Oxygen is an element, not a compound, and can be found on the periodic table with the atomic number eight. Oxygen is primarily found in two common forms, oxygen gas and in air. Oxygen gas molecules are made of two oxygen atoms covalently bonded together, but it is still considered an element; air is a mixture of mainly oxygen and nitrogen, which makes it a compound.
Elements are pure substances that form a single atom. Atoms can then be bonded to each other in order to form compounds. For example, carbon dioxide is a compound composed of one carbon atom and two oxygen atoms. Finally, mixtures describe substances that are composed of two or more individual compounds or elements.