Here's why:

* Atoms are the basic building blocks of matter. Oxygen (O) is an atom.

* Molecules are formed when two or more atoms bond together. Oxygen gas, which we breathe, exists as a molecule with two oxygen atoms bonded together (O₂).

Oxygen compounds are different. They are formed when oxygen atoms bond with other types of atoms. Examples of oxygen compounds include:

* Water (H₂O): Two hydrogen atoms bonded to one oxygen atom.

* Carbon dioxide (CO₂): One carbon atom bonded to two oxygen atoms.

So, while oxygen is a molecule (O₂), it can also be found in many different compounds.