Packed ions

How much can we bend this bond Well, the electrons of each ion occupy complicated three-dimensional regions (or orbitals ) around the nuclei. But at an approximate level we can assume the ions to be spherical, and there is then considerable freedom in the way we pack the ions round each other. The ionic bond therefore lacks directionality, although in packing ions of opposite sign, it is obviously necessary to make sure that the total charge (+ and -) adds up to zero, and that positive ions (which repel each other) are always separated by negative ions. 

The ultimate goal in the development of any separation medium, i.e., its use for the separation of real-life samples, has also been demonstrated by the separation of baker s yeast (Saccharomyces cerevisiae) extract. This separation compares favorably to those obtained with commercial packed ion-exchange... 

Fig. 5. Diffusion spectra for water traveling through close-packed ion-exchange resin beads (50-100 mesh) at flow rates of 13 ml hr" (solid circles), 25 ml hr" (open squares), 50 ml hr" (open circles), and 100 ml hr" (solid squares). The crosses represent the measured spectrum for stationary water. The lines are to guide the eye. The pronounced peak is believed to arise from the oscillatory motion of water around the beads, while the low-frequency plateau is due to perfusive spreading of the flow. [Reproduced by permission from Callaghan and Stepisnik, 1995a.]...

The sudden increase in atomic volume at americium is interesting and paralleled by a similar increase at the corresponding lanthanide, europium. It probably results from the change to close-packed ions, from the ions of the preceding elements. 

For close-packed structures, N close-packed ions lead to N octahedral sites. Therefore, if we assume three close-packed anions (A), then there should also be three octahedral holes. Since only two thirds are filled with cations, there are two cations (X) in two octahedral sites and the stoichiometry for the solid is X2A3. 

E3,6 Remember that in any close-packed structure with N close-packed ions there are N octahedral holes and 2N tetrahedral holes (see Section 3,3),... 

All chemical compounds can be classed as either ionic compounds or covalent compounds, which are categories based on the kind of bonding that holds them together. Ionic compounds are crystalline solids formed by neatly packed ions of opposite charge. Covalent compounds might be solid, liquid, or gas at room temperature, and they exist as molecules. For that reason, covalent compounds often are referred to as molecular compounds. 

Specific and local structure-determining interactions were neglected in this analysis of the salts as close-packed ions. Whereas the coulombic contributions as evaluated by Rohl and Mingos will favour the symmetrical arrays, as occur... 

The fluoride ion. There are two tetrahedral sites per closest-packed ion. If all the tetrahedral sites are occupied, the formula must be either MX2 or M2X. Normally, we expect the anion to be closest packed because generally anions are larger than the cations. In the case of Cap2 the fluoride ion, with an ionic radius of 1.33 A, is larger than the calcium ion (r = 1.00 A), but the structure can be analyzed by assuming that the Ca ions are closest-packed. Thus, in the illustration above (and in those below), the lighter spheres are F and the black spheres are Ca. ... 

Fig. 28.4 Two unit cells of body-centred packed ions, showing one tetrahedral hole (red lines).

Table 4.1. Crystal Structures Based on Close-Packed Ion Stacking with Ions in Its Available Structural... 

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