Consider the reaction, S2O8-2+I-→Fe+3SO4-2+I2 ;  The element undergoing oxidation is ------- and the element undergoing reduction is------

Iodide oxidizes to iodine;In persulphate ion there are six oxygen atoms with -2 state and 2 oxygen atoms with -1 state. In sulphate ion all oxygen atoms are in -2 state;Thus persulphate reduces to sulphate where oxygen atom undergoes reduction;