 Seconds!

 

❓ Question

Why is Al³⁺ stable, Tl⁺ highly stable, and Tl³⁺ a strong oxidising agent?
Let’s break the entire concept in one crisp explanation.

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✍️ Short Explanation

The inert pair effect refers to the reluctance of the ns² electrons (the "inert pair") to participate in bonding for heavy p-block elements.
This means as we go down a group, especially in groups 13–16, the lower oxidation state becomes more stable, and the higher oxidation state becomes less stable.

Let's apply this to Group 13: Al → Ga → In → Tl.

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🔹 1. Why Al³⁺ is Stable

• Aluminium is in the 3rd period.

• Relativistic effects are very small.

• The 3s² electrons are not inert — they easily participate in bonding.

• So Al prefers oxidation state +3 (Al³⁺).

👉 Al³⁺ is very stable, does not get reduced easily.

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🔹 2. Why Tl⁺ is More Stable than Tl³⁺

• Thallium (Tl) is in the 6th period.

• The 6s² electrons are held tightly due to relativistic contraction.

• They behave like an “inert pair.”

• Tl finds it easier to lose only the 6p electron, giving Tl⁺,
  rather than losing all 6s + 6p electrons (which would give Tl³⁺).

👉 Tl⁺ is the most stable oxidation state of thallium.

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🔹 3. Why Tl³⁺ is a Powerful Oxidising Agent

• Tl³⁺ has lost the inert 6s² pair, making it highly unstable.

• It strongly wants to gain electrons to return to the stable Tl⁺ state.

• This means it oxidises other substances while getting reduced to Tl⁺.

👉 Tl³⁺ = strong oxidising agent
because

$$\mathrm{Tl^{3+} \rightarrow Tl^{+}}$$

is strongly favoured.

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🧮 Solution Summary

Species	Oxidation State	Stability	Reason
Al³⁺	+3	Highly stable	3s² electrons participate easily in bonding
Tl⁺	+1	Most stable	6s² inert pair does not participate
Tl³⁺	+3	Unstable	Loses inert s-pair → strong oxidising agent

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✅ Conclusion & Video Solution

📌 Final Concept Summary:

• Al³⁺ is very stable → no inert-pair effect.

• Tl⁺ is the most stable oxidation state → strong inert-pair effect.

• Tl³⁺ is unstable and a strong oxidising agent → reduces itself back to Tl⁺.

$$\boxed{\text{Inert Pair Effect increases down the group → Stability shifts from +3 to +1.}}$$