* Elements are made up of only one type of atom. For example, oxygen (O) is an element.

* Compounds are formed when two or more different elements combine chemically.

O2 represents a molecule of oxygen gas, which is a diatomic element. The "2" indicates that there are two oxygen atoms bonded together.

The formula "2O2" would imply two separate oxygen molecules, but that's not a standard way to represent them. It's redundant.