Ionic bond formation

It has to date been recognized that the breaking and forming of bonds in solution are in principle influenced by three major factors electronic, steric and solvent effects. Thus, in a quantitative examination to differentiate covalent and ionic bond formation, it is necessary first to investigate the electronic effect alone, separate from steric and solvent effects. 

As already mentioned, the criterion of complete ionization is the fulfilment of the Kohlrausch and Onsager equations (2.4.15) and (2.4.26) stating that the molar conductivity of the solution has to decrease linearly with the square root of its concentration. However, these relationships are valid at moderate concentrations only. At high concentrations, distinct deviations are observed which can partly be ascribed to non-bonding electrostatic and other interaction of more complicated nature (cf. p. 38) and partly to ionic bond formation between ions of opposite charge, i.e. to ion association (ion-pair formation). The separation of these two effects is indeed rather difficult. 

The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

Figure 2.3 The potential-energy curve for Li+ + F ionic bond formation according to quantum mechanics (solid line) or classical electrostatics (dotted line).

Figure 2.5 Charge transfer (CT percentage of e charge) from F to Li+ during ionic-bond formation. For reference, a dotted vertical line marks the equilibrium bond length. Note the steep rise corresponding to the onset of the ionic-covalent transition near R = 1 A.

Still another aspect of the Li and F valence orbitals is modified by ionic-bond formation. In an isolated ionic or neutral species, each NAO retains the characteristic angular shape of the pure s and p hydrogenic orbitals shown in Fig. 1.1, reflecting the full rotational symmetry of atoms. However, in the presence of another atom or ion this symmetry is broken, and the optimal valence orbitals acquire sp hybrid form (assumed for simplicity to include only valence s and p orbitals), as represented mathematically by... 

Figure 2.7 Percentage p character of Li and F bonding hybrids in ionic-bond formation.

For Li—F, the quantal ionic interaction can be qualitatively pictured in terms of the donor-acceptor interaction between a filled 2pf. orbital of the anion and the vacant 2su orbital of the cation. However, ionic-bond formation is accompanied by continuous changes in orbital hybridization and atomic charges whose magnitude can be estimated by the perturbation theory of donor-acceptor interactions. These changes affect not only the attractive interactions between filled and unfilled orbitals, but also the opposing filled—filled orbital interactions (steric repulsions) as the ionic valence shells begin to overlap. 

This chapter consists of two sections, one being a general discussion of the stable forms of the elements, whether they are metals or non-metals, and the reasons for the differences. The theory of the metallic bond is introduced, and related to the electrical conduction properties of the elements. The second section is devoted to a detailed description of the energetics of ionic bond formation. A discussion of the transition from ionic to covalent bonding in solids is also included. 

The first three terms of equation 7.12 are positive and can only contribute to the feasibility of ionic bond production by being minimized. The last two terms are negative for uni-negative ions (jE ea represents energy released), and for ionic bond feasibility should be maximized. From such considerations it is clear that ionic bond formation will be satisfactory for very electropositive and easily atomized metals with very electronegative and easily atomized non-metals. 

A second argument, that the hydrides and nitrides of the metals beyond the third group are stabilized by non-ionic bond formation,... 

The suggestion of ionic bond formation (6-8) has been strongly criticized by Emmett and Teller (20), and later by Couper and Eley (21), who calculated theoretically that all possible ionization processes are... 

Using infrared analysis they showed that one of the binding mechanisms is ionic bond formation, following proton transfer from the humic acids to the s-triazine molecules. Another mechanism found by Senesi and Testini (1982) was hydrogen bonding. 

Fig. 13.1. Ionic bond formation between nylon 66 and C.l. Acid Orange 7.

Fig. 13.5. Ionic bond formation between polyacrylonitrile and C.l. Basic Red 18.

An attraction between ions of opposite charge. Potassium bro-i mide consists of potassium ions (K ) ionically bound to bro- mide ioas (Br). Unlike covalent i bonds, ionic bond formation involves transfer of electrons,... 

Chemisorption occurs when strong interactions, including hydrogen bonding and covalent and ionic bond formation, occur between the adsorbate and the solid surface. Chemisorption typically occurs even at very low concentrations, and the chemisorbed species are often irreversibly bound to the surface, i.e., they will not readily desorb under ambient temperature conditions. The endpoint for chemisorption is when alt the active sites on the solid surface are occupied by chemisorbed molecules. 

Of the various compatibilization strategies that have been devised, an increasingly common method is either to add a block, graft, or crosslinked copolymer of the two (or more) separate polymers in the blend, or to form such copolymers through covalent or ionic bond formation in situ during the Reactive Compatibilization step. The first of these processes was described in Chapter 4 of this Handbook, Interphase and Compatibilization by Addition of a Compatibilizer, while the second method is the topic of this Chapter. 

As defined in Appendix 5 compatibilization means A process of modification of interfacial properties of an immiscible polymer blend, leading to creation of polymer alloy . A polymer alloy in turn is defined as An immiscible polymer blend having a modified interface and/or morphology , whereas a polymer blend is simply A mixture of at least two polymers or copolymers . In other words, all polymer alloys are blends, but not all polymer blends are alloys. A somewhat more elaborate definition of a polymer alloy would describe a blend of at least two immiscible polymers stabilized either by covalent bond or ionic bond formation between phases, or by attractive intermolecular interaction, e.g., dipole-dipole, ion-dipole, charge-transfer, H-bonding, van der Waals forces, etc. 

Presence of reactive functionality for covalent or ionic bonds formation. 

Summarize ionic bond formation by correctly pairing these terms cation, anion, electron gain, and electron loss. 

I Categorize ionic bond formation as exothermic or endothermic. 

Interactive Figure To see an animation of sodium chloride ionic bond formation, visit qlencoe.com. 

Describe the energy change associated with ionic bond formation, and relate it to stability. 

Now let us take a look at an example to understand this better. The sodium fluoride (NaF) molecule is a result of ionic bond formation between sodium and fluoride ions. Together with this, you should also try to understand Lewis dot stmctures. Lewis structure will be discussed in detail later in this chapter. 

The energetics of ionic bond formation helps explain why many ions tend to have noble-gas electron configurations. For example, sodium readily loses one electron to form Na, which has the same electron configuration as Ne ... 

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