atomradien

Atomradien, or atomic radius, is a measure of the size of an atom. Because electrons form a cloud rather than a hard boundary, there is no single fixed diameter. Instead several conventions are used to assign a representative radius. The covalent radius is defined as half the distance between two identical atoms bonded covalently. The van der Waals radius is half the distance between nonbonded atoms in a close approach, such as in a crystal. The metallic radius applies to atoms in a metallic lattice and is roughly half the distance to the nearest neighbor in the crystal. All of these radii depend on bonding, coordination and the environment of the atom.

Because the electron cloud depends on chemical state, atomic radius values are not universal constants. In practice, radii reflect the chosen convention and the coordination of the atom. General periodic trends apply: across a period, atomic radius tends to decrease from left to right due to increasing effective nuclear charge pulling electrons closer. Down a group, radii increase as additional electron shells are added. There are deviations, particularly among transition metals, where d-electron shielding and other effects modify the trend. The lanthanide contraction also reduces sizes across the lanthanide series. Ions differ as well: cations are typically smaller than their neutral atoms, while anions are larger.

Measurements rely on methods such as X-ray crystallography and electron diffraction to determine radii in solids, complemented by spectroscopic data and theoretical calculations. Atomic radii are used to estimate bond lengths, predict molecular geometry, and understand periodic trends and reactivity. The term atomradien is used in several languages to denote this concept, with the specific convention stated to avoid ambiguity.