Animated Solution for Chemistry - s and p-Block Elements: Aluminium is usually found in +3 oxidation state. In contrast, thallium exists in +1 and +3 oxidation states. This is due to

Have you ever wondered why elements in the same group of the periodic table sometimes behave so differently? Take Group 13, for example. It includes Boron, Aluminium, Gallium, Indium, and Thallium. They all share the same outermost electronic configuration: $n{s}^{2}n{p}^1$. With three valence electrons, you would naturally expect them all to happily form $+3$ ions.

And for the lighter elements like Aluminium, this is exactly what happens. Aluminium loves being in the $+3$ oxidation state. But as we travel down the group to the heavier elements like Thallium, a strange phenomenon occurs. Thallium often prefers to lose only one electron, forming a $+1$ state, while its $+3$ state becomes surprisingly unstable. Why does this happen? The answer lies deep within the atom, in a concept known as the inert pair effect.

Poor Shielding

To understand the inert pair effect, we need to look at the inner electrons. As we move down a group, the atomic number increases, meaning the nucleus gets more protons and becomes more positively charged. At the same time, new electron shells are added.

For heavier elements like Thallium, the inner shells include $d$ and $f$ orbitals. Here is the catch: $d$ and $f$ orbitals are large and diffuse. They are terrible at shielding the outermost electrons from the pull of the nucleus. Because these inner electrons fail to block the nuclear charge effectively, the outermost $s$ electrons (the $n{s}^2$ pair) feel a much stronger pull from the nucleus than expected.

Because the $n{s}^2$ electrons are pulled so tightly towards the nucleus, they become chemically 'inert' or reluctant to participate in bonding. They prefer to stay paired up in their orbital rather than being shared or lost.

This means that for Thallium, it is relatively easy to lose the single $p$ electron to form a $T{l}^{+}$ ion. However, trying to remove the two $s$ electrons to form a $T{l}^{3+}$ ion requires a massive amount of energy—often more energy than is recovered by forming chemical bonds.

This is why the stability of the $+3$ oxidation state decreases down the group:

Conversely, the stability of the $+1$ oxidation state increases down the group:

Aluminium, being higher up in the group, doesn't have these poorly shielding $d$ and $f$ electrons in its core. Its $s$ electrons are readily available for bonding, making the $+3$ state its most common and stable form. Thallium, burdened by the poor shielding of its massive core, falls victim to the inert pair effect, making its $+1$ state highly prominent.

Understanding the inert pair effect unlocks the secret to predicting the chemical behavior of heavy p-block elements!