Monoatomic

Monoatomic describes a chemical species that consists of a single atom and is not bonded to other atoms as part of a molecule. The term is commonly applied to atoms that exist in isolation, such as in certain gases or ions, rather than as components of molecules.

Monoatomic gases are the classic context in which the term is used. Noble gases—helium, neon, argon, krypton, xenon, and radon—exist as monoatomic gases at standard conditions due to their complete outer electron shells, which lead to very weak interatomic bonding. Most other elements form diatomic or larger molecules in the gaseous state at ambient conditions, so they are not described as monoatomic gases. Monoatomic ions, such as Na+, Cl−, and Fe3+, are single atoms that carry a charge and occur freely in solution or as constituents of crystalline salts, distinct from neutral atoms in molecules.

In terms of physical properties, a monoatomic ideal gas has translational motion as its only degrees of freedom, yielding a molar internal energy of (3/2)RT, Cp,m = (5/2)R, and Cv,m = (3/2)R. Such gases exhibit weak intermolecular forces and approximate ideal gas behavior under low pressures and moderate temperatures.

In condensed phases, atoms are typically part of lattices or molecules, so the term monoatomic is less applicable. The concept remains most relevant for describing isolated noble gas atoms in the gas phase and certain ions in solution.