Atomic radius

Atomic radius is an operational measure of atomic size, inferred from distances between nuclei in bonded or neighboring atoms. Its value depends on the chemical environment and the radius convention used.

Metallic radius: Half the nearest-neighbor nuclear distance in a metallic crystal. Metallic bonding supplies a distinct operational definition of atomic size.
Effective nuclear charge: The net positive charge experienced by an electron after shielding by other electrons. Across a period, increasing effective nuclear charge generally pulls the electron cloud inward.
Bond length: The equilibrium distance between the nuclei of two bonded atoms. Bond lengths are a principal source of covalent-radius estimates.
Covalent radius: Half the distance between the nuclei of two identical atoms joined by a covalent bond. Its bonded-atom basis differs from nonbonded radius estimates.
Ionic radius: An assigned size for an ion in a crystal, inferred from interionic distances and a radius model. Removing or adding electrons changes atomic size and requires a separate radius convention.
Electron shielding: The reduction of an electron's attraction to the nucleus caused by repulsion from other electrons. Shielding weakens nuclear attraction and helps explain size changes down a group.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule. Atomic size helps interpret how closely atoms can approach within molecular structures.
Van der Waals radius: Half the closest distance between nonbonded atoms of the same element. It is commonly larger than a covalent radius because nonbonded atoms remain farther apart.
Crystal structure: The ordered three-dimensional arrangement of atoms, ions, or molecules in a crystalline solid. Crystal geometry provides the neighbor distances used to estimate metallic and ionic radii.
Electron configuration: The distribution of an atom's electrons among shells, subshells, and orbitals. Which orbitals are occupied affects how far the electron cloud extends.

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