Selenium trioxide (SeO3) is a chemical compound used in the manufacture of photoelectric cells and solar energy devices. A laboratory preparation is to react anhydrous K2SeO4 and SO3. The compound is thermodynamically unstable with respect to selenium dioxide. The solid consists of cyclic tetramers, Se4O12. In the gas phase there is some dissociation into the monomer. It is a strong oxidizing agent, as is selenic acid and all selenates.The molecular shape of selenium trioxide is triagonal planar. Selenium trioxide is a nonpolar molecule. the bonds between the selenium and the oxygen are double bonds. The bond angles are all 120°, and the molecule is flat. There are no lone bonding pairs.