These elements form monoxides (MO), dioxides (MO2). Ozone (O3) and sulphur dioxide (SO2) are gases while selenium dioxide (SeO2) is solid. Reducing property of dioxide decreases from SO2 to TeO2 ; SO2 is reducing while TeO2 is an oxidizing agent. Besides MO2 type oxides sulphur, selenium and tellurium also form MO3 type oxides (SO3 , SeO3 , TeO3). Both types of oxides are acidic in nature. Increasing order of acidic nature of oxides is TeO3 < SeO3 < SO3