Oxygen gas is classified as an element. An element is a pure substance that cannot be broken down into simpler substances by chemical means. Oxygen exists in nature primarily as diatomic molecules (O2), which consist of two oxygen atoms bonded together. This makes it a pure substance, confirming its status as an element.
In contrast, a compound is a substance formed when two or more different elements are chemically bonded together, such as water (H2O) or carbon dioxide (CO2). A mixture, on the other hand, consists of two or more substances that are physically combined but not chemically bonded, like air or a salad. Since oxygen gas is composed solely of oxygen molecules and does not contain other substances or elements, it is not a compound or a mixture, but rather an element.