Atomic radii are the distance between the outermost occupied probability region of an atom and its nucleus.
Atomic radii are useful in determing the average bond length between two atoms.
Indeed atomic radii are usually calculated using X-ray diffraction data that is collected from a crystal of the substance. If ion particles are involved, this results in ionic radii. Metallic elements generally have the largest atomic radii and non-metallic elements the smallest. Ionic radii of cations are always smaller than the atomic radii because the positive charge pulls the remaining electrons closer to the nucleus.