Keskususatomi

Keskususatomi, or central atom, is a term used in chemistry to describe the atom at the center of a molecule or coordination complex to which other atoms or ligands are directly bonded. In Finnish-language chemistry literature the term Keskususatomi is used to refer to the atom that largely determines the local geometry of the system.

In simple covalent molecules, the keskususatomi is the atom that forms bonds with the surrounding substituents. The choice of central atom influences the overall shape according to the arrangement of bonding and lone electron pairs. According to VSEPR theory, the central atom’s valence electron pairs (bonding and lone pairs) determine the molecular geometry, with common examples including tetrahedral carbon in methane (CH4), trigonal pyramidal nitrogen in ammonia (NH3), and bent water (H2O) with oxygen as the central atom.

In coordination chemistry, the keskususatomi is typically a metal ion around which ligands are arranged. The central atom's oxidation state and coordination number help define the geometry and properties of the complex, producing shapes such as octahedral, tetrahedral, or square planar, depending on the metal and ligands involved.

Determination of the central atom’s role and geometry relies on structural methods such as X-ray crystallography and spectroscopy, as well as computational modeling in modern chemistry. The concept is also used when discussing hybridization, bond angles, and reactivity patterns around the central atom.

See also: VSEPR theory, molecular geometry, coordination chemistry, Lewis structures, ligands, oxidation state.