The oxidizing power of XO2 (X= C, Si, Sn or Pb) increases as the group is descended. PbO2 can oxidize conc. HCl to Cl2 but SnO2 & SiO2 cannot. This is because with increasing atomic radius and thus a decreasing attraction for the outer electrons.This cause increasing delocalisation, which is characteristic of metals. Also, +2 oxidation state stability increases down the group.