Coordination number

The number of nearest neighboring atoms or ions surrounding a selected atom or ion in a structure. It describes local geometry and depends on how neighbors are identified.

Chemical bond: An interaction that holds atoms or ions together in molecules, crystals, or other materials. Bonding often helps distinguish a coordinated neighbor from a more distant atom.
Radial distribution function: A function describing how particle density varies with distance from a reference particle. Its first minimum is often used to set the outer boundary of the neighbor shell.
Tetrahedral coordination: A local arrangement in which four neighbors occupy the vertices of a tetrahedron around a central particle. It is a common four-neighbor geometry in crystals and molecular structures.
Oxidation state: A formal charge assigned to an atom under a set of electron-allocation rules. It describes electron accounting, not the number of nearby atoms or ions.
Interatomic distance: The distance between the nuclei of two atoms in a material or molecule. Neighbor identification commonly uses distances between the selected atom and surrounding atoms.
Voronoi tessellation: A partition of space into regions closest to each of a set of generating points. Voronoi cells identify geometric neighbors without requiring a fixed distance cutoff.
Octahedral coordination: A local arrangement in which six neighbors occupy the vertices of an octahedron around a central particle. It illustrates how six neighbors form a recognizable coordination polyhedron.
Coordination complex: A chemical species with a central metal atom or ion bound to surrounding ligands. In complexes, coordination number usually counts donor atoms bonded to the central metal.
Crystal structure: The ordered arrangement of atoms, ions, or molecules in a crystalline solid. Coordination numbers summarize local arrangements within a crystal structure.
Bond valence method: A method that estimates atomic valence by summing contributions from bonds, weighted by their lengths. Bond-length contributions can help assess whether nearby atoms are chemically coordinated.

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