monatomic

Monatomic refers to chemical species that consist of single atoms rather than molecules formed by bonds between atoms. In chemistry, most elements exist as diatomic or polyatomic molecules under ordinary conditions, but some elements can appear as monatomic units, especially in the gas phase as noble gases.

Monatomic gases are atoms that do not bond to other atoms under the conditions in which they are observed. The noble gases—helium, neon, argon, krypton, xenon, radon, and oganesson—are the principal examples of monatomic gases at standard conditions. They are characterized by chemically inert behavior due to their complete valence electron shells, very low reactivity, and weak interatomic forces. Their physical properties include low melting and boiling points and a colorless, odorless appearance in the gaseous state. In condensed phases, noble gases form crystalline solids consisting of individual atoms separated by weak interactions rather than molecules.

Monatomic species are used in various applications that require inert environments. Argon is widely used as a shielding gas in welding; helium is used in cryogenics and for lifting balloons; neon and xenon are used in lighting and medical imaging; krypton is used in certain lasers. Radon is radioactive and hazardous, while oganesson remains primarily of theoretical and experimental interest due to its extreme radioactivity and short half-life.

In theoretical contexts, monatomic is a useful idealization in models of gases and solids, where particles are treated as nonbonded, point-like entities. The term is contrasted with diatomic and polyatomic species.