Periodic trends in ionic radii

The structures of many ionic crystals can be rationalized to a first approximation by considering the relative sizes and relative numbers of the ions present. For monatomic ions, cations are usually smaller than anions (see Appendix 6), although examples such as KF and CsF show that this is not 

Value of — r Predicted coordination number of cation Predicted coordination geometry of cation 

For a given compound stoichiometry, predictions about the coordination type of the cation necessarily make predictions about the coordination type of the anion. Use of radius ratios meets with some success, but there are many limitations. We 

For LiF, the radius ratio is 0.57 and so an octahedral coordination around the Li cation is predicted. This corresponds to an NaCl-type structure (Fig. 6.16), in agreement with that observed. Each of the group 1 halides (except CsCl, CsBr and Csl) at 298 K and 1 bar pressure adopts the NaCl-type structure CsCl, CsBr and Csl adopt the CsCl-type structure (Fig. 6.17). Radius ratio rules predict the correct stmctures in only some cases. They predict tetrahedral coordination for the cations in liBr and LU, octahedral coordination in LiF, LiCl, NaCl, NaBr, NaL KBr and Kl, and cubic coordination in NaF, KF, KCl, RbF, RbCl, RbBr, CsF, CsCl, CsBr and Csl. Radius ratio rules give only one prediction for any one ionic crystal, and some compounds undergo phase changes under the influence of temperature and pressure, e.g. when CsCl is sublimed onto an amorphous surface, it crystallizes with the NaCl structure and, under high-pressiue conditions, RbCl adopts a CsCl-type structure. 

The right-hand side of Fig. 6.15 illustrates the small variation in size for and ions of the tf-block metals. As expected, the decrease in nuclear charge in 

EXERCISE 7.3 Predicting Relative Sizes of Atomic and Ionic Radii 

Cations are smaller than their parent atoms, and so Ca Ca. Because Ca is below Mg in group 2 A, Ca is larger than Mg. Consequently, Ca Ca Mg.  

How do cations of the same charge change in radius as you move down a coiumn in the periodic tabie  

Notice the positions and atomic numbers of these elements in the periodic table. The nonmetal anions precede the noble gas Ne in the table. The metal cations follow Ne. Oxygen, the largest ion in this isoelectronic series, has the lowest atomic number, 8. Aluminum, the smallest of these ions, has the highest atomic number, 13. 

Ionic size plays a major role in determining the properties of devices that rely on movement of ions. Lithium-ion batteries, which have become common energy sources for electronic devices such as cell phones, iPads, and laptop computers, rely in part on the small size of the lithium ion for their operation. 

CHEMICAL AND THEORETICAL BACKGROUND Box 6.4 Radius ratio rules 

---

General trends in ionic radii of A group elements with position in the periodic table. 

Knowledge Required (1) The periodic trends in atomic radii. (2) The periodic trends in ionic size. 

The data in Table 3.14 shows the following trends in ionic radii across period 3 ... 

Recall that ions are simply atoms (or groups of atoms) that have lost or gained electrons, hi this section, we examine periodic trends in ionic electron configurations, magnetic properties, ionic radii, and ionization energies. 

The data include metallic radii, ionic radii, covalent radii, and van der Waals radii. Although the various types of radii cannot be directly compared, the figure does illustrate the periodic trends in sizes. 

E2.10 Table 2.7 lists selected covalent radii of the main group elements. As we can see from the table, the trends in covalent radii follow closely the trends in atomic and ionic radii covered in Chapter 1 (Section L7(a)). Considering first the horizontal periodic trends we can see that the single bond covalent radii decrease if we move horizontally from left to right in the periodic table. Recall that the atomic radii decrease while the increases in... 

Use the periodic table to predict the trends in atomic radii, ionic radii, ionization energy, and electron affinity. (Sections 7.2,7.3,7.4, and 7.5)... 

This chapter and the remaining chapters offer many opportunities to relate new information to principles presented earlier in the text. Ideas of atomic structure, periodic trends in atomic and ionic radii, chemical bonding, and thermodynamics will help us to understand the chemical behavior of the... 

The radii of cations and anions derived from atoms of the main-group elements are shown at the bottom of Figure 6.13. The trends referred to previously for atomic radii are dearly visible with ionic radius as well. Notice, for example, that ionic radius increases moving down a group in the periodic table. Moreover the radii of both cations (left) and anions (right) decrease from left to right across a period. 

Identify trends in the periodic table for IE, EA, electronegativity, and atomic/ionic radii. 

Figure 1.2 Some important trends in the periodic table for (a) ionization energy, (b) electron affinity, (c) atomic and ionic radii, and (d) electronegativity. Increasing values are in the direction of the arrow.

If we examine the distances listed in Table 7.2 some interesting facts emerge. For a given metal A. the A—P distance is constant as we might expect for an ionic alkaline earth metal-phosphide bond. Furthermore, these distances increase calcium < strontium < barium in increments of about 15 pm os do the ionic radii of Ca2+. St7, and Ba- (Table 4.4). However, the B—P distances vary somewhat more with no periodic trends (Mn. Cu larger Ni, Fe, Co smaller). Most interesting, however, is the huger variability in the P—P distance from about 380 pm (Mn. Fe) to 225 pm (Cu). As it Luros Out, the lower limit of 225 pm (Cu) is a typical value for a P— P bond (Table E.l,... 

The covalent radii differ from ionic radii because the attractive and repulsive forces differ in the two kinds of bonds and therefore a different equilibrium internuclear distance (Fig. 3.1) will be achieved in the two cases. Nevertheless, the variation of covalent radii over the periodic table shows the same trends as the variation of ionic radii. 

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... 

A common trend in the ionic radii of the transition elements is that they tend to decrease with increasing atomic number in a period. This... 

Sketch a simplified periodic table and use arrows and labels to compare period and group trends in atomic and ionic radii, ionization energies, and electronegativities. 

---