Trend 3.2 of the Boron Group (Group IIIA) Trends is that Aluminum, Gallium, Indium, and Thallium all form a range of compounds in the +3 oxidation state and compounds in the +1 oxidation state become progressively more stable down group III.
Why do Gallium, Indium, and Thallium form +3 oxidation states?
Aluminum, Gallium, Indium and Thallium all form stable compounds in the +3 oxidation state because when they lose three electrons they gain a Neon core. In other words, they goes back to having 8 valence electrons because they lose their one p electron and two s electrons. Looking at Gallium, Indium, and Thallium this may not make sense until you realize that electrons are lost from the outer shells first and that the d orbitals are inside the s orbitals because d orbitals go down in energy during ionization.
But why is the +1 oxidation state stable?
However, this does not explain why the +1 oxidation state becomes more stable down the column. In reality only Thallium is stable in the +1 oxidation state. Indium can occasionally form a +1 oxidation state but it is much, much more stable in +3 oxidation state. Gallium and Aluminum are very unstable in +1 oxidation state.
Why is Thallium okay with a +1 oxidation state?
Thallium is in the 6th row. In the 6th row are the Lanthanides which have 4f orbitals. Because the Lanthanides exist between Thallium and Cesium/Barium, there is a lot more shielding from the charge in the nucleus. This is known as the 6s inert pair effect. This means that only the p electron can participate in bonding and thus a +1 oxidation state.
But can’t Thallium be stable in a +3 oxidation state too?
Yes. Thallium can still be stable in a +3 oxidation state because when it will lose its outer most electrons first. The 6p and both 6s electrons would be lost bringing it to the Xenon core before the 4f orbitals and 5d orbitals would be lost. This is because during ionization the d orbitals go down in energy. This is also related to the relativistic effect . However, the idea that d orbitals go down in energy is enough to understand the trend.
Okay, but why is Indium occasionally stable in a +1 oxidization state?
When Indium loses one electron it leaves two s orbitals on the outside. This is best explained by the relativistic effect. It can also experience additional shielding from the d block.