# Why is the oxidation state of Te in H2Te equal to -2?

Hydrogen has electronegativity of 2.2 whereas Telenium has electronegativity equal to 2.1. So since hydrogen is more electronegative than Telenium, as per what I have read, oxidation state of Te should be +2 in H2Te.

But actually it is -2, so where is my reasoning wrong?

• Electronegativity doesn't work in this manner for elements lower than second period. Jun 25 '21 at 3:19
• Oxidation states are merely a human convention. Jun 25 '21 at 6:22
• @Nisarg could you cite the source of your answer or explain why it doesn't work for elements of period 3 and higher? thanks... Jun 25 '21 at 6:43

Electronegativity is not a constant, but is compound specific. It depends e.g. on the oxidation state.

There are multiple methods of electronegativity calculation, where some elements may switch the order.

The most used Pauling electronegativity is explicitly based on energies of particular bonds, but for small differences it is ambiguous about the difference sign.

The difference in electronegativity between atoms A and B is given by: $$|\chi_{\rm A} - \chi_{\rm B}| = ({\rm eV})^{-1/2} \sqrt{E_{\rm d}({\rm AB}) - \frac{E_{\rm d}({\rm AA}) + E_{\rm d}({\rm BB})} 2}$$ where the Bond dissociation energy $$E_\mathrm{d}$$ of the A–B, A–A and B–B bonds are expressed in $$\mathrm{eV}$$, the factor $$(eV)^{- \frac 12}$$ being included to ensure a dimensionless result. Hence, the difference in Pauling electronegativity between hydrogen and bromine is 0.73 (dissociation energies: H–Br, 3.79 eV; H–H, 4.52 eV; Br–Br 2.00 eV)

$$\ce{H2Te}$$ is relatively strong acid with $$\mathrm{p}K_\mathrm{a} = 2.6$$, what gives a hint that in context of water $$\ce{H2Te}$$ dissociation, $$\ce{Te}$$ has higher electronegativity than $$\ce{H}$$. If the opposite had been the case, $$\ce{HTe+}$$ and $$\ce{H- }$$ would have been created by the dissociation, with following up $$\ce{H- + H2O -> H2 + OH-}$$.

Aside of that, oxidation state (OS) is a formal quantity. As a formal one, it is advantageous to keep the same OS for the analogical compounds along the group. For some , typically mostly non metal ones, hydrogen is kept at +1, for some others, like electropositive metals, as -1.

Another moot case is $$\ce{PH3}$$ and down the group. OS of these elements is conventionally kept as -3 here.

Be aware the OS is the tool to simplify things like compound categorization and redox equation enumeration. Keeping the strict rule of universal electronegativity for OS would complicate things by making outlying exceptions.