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In chemistry, an atom cluster is an ensemble of bound atoms or molecules that is intermediate in size between a simple molecule and a nanoparticle; that is, up to a few nanometers in diameter. The term microcluster may be used for ensembles with up to couple dozen atoms.
Clusters with a definite number and type of atoms in a specific arrangement are often considered a specific chemical compound and are studied as such. For example, fullerene is a cluster of 60 carbon atoms arranged as the vertices of a truncated icosahedron, and decaborane is a cluster of 10 boron atoms forming an incomplete icosahedron, surrounded by 14 hydrogen atoms.
The term is most commonly used for ensembles consisting of several atoms of the same element, or of a few different elements, bonded in a three-dimensional arrangement. Transition metals and main group elements form especially robust clusters. Indeed, in some contexts, the term may refer specifically to a metal cluster, whose core atoms are metals and contains at least one metallic bond. In this case, the qualifier poly specifies a cluster with more than one metal atom, and heteronuclear specifies a cluster with at least two different metal elements. Naked metal clusters have only metal atoms, as opposed to clusters with outer shell of other elements. The latter may be functional groups such as cyanide or methyl, covalently bonded to the core atoms; or many be ligands attached by coordination bonds, such as carbon monoxide, halides, isocyanides, alkenes, and hydrides.
However, the term is also used for ensembles that contain no metals and whose core atoms are held together by covalent or ionic bonds. It is also used for ensembles of atoms or molecules held together by Van der Waals or hydrogen bonds, as in water clusters.