# Why does thallium form TlF and TlF3 but aluminium only forms AlF3?

Does this have something to do with the inert pair effect?

• Yes, that's exactly right. – Geoff Hutchison May 4 '15 at 15:11

Ahh, Thallium is a very nice element. Because it's weird.

The +3 oxidation states are favorable except for the heavier elements, such as Tl, which prefer the +1 oxidation state due to its stability; this is known as the inert pair effect. Chemwiki.UCDavis

As an example in group 13 the +1 oxidation state of Tl is the most stable and Tl(III) compounds are comparatively rare. Inert pair effect, Wikipedia

## So, why?

Inert pair effect was introduced to describe why the facts above are correct. However, the original explanation, provided by Sidgwick, relied on the fact that the $$s$$ subshell has an "inert" pair of electrons. This can't be correct, and Wikipedia uses the trend in ionization potentials of "$$s$$" electrons to prove it wrong: $$\ce{In < Al < Tl < Ga}$$

A better explanation lies within the lower bond enthalpies of $$\ce{MX}$$ where M is one of the p-block heavy elements.

To reach a high oxidation number, elements will need to have a strong bond. They wanna bond in order to lose energy (i.e. become more stable) after all! If the bond is weak, this energy isn't provided. Thus, a heavy element is better off with being less oxidised.

See this and this. (Sorry, they might be behind a paywall)

• -1, inert pair effect is definitely a real thing; it's due to the lanthanide contraction for the 6p elements + relativistic 6s contraction (the "inert pair"). Also, the thallium halides aren't molecular so invoking bond enthalpies doesn't give the full picture either. – J. LS May 4 '15 at 15:57
• Hmm...@J.LS I never said it's not real. Just that the description isn't very authentic. (Or rather, the first description) How do you explain the trend in ionization potentials? – M.A.R. May 4 '15 at 16:01
• @J.LS Figures out, the pattern in ionization potentials is due to the relativistic effects you mention. – M.A.R. May 4 '15 at 16:24
• No, the pattern in ionization potentials is due to the transition metal and lanthanide contractions and the associated decrease in shielding for Ga/Tl respectively; relativistic effects are only important for Tl and probably are of secondary importance. – J. LS May 4 '15 at 16:51