Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason

Q: Why isn't $\text{TlI}_3$ formed by $\text{Tl}^{3+}$ and three $\text{I}^-$ ions?

$\text{Tl}^{3+}$ is a very strong oxidizing agent, and $\text{I}^-$ is a good reducing agent. If they were to exist together, $\text{Tl}^{3+}$ would immediately oxidize $\text{I}^-$ to $\text{I}_2$, reducing itself to $\text{Tl}^+$. Hence, it exists as $\text{Tl}^+$ and $\text{I}_3^-$.

Q: What exactly is the Inert Pair Effect?

It is the reluctance of the outermost $s$-electrons to participate in chemical bonding in heavier post-transition metals. This happens because poor shielding by inner $d$ and $f$ electrons causes the nucleus to hold the $s$-electrons very tightly.

Q: Does the Inert Pair Effect only apply to Thallium?

No, it applies to all heavier elements in the p-block, such as Lead (Pb) in Group 14 preferring $+2$ over $+4$, and Bismuth (Bi) also preferring $+3$ over $+5$.

Q: Why do $d$ and $f$ orbitals provide poor shielding?

Shielding depends on electron density near the nucleus. $s$ and $p$ orbitals are more penetrating and spherical/directional, whereas $d$ and $f$ orbitals are highly diffused and spread out, leaving "gaps" for the nuclear charge to leak through.

Q: What does "isomorphous" mean in this context?

Isomorphous means having the same crystal structure. $\text{TlI}_3$ and $\text{CsI}_3$ crystallize in the same lattice pattern because $\text{Tl}^+$ and $\text{Cs}^+$ have similar ionic radii and both form compounds with the linear $\text{I}_3^-$ anion.

Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) In $	ext{TlI}_3$, isomorphous to $	ext{CsI}_3$, the metal is present in $+1$ oxidation state.
Reason (R) Tl metal has fourteen f-electrons in the electronic configuration.
In the light of the above statements, choose the most appropriate answer from the options given below.

Options:

(A) A is correct but R is not correct.

(B) Both A and R are correct and R is the correct explanation of A.

(C) A is not correct but R is correct.

(D) Both A and R are correct but R is not correct explanation of A.

Sigma Percentile · Accepted Answer

### Analyzing the Setup

When we first encounter a compound like $	ext{TlI}_3$, our immediate instinct is to assign an oxidation state based on simple stoichiometry. Since there are three iodine atoms, it is tempting to assume that Thallium is in a $+3$ oxidation state, existing as $	ext{Tl}^{3+}$ and three $	ext{I}^-$ ions. However, chemistry is full of fascinating exceptions!

Thallium triiodide is actually a polyhalide. It is isomorphous to Cesium triiodide ($	ext{CsI}_3$). In reality, it does not exist as a simple ionic compound of $	ext{Tl}^{3+}$. Instead, it is composed of a Thallium cation ($	ext{Tl}^+$) and a triiodide anion ($	ext{I}_3^-$).

Therefore, the Assertion (A) is absolutely correct: Thallium is indeed present in the $+1$ oxidation state. But why does Thallium, a member of Group 13 (which typically shows a $+3$ state like Aluminum), prefer the $+1$ state?

### The Master Equation: Electronic Configuration

To understand this anomaly, we must dive into the electronic structure of Thallium. With an atomic number of $Z = 81$, its electronic configuration is:

$$_{81}	ext{Tl} : [	ext{Xe}] 4f^{14} 5d^{10} 6s^{2} 6p^{1}$$

Notice the presence of the fourteen $4f$ electrons in the anti-penultimate shell and the ten $5d$ electrons in the penultimate shell. This is where the magic of the **Inert Pair Effect** begins.

### The Inert Pair Effect

In multi-electron atoms, inner electrons shield the outer electrons from the attractive pull of the nucleus. However, not all orbitals shield equally. The $s$ and $p$ orbitals provide good shielding, but the $d$ and $f$ orbitals have highly diffused, complex shapes. Because of this diffused nature, the $4f$ and $5d$ electrons offer very **poor shielding**.

Due to this ineffective shielding, the outermost $6s$ electrons experience a much higher **Effective Nuclear Charge** ($Z_{	ext{eff}}$). The nucleus pulls these $6s^2$ electrons tightly towards itself, making them highly stable and reluctant to participate in chemical bonding. They essentially become "inert."

### Final Conclusion

Because the $6s^2$ pair is locked away by the strong nuclear pull, only the single $6p^1$ electron is easily available for bonding. This is why Thallium forms highly stable $+1$ compounds, while its $+3$ state is strongly oxidizing and relatively unstable.

The Reason (R) states that Thallium has fourteen $f$-electrons, which is the root cause of this poor shielding and the subsequent inert pair effect. Thus, both the Assertion and the Reason are correct, and the Reason is the perfect explanation for the Assertion!

Select Answer:

Visualized Solution

$Analyzing Assertion (A)$

$The Reality of TlI_{3}$

$Why + 1 State?$

$Electronic Configuration of Tl$

$The Shielding Effect$

$The Inert Pair Effect$

$Final Conclusion$

The Sigma Insight: Group 13 Elements

Analyzing the Setup

The Master Equation

The Inert Pair Effect

Final Conclusion

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