Here's why:

* Oxidation: Oxygen is a highly reactive element, and it readily combines with other elements in a process called oxidation.

* Metal Oxides: When oxygen reacts with metals, the metal atoms lose electrons and form positively charged ions (cations). The oxygen atoms gain electrons and form negatively charged ions (anions). These oppositely charged ions attract each other and form an ionic compound called a metal oxide.

Examples of Metal Oxides:

* Iron oxide (rust): Fe₂O₃

* Aluminum oxide: Al₂O₃

* Copper oxide: CuO

* Sodium oxide: Na₂O