<P> Under most definitions the radii of isolated neutral atoms range between 30 and 300 pm (trillionths of a meter), or between 0.3 and 3 ångströms . Therefore, the radius of an atom is more than 10,000 times the radius of its nucleus (1--10 fm), and less than 1 / 1000 of the wavelength of visible light (400--700 nm). </P> <P> For many purposes, atoms can be modeled as spheres . This is only a crude approximation, but it can provide quantitative explanations and predictions for many phenomena, such as the density of liquids and solids, the diffusion of fluids through molecular sieves, the arrangement of atoms and ions in crystals, and the size and shape of molecules . </P> <P> Atomic radii vary in a predictable and explicable manner across the periodic table . For instance, the radii generally decrease along each period (row) of the table, from the alkali metals to the noble gases; and increase down each group (column). The radius increases sharply between the noble gas at the end of each period and the alkali metal at the beginning of the next period . These trends of the atomic radii (and of various other chemical and physical properties of the elements) can be explained by the electron shell theory of the atom; they provided important evidence for the development and confirmation of quantum theory . The atomic radii decrease across the Periodic Table because as the atomic number increases, the number of protons increases across the period, but the extra electrons are only added to the same quantum shell . Therefore, the effective nuclear charge towards the outermost electrons increases, drawing the outermost electrons closer . As a result, the electron cloud contracts and the atomic radius decrease . </P> <P> In 1920, shortly after it had become possible to determine the sizes of atoms using X-ray crystallography, it was suggested that all atoms of the same element have the same radii . However, in 1923, when more crystal data had become available, it was found that the approximation of an atom as a sphere does not necessarily hold when comparing the same atom in different crystal structures . </P>

Is there a relationship between atomic number and atomic radii for elements in a single period