Animated Solution for Chemistry - s and p-Block Elements: The relative stability of $+1$ oxidation state of group 13 elements follows the order

Why Thallium Prefers $+1$

When we study the periodic table, we often expect elements in the same group to behave identically. Group 13 elements—Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), and Thallium (Tl)—all share the same general valence electronic configuration: $n{s}^{2}n{p}^1$.

Naturally, you would expect them to lose all three of these valence electrons to form a stable $+3$ oxidation state. And for the lighter elements like Boron and Aluminium, this is absolutely true. However, as we journey down the group towards the heavier elements, a fascinating plot twist occurs. The $+1$ oxidation state begins to emerge from the shadows, eventually becoming the dominant and most stable state for Thallium.

To understand this anomaly, we have to look deep inside the atomic structure. As we move from Aluminium to Gallium, and further down to Indium and Thallium, the inner electron shells begin to fill up with $d$ and $f$ electrons.

Here is the catch: unlike the spherical $s$ orbitals or the dumbbell-shaped $p$ orbitals, the $d$ and $f$ orbitals have highly diffused and complex shapes. Because they are so spread out, they act like a very poor, transparent shield. They fail to effectively screen the outermost valence electrons from the pulling power of the positively charged nucleus.

Because of this poor shielding, the effective nuclear charge ($Z_{\text{eff}}$) felt by the outermost electrons increases significantly. The nucleus acts like a strict teacher, grabbing hold of the inner $n{s}^2$ electrons with an iron grip.

These two $s$-electrons are pulled so tightly towards the nucleus that they refuse to participate in chemical bonding. They effectively become 'inert'. This phenomenon is famously known as the Inert Pair Effect.

As a result, when a heavy atom like Thallium reacts, it only easily lets go of its single, loosely held $n{p}^1$ electron. Losing just this one electron results in the $+1$ oxidation state.

The reluctance of the $n{s}^2$ electrons to bond increases progressively as we go down the group because the shielding gets progressively worse (especially after the introduction of $4f$ electrons in Thallium).

Therefore, the stability of the $+1$ oxidation state strictly increases down the group. Aluminium is the least stable in the $+1$ state, while Thallium is the most stable.

A crucial tip for your exams: Because Thallium is so stable in the $+1$ state, its $+3$ state (${\text{Tl}}^{3+}$) is highly unstable. It desperately wants to gain two electrons to revert to ${\text{Tl}}^{+}$, making ${\text{Tl}}^{3+}$ a very powerful oxidizing agent!