* Compounds are formed when two or more different elements chemically combine in a fixed ratio.

Let's break down each option:

* Water (H₂O): A compound made of hydrogen and oxygen.

* Carbon Dioxide (CO₂): A compound made of carbon and oxygen.

* Gas: This is a state of matter, not a specific compound. There are many gases, including oxygen gas.

* Silicon (Si): This is an element, a pure substance that cannot be broken down into simpler substances.

* Oxygen Gas (O₂): A compound made of two oxygen atoms.

Therefore, silicon is the only option that is not a compound.