Atomic radius ionic size compared

Figure 1.37 illustrates some ionic radii, and Fig. 1.38 shows the relative sizes of some ions and their parent atoms. All cations are smaller than their parent atoms, because the atom loses one or more electrons to form the cation and exposes its core, which is generally much smaller than the parent atom. For example, the atomic radius of Li, with the configuration ls22s, is 157 pm, but the ionic radius of Li+, the bare heliumlike Is2 core of the parent atom, is only 58 pm. This size difference is comparable to that between a cherry and its pit. Like atomic radii, cation radii increase down each group because electrons are occupying shells with higher principal quantum numbers. 

Also as a result of the lanthanide contraction, yttrium has an ionic radius comparable to that of the heavier REE species in the holmium-erbium region. If the effective ionic radius (Shannon 1976) of is plotted (0.90 A)., it plots in between element 67 (Ho) and 68 (Er). Scandium (effective ionic radius is 0.745 A), plots outside of the Lanthanide series. As also the outermost electronic arrangement of yttrium is similar to the heavy rare earths, the element behaves chemically like the heavy rare earths. It concentrates during (geo)chemical processes with the heavier REEs, and is difhcult to separate from the heavy REEs. Scandium, on the other hand, has a much smaller atomic radius, and the trivalent ionic size is much smaller than that of the heavy rare earths. Therefore, scandium does not occur in rare earth minerals, and in general has a chemical behavior that is significantiy different from the other rare earth elements (Gupta and Krishnamurthy 2005). 

These three structures are the predominant structures of metals, the exceptions being found mainly in such heavy metals as plutonium. Table 6.1 shows the structure in a sequence of the Periodic Groups, and gives a value of the distance of closest approach of two atoms in the metal. This latter may be viewed as representing the atomic size if the atoms are treated as hard spheres. Alternatively it may be treated as an inter-nuclear distance which is determined by the electronic structure of the metal atoms. In the free-electron model of metals, the structure is described as an ordered array of metallic ions immersed in a continuum of free or unbound electrons. A comparison of the ionic radius with the inter-nuclear distance shows that some metals, such as the alkali metals are empty i.e. the ions are small compared with the hard sphere model, while some such as copper are full with the ionic radius being close to the inter-nuclear distance in the metal. A consideration of ionic radii will be made later in the ionic structures of oxides. 

This sort of variation is to be expected because of the lack of definition of the radius of the atom. In making comparisons of the sizes of atoms, situations should be chosen in which the atoms have about the same environment. It is possible to construct consistent tables of ionic radii, for example, or of covalent radii or metallic radii. If possible, it is best to compare the radii for situations in which the atoms have the same number of neighbors. 

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