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Why is thallium(III) oxide stable while thallium(III) chloride unstable

Both the compounds have +3 oxidation state of thallium and the electronegativity difference between chlorine and oxygen is not very large. Then why is there such a large difference between the stability of the 2 compounds?

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    $\begingroup$ The difference between standing 1 inch from a cliff and 1 inch over the cliff is not very large either, yet it may mean a great deal in the end. $\endgroup$ – Ivan Neretin Feb 28 '17 at 16:48
  • $\begingroup$ @IvanNeretin so is this difference basically due to electronegativity? How can we predict the stability of such species in the absence of experimental data? $\endgroup$ – Osheen Sachdev Feb 28 '17 at 17:26
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    $\begingroup$ This difference is due to oxygen and chlorine being different elements. They differ in size, in weight, in all aspects of chemistry and in pretty much everything else. It's not like we can take an element and tune its electronegativity to a different value, while leaving all other properties intact. $\endgroup$ – Ivan Neretin Feb 28 '17 at 17:53
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    $\begingroup$ As for predicting stability, I'd take a look at the oxidation potentials ($\ce{Tl^3+/Tl+, Cl2/Cl-,\text{ and }O2/OH-}$). True, they are measured in water solutions, and hence not applicable directly to solid compounds, but still they may give us a vague idea of what to expect. $\endgroup$ – Ivan Neretin Feb 28 '17 at 17:56
  • $\begingroup$ @IvanNeretin basically I came across this question which asked me to compare the stability of these 2 and a few other compounds but I didn't know what basis to use to do so in the absence of any data given to me. The oxidation states were same. So how can I compare their stabilities using periodic trends and no other data? $\endgroup$ – Osheen Sachdev Feb 28 '17 at 18:14
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The stability of thallium(III) oxide and thallium(III) chloride can be compared by observing two properties:

  1. Size of corresponding ions

Size of $\ce{Tl^3+}$ is 0.95 Å, size of $\ce{O^2-}$ is 1.44 Å and that of $\ce{Cl-}$ is 1.8 Å. So, it pretty seems that difference in size between thallium and oxygen atoms is less than that of difference in size between thallium and chlorine atoms. So, thallium(III) oxide is stable than that of thallium(III) chloride. (data)

  1. Difference in electronegativity

Electronegativity of thallium is 2.04, oxygen is 3.44 and that of chlorine is 3.16. So, difference in electronegativity of thallium-oxygen is more than that of thallium-chlorine. So, thallium(III) oxide is stable than that of thallium(III) chloride. (data)

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