Ionic radii predicting relative sizes

The electron configuration or orbital diagram of an atom of an element can be deduced from its position in the periodic table. Beyond that, position in the table can be used to predict (Section 6.8) the relative sizes of atoms and ions (atomic radius, ionic radius) and the relative tendencies of atoms to give up or acquire electrons (ionization energy, electronegativity). 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

Ionic Size vs. Atomic Size The ionic radius is an estimate of the size of an ion in a crystalline ionic compound. You can picture it as one ion s portion of the distance between the nuclei of neighboring ions in the solid (Figure 8.21). From the relation between effective nuclear charge and atomic size, we can predict the size of an ion relative to its parent atom ... 

These rules enable us to predict the structure of the compound from the relative sizes of the two ions. Applied to many different ionic crystals of different valence types, the rules are quite good. There are at least two reasons for the exceptions to the radius ratio rules (1) the ions are not rigid spheres (2) the ions of opposite charge are not in contact. 

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