Periodic property ionic radii

In this section we will consider how the periodic table can be used to correlate properties on an atomic scale. In particular, we will see how atomic radius, ionic radius, ionization energy, and electronegativity vary horizontally and vertically in the periodic table. 

One property of a transition metal ion that is particularly sensitive to crystal field interactions is the ionic radius and its influence on interatomic distances in a crystal structure. Within a row of elements in the periodic table in which cations possess completely filled or efficiently screened inner orbitals, there should be a decrease of interatomic distances with increasing atomic number for cations possessing the same valence. The ionic radii of trivalent cations of the lanthanide series for example, plotted in fig. 6.1, show a relatively smooth contraction from lanthanum to lutecium. Such a trend is determined by the... 

Filling of the inner 4f electron shell across the lanthanide series results in decreases of ionic radii by as much as 15% from lanthanum to lutetimn, referred to as the lanthanide contraction (28). While atomic radius contraction is not rmique across a series (i.e., the actinides and the first two rows of the d-block), the fact that all lanthanides primarily adopt the tripositive oxidation state means that this particular row of elements exhibits a traceable change in properties in a way that is not observed elsewhere in the periodic table. Lanthanides behave similarly in reactions as long as the mnnber of 4f electrons is conserved (29). Thus, lanthanide substitution can be used as a tool to tune the ionic radius in a lanthanide complex to better elucidate physical properties. 

The lanthanide contraction, however, has also effects for the rest of the transition metals in the lower part of the periodic system. The lanthanide contraction is of sufficient magnitude to cause the elements which follow in the third transition series to have sizes very similar to those of the second row of transition elements. Due to this, for instance hafnium (Hf ) has a 4" -ionic radius similar to that of zirconium, leading to similar behavior of these elements. Likewise, the elements Nb and Ta and the elements Mo and W have nearly identical sizes. Ruthenium, rhodium and palladium have similar sizes to osmium iridium and platinum. They also have similar chemical properties and they are difficult to separate. The effect of the lanthanide contraction is noticeable up to platinum (Z = 78), after which it no longer noticeable due to the so-called Inert Pair Effect (Encyclopedia Britannica 2015). The inert pair effect describes the preference of post-transition metals to form ions whose oxidation state is 2 less than the group valence. 

The properties which depend on the external electronic shell stmcture vary periodically with the Z number. The most important periodic properties are the atomic radius, the atomic volume, the ionic radius, the ionic volume, the melting point, the boiling point, the ionization energy, the electron affinity, the electronegativity, the valence, the acid-base character etc. (Aldea et al., 2000). 

A biologist can use the periodic table in same way as a chemist. It can be used to find elements with similar chemical properties, predict chemical formulas, predict charges on simple ions, predict electron structures of atoms and ions, find simple ions of similar ionic radius, predict physical and chemical properties, and relative atomic masses can be used in calculations involving the mole concept. 

The groups and periods of the periodic table display general trends in the following properties of the elements electron affinity, electronegativity, ionization energy, atomic radius, and ionic radius. 

Figure 19.2 shows the noble gases superimposed on the network of interconnected ideas. Table 19.1 is a slightly amended version of the usual table of periodic properties. Note that these properties are exactly as expected on the basis of effective nuclear charge and the distance of the valence electrons from that charge. Consistent with the noble nature of these elements, the usual entries for atomic and ionic radii have been replaced by van der Waals radii. Only two entries, for xenon and krypton, have been made in the table under covalent radii. (Several radon compounds are known, but the covalent radius has not been well-established.) As expected, these radii increase regularly down the group. 

Since element 103, Lr, is the last member of the actinide or 5f series of elements, element 104 was expected to be the first member of the next group of the periodic table, i.e. the group IV B elements. Indeed, from the results of relativistic Dirac-Fock calculations, the electronic ground-state configuration for a neutral free atom of element 104 was predicted to be 5f 6d 7s [75]. It is expected to have a valence and ionic radius similar to Zr and Hf and to exhibit similar chemical properties [76, 77], Using equations developed by Jorgensen, Penneman and Mann have predicted that element 104 should be predominantly tetravalent in aqueous solution and solid compounds however, the chemistry of element 104 may involve 2+ and 3+ as well [78]. Further, the hydrolytic properties and the solubility of compounds of element 104 are expected to be similar to Hf [75]. Some of the chemical properties predicted for element 104 are given later in Table 13.10. 

The following properties of astatine have been measured or estimated (a) covalent radius (b) ionic radius (At ) (c) first ionization energy (d) electron affinity (c) electronegativity (f) standard reduction potential. Based on periodic relationships and data in Table 22.4, what values would you expect for these properties ... 

---