Oxygen is classified as an element. An element is a pure substance that cannot be broken down into simpler substances by chemical means. Oxygen, with the chemical symbol O, is found on the periodic table and is essential for respiration in most living organisms.
In contrast, a compound consists of two or more elements that are chemically bonded together, like water (H2O) which is made of hydrogen and oxygen. A heterogeneous mixture is one where the components remain separate and can be seen, such as a salad, while a homogeneous mixture has a uniform composition throughout, like saltwater.
Since oxygen occurs naturally as a single type of atom and is not composed of different atoms or elements bonded together, it is categorized distinctly as an element.