Here's why:

* Elements are the simplest pure substances that cannot be broken down further by chemical means. They are made up of only one type of atom.

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

Oxygen (O) exists as a diatomic molecule in its natural state (O₂), meaning two oxygen atoms are bonded together. However, it is still considered an element because it consists of only one type of atom.

Here's an analogy: Think of a LEGO set.

* Elements are like individual LEGO bricks (oxygen atoms).

* Compounds are like structures built from multiple different LEGO bricks (like water, which is made of hydrogen and oxygen atoms).

So, while oxygen exists as a diatomic molecule (O₂), it's still classified as an element because it only contains one type of atom: oxygen.