Why does reducing property of dioxide decreases from SO2 to TeO2 and SO2 is reducing while TeO2 is an oxidisng agent.
The dioxide molecules contain pπ–pπ bonds which become weaker with increase in atomic number because, increase in atomic no. increases size along a group and hence there occurs an increase in the bond length and so the reducing character decreases down the group.
Due to presence of d-orbitals(vacant) sulphur can extend its co valency and show oxidation states till +6, which is stable in sulphur (eg: SF6). Hence it acts as a reducing agent, and now the character decreases down the group because of decrease in strength of the pπ–pπ bonds while, unlike sulphur Te cannot show +6 oxidation state as it is highly unstable due to inert pair effect. Therefore there can only be a decrease in it oxidation state (it can decrease to -2, +2 etc.
Hence, TeO2 acts as oxidizing agent.