Sulphur dioxide is a reducing agent but tellurium dioxide is an oxidizing agent, why?
Oxidation no. of sulphur changes from 0 to +4 in SO2. Due to presence of d-orbitals(vacant) sulphur can extend its covalency & show oxidation states till +6, which is stable in sulphur (eg: SF6).
Hence it acts as a reducing agent. (PS it acts as both oxidising and reducing agent)
Oxidation no. of Te changes from 0 to +4 in TeO2. 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). Also, TeO2is a polymeric oxide which is not very much stable.
Hence it acts as an oxidising agent.