Which of the following is NOT considered a compound?

The correct choice, which identifies what is not considered a compound, is based on the definition of a compound. A compound is a substance formed when two or more different elements are chemically bonded together.

In this context:

• Water is a compound consisting of two hydrogen atoms and one oxygen atom (H2O).
• Carbon dioxide is also a compound made up of one carbon atom and two oxygen atoms (CO2).
• Table salt, or sodium chloride, is a compound formed from sodium and chlorine (NaCl).

Air, however, is a mixture rather than a compound. It is composed of various gases—primarily nitrogen and oxygen—along with small amounts of other gases such as carbon dioxide and argon. These components retain their individual properties and are not chemically bonded together, which is a key indicator that air does not qualify as a compound. Thus, the identification of air as the correct answer reflects an understanding of the distinction between mixtures and compounds.