sulfur trioxide

Pure sulfur trioxide is a white solid with a low melting and boiling point. It reacts very rapidly with water vapour in the air to form sulfuric acid. That means that if you make some in the lab, you tend to see it as a white sludge which fumes dramatically in moist air (forming a fog of sulfuric acid droplets). Gaseous sulfur trioxide consists of simple SO₃ molecules in which all six of the sulfur's outer electrons are involved in the bonding.

There are various forms of solid sulfur trioxide. The simplest one is a trimer, S₃O₉, where three SO₃ molecules are joined up and arranged in a ring.

There are also other polymeric forms in which the SO₃ molecules join together in long chains. For example:

It is difficult to draw this convincingly. In fact, on each sulfur atom, one of the double bonded oxygens is coming out of the diagram towards you, and the other one is going back in away from you.

The fact that the simple molecules join up in this way to make bigger structures is what makes the sulfur trioxide a solid rather than a gas.