This factor overweighs the first because of the shielding effect of inner electrons as you proceed downward. Even though the nuclear charge is increasing as you move downward there are more electrons between the valence outer electrons and the nucleus. These inner electrons will shield the valence electrons from the attractive influence of the positive nucleus. Therefore the distance factor is the dominate factor and the energy requirement for removing a valence electron decreases. Atomic radii are the distance between the outermost occupied probability region of an atom and its nucleus.
Atomic radii are useful in determining 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. Ionization Potential is the energy required to remove an electron from an atom.