Oxygen, a gas we breathe, is often associated with air, which is understood to be a blend of multiple components. This relationship causes confusion regarding oxygen’s chemical classification. To classify pure oxygen definitively, it is necessary to establish the chemical definitions for the categories of matter: element, compound, and mixture. This framework allows for a clear understanding of oxygen’s structure and behavior, distinguishing it from the gaseous environment it helps to create.
Defining the Categories: Element, Compound, Mixture
The three fundamental categories of matter describe how atoms are organized and connected. An element represents the simplest form of matter, composed entirely of only one type of atom. Every particle within an element has the same number of protons in its nucleus, which is the defining characteristic of that substance. Elements cannot be broken down into simpler chemical substances by ordinary chemical means.
A compound is formed when two or more different types of elements are chemically bonded together. The atoms must be linked in a precise, fixed ratio, such as the two hydrogen atoms and one oxygen atom that form a water molecule ($\text{H}_2\text{O}$). This chemical bond results in a new substance with properties entirely unlike the elements from which it was formed. Breaking a compound requires a chemical reaction that breaks the atomic bonds.
A mixture involves two or more substances that are physically combined but are not chemically bonded. The components retain their individual chemical identities and properties, meaning the oxygen still behaves like oxygen and the nitrogen still behaves like nitrogen. A mixture does not have a fixed composition; the ratio of the components can be varied continuously.
Classifying Pure Oxygen Gas
Pure oxygen gas, represented by the chemical formula $\text{O}_2$, is not a mixture. It is a diatomic molecule, meaning it consists of two oxygen atoms chemically bonded together by a double covalent bond. Because it is exclusively made up of only one type of atom—oxygen—it belongs to the element category.
The two oxygen atoms are linked in a fixed 1:1 ratio, and this strong chemical bond means that $\text{O}_2$ must be separated by chemical means, not simple physical processes. While the $\text{O}_2$ molecule is the molecular form of the element oxygen, its structure means it adheres to the rules of a single substance with uniform properties. $\text{O}_2$ is thus classified as a pure substance, which is a category that encompasses both elements and compounds, and is the opposite of a mixture.
The existence of pure oxygen as a chemically bonded $\text{O}_2$ molecule prevents it from being defined as a mixture. If it were a mixture, the components could be easily separated without breaking a chemical bond, which is not possible for $\text{O}_2$.
Why Atmospheric Air is a Mixture
The source of the common misconception about oxygen’s classification is atmospheric air, which is definitively a mixture. Air is a physical blend of several different gases, primarily nitrogen ($\text{N}_2$) and oxygen ($\text{O}_2$), along with argon ($\text{Ar}$) and trace amounts of others like carbon dioxide ($\text{CO}_2$) and neon. These gases are not chemically bonded to one another; they simply occupy the same space. The defining characteristic of air as a mixture is the variability of its composition, as the concentration of components like water vapor can vary dramatically.
The components of air can be physically separated using techniques that do not require breaking chemical bonds. Industrial separation relies on physical processes like cryogenic distillation, which cools air until it liquefies and separates the components based on their different boiling points. Nitrogen boils off first at $-196\,^{\circ}\text{C}$, followed by argon and then oxygen at $-183\,^{\circ}\text{C}$. This ability to isolate components using differences in physical properties reinforces that air is a physical combination—a mixture.