Trioxides : Sulfur, selenium and tellurium can form trioxides also.

(a) Sulfur trioxide, SO₃ : In the gaseous state monomeric SO₃ has a planar structure with S–O bond distance of 143 pm and O–S–O bond angle of 120^o.SO₃ molecule is a resonance hybrid of following structures.

In the solid phase sulfur trioxide polymerizes to cyclic trimer or to a stable linear chain structure. SO₃ is the anhydride of H₂SO₄ . It is acidic in nature and acts as oxidizing agent.

(b) Selenium trioxide, SO₃ : it is a solid substance which exists as a cyclic tetramer, however in the vapor phase it exists as a monomer

(c) Tellurium trioxide, TeO₃ : It is a solid at room temperature existing as a polymer.

The increasing order of acidic nature of oxides is TeO₃ < SeO₃ < SO₃