Ionic or covalent bonding

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

As an example, let us pose the question why does BF3 adopt a molecular structure, while A1F3 is apparently ionic As shown in Table 5.2, the ionic model (using the Kapustinskii equation) gives a fair approximation to the thermochemistry of formation of A1F3. Let us estimate the enthalpy of formation of a hypothetical ionic substance BF3(s), having a structure similar to that of A1F3. The lattice energy can be estimated by means of the Kapustinskii equation. We require the 

The formation of BF3(s) from the elemental substances is apparently exothermic but the experimental enthalpy of formation of BF3(g) is -1137 kJ mol-.  

Between the extremes of ionic and molecular substances, we have 

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Most ceramics are intrinsically hard ionic or covalent bonds present an enormous lattice resistance to the motion of a dislocation. Take the covalent bond first. The covalent bond is localised the electrons which form the bond are concentrated in the region between the bonded atoms they behave like little elastic struts joining the atoms (Eig. 17.1b). When a dislocation moves through the structure it must break and reform... 

Questions such as, for example, whether sphalerite contains Zn++ and S= ions or has a covalent structure similar to that of diamond, and whether ionic or covalent bonds are present in complexes such as [FeF% —, [Fe(CN)e]=, etc., have been extensively discussed it has, indeed, until recently not been at all clear whether or not they could be definitely... 

This is the last bond type to be considered. Let s start with a question What holds a metal together A bar of copper or magnesium has properties that are entirely different from substances held together by ionic or covalent bonds. Metals are dense structures that conduct electricity readily. They are malleable, which means that they can be easily twisted into shapes. They are ductile, which allows them to be drawn into wires. No substances with ionic or covalent bonds, such as salt or water, behave anything like metals. 

One clue to understanding the nature of metallic bonds comes from their high electrical conductivity. Like most substances held together by ionic or covalent bonds, pure water and pure salt do not conduct electricity well. But pure copper does. Electrical conductivity is a measure of how free the electrons are to move. The high conductivity of metals indicates that their electrons are freer to move than the electrons are in salt or water. 

The light incident on a mineral surface divides itself in a number of ways. It is partly reflected, partly transmitted, and partly absorbed. Minerals exhibiting metallic bonding have a high reflectivity. Those characterized by ionic or covalent bonding have a high transmissivity. The optical properties are anisotropic they are different in different directions and depend on the arrangement of atoms in the crystals. 

Lanthanides form soluble complexes with many inorganic and organic substances however, the nature of the bonding in these complexes has not been completely determined. There is evidence for either ionic or covalent bond formation or a combination of both. Lanthanides are complexed by inorganic ions, but not as readily as are the transition elements. The inorganic complexes are not as important... 

An antisite defect is an atom on a site normally occupied by a different chemical species that exists in the compound. Antisite defects are a feature of a number of important materials, especially weakly ionic or covalently bonded ones. In a compound of formula AB the antisite defects that can occur are an A atom on a site normally occupied by a B atom (Fig. 1.16a), or a B atom on a site normally occupied by an A atom (Fig. 1.16b). 

The adsorption of gases on solids can be classified into physical and chemical adsorption. Physical adsorption is accompanied by a low enthalpy of adsorption, and the adsorption is reversible. The adsorption/desorption characteristics are in these cases often described by adsorption isotherms. On the other hand, chemical adsorption or segregation involves significantly larger enthalpies and is generally irreversible at low temperatures. It is also often accompanied by reconstruction of the surface due to the formation of strong ionic or covalent bonds. 

C—Sublimation usually does not involve bond breaking. In any case, Zn is a metal, and it has no ionic or covalent bonds to break. 

Nonspecific effects can also limit the availability of solutes to undergo chemical reactions. The usual case is that the long-range electrostatic interactions between ions leads to a reduction in their availability to form ionic or covalent bonds. As with solvent-ion... 

Their unique characteristics are a result of their outer shells having seven electrons, and thus requiring only one electron to become complete. This -1 oxidation state makes them extremely reactive with both metals and some nonmetal elements that form negative ions, and they may form either ionic or covalent bonds. They can also form compounds with each other these binary compounds of the halogens are called halides. ... 

Adhesion in which two bodies are held together at an interface by ionic or covalent bonding between molecules on either side of the interface. 

In an adsorption process involving ionic or covalent bonding, the adsorption heats of principal interest are — A//o, the heat of adsorption at zero coverage and S( — AH), the decrease in the heat of adsorption with coverage. It is in connection with the latter that the role of the work func-... 

What element is well suited to forming either ionic or covalent bonds ... 

On the other hand, the results generally seemed promising and gave impetus to further development in this field. This resulted in various procedures for making HCP with heparin attached to the polymer via either ionic or covalent bonds. 

The strong interactions that result in either ionic or covalent bonds determine the primary structure of molecules. Molecules often interact, not by forming more bonds of this type,... 

Ceramic materials have strong ionic or covalent bonds and that is why sliding processes as occur in metals are not or only slightly possible. They lack the necessary plastic behaviour. 

The application of ceramics has infiltrated almost all fields in the last 20 years, because of their advantages over metals due to their strong ionic or covalent bonding. But it is just this bonding nature of ceramics that directly results in their inherent brittleness and difficulty in machining. In other words, ceramics show hardly any macroscopic plasticity at room temperature or at low temperatures like metals. Hence, superplasticity at room temperature is a research objective for structural ceramics. In recent years, many researches have been carried out to investigate nanophase ceramic composites. 

It should be noted that the retinol-based and the retinal-based systems represent two different situations the retinal system requires that it be associated with another atom or molecule (at least a hydrogen atom) through ionic or covalent bonding. However, the retinol system is complete unto itself by merely substituting a polar auxochrome for Z in the molecule, it will exhibit a peak absorption in the desired spectrum under the appropriate conditions. 

Intramolecular charge distribution depends on the number of valence electrons and electronegativity differences. In some extreme cases it resembles the interactions, traditionally considered to define van der Waals, metallic, ionic or covalent bonding. The true interaction in all cases however, contains elements of all four extremes. 

Crosslinked polymer—A three-dimensional polymer created when intermo-lecular forces connect adjacent chains the forces may be hydrogen bonds, dipole interactions, van der Waals forces, or ionic or covalent bonds. 

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