Ionic compounds definition

The elements show increasing metallic character down the group (Table 14.6). Carbon has definite nonmetallic properties it forms covalent compounds with nonmetals and ionic compounds with metals. The oxides of carbon and silicon are acidic. Germanium is a typical metalloid in that it exhibits metallic or nonmetallic properties according to the other element present in the compound. Tin and, even more so, lead have definite metallic properties. However, even though tin is classified as a metal, it is not far from the metalloids in the periodic table, and it does have some amphoteric properties. For example, tin reacts with both hot concentrated hydrochloric acid and hot alkali ... 

For a given molecule and a given intemuclear separation a would have a definite value, such as to make the energy level for P+ lie as low as possible. If a happens to be nearly 1 for the equilibrium state of the molecule, it would be convenient to say that the bond is an electron-pair bond if a is nearly zero, it could be called an ionic bond. This definition is somewhat unsatisfactory in that it does not depend on easily observable quantities. For example, a compound which is ionic by the above definition might dissociate adiabatically into neutral atoms, the value of a changing from nearly zero to unity as the nuclei separate, and it would do this in case the electron affinity of X were less than the ionization potential of M. HF is an example of such a compound. There is evidence, given bdow, that the normal molecule approximates an ionic compound yet it would dissociate adiabatically into neutral F and H.13... 

By definition, a salt is an ionic compound made up of a cation other than hT and an anion other than OH or 02. However, in these examples a "salt" represents any ionic (metal-nonmetal) compound. 

D. Molecular Orbital Calculations. Pharmacological Activity of Meso-ionic Compounds Meso-ionic— Definition and Delineation... 

One hundred and forty-four meso-ionic heterocycles of type A (13, 19) and 84 meso-ionic heterocycles of type B (14, 20) are possible. The numbers of these two types which are now known (Table I type A, 44 representatives) and (Table II type B, 8 representatives) encourage us to put forward the proposal that the term meso-ionic should in future be restricted to five-membered heterocycles belonging to type A (13, 19) and type B (14,20). This clear restriction upon the use of the term meso-ionic has obvious advantages. It still embraces 228 different classes of heterocycles with a common structural characteristic, and the many types of meso-ionic compounds included in the authoritative review by Ohta and Kato " are included. Needless to say, the restriction upon the definition of the term meso-ionic to five-mem red heterocycles of type A and type B still includes, for example, benz derivatives such as the compounds 67, 71, 110, 123, 133, 151, 206, 209, 226, 255, and 258. 

From 1975 to 1990, scientists at the University of Kansas utilized a rational synthetic design for the definition of a new excipient, the SBE derivative of P-CD (SBE7-P-CD CAPTISOL ). Designing renal safety into the CD was approached by introducing anionic substituents onto the CD structure. This approach capitalized on the increased water solubility that would be realized with the introduction of an ionic substituent. Higher intrinsic water solubility was expected to help minimize the potential precipitation of the CD, if concentrated in the kidney cell, and the charged substituent was expected to capitalize on the ability of the kidney to efficiently excrete ionic compounds into the urine, thus reducing residence time and exposure of the kidney cells to the CD. 

It is once more apparent that not all compounds can be brought within the simple scheme of the ionic bond and it is difficult to say precisely where the division should be made. Carbon tetrachloride, for example, would be a volatile compound even if it were an ionic compound that in actual fact it is volatile does not necessarily prove that it is ionic, because it is known that there are other volatile compounds which are definitely not ionic. 

This brings us to a class of compounds too often overlooked in the discussion of simple ionic compounds the transition metal halides. In general, these compounds (except fluorides) crystallize in structures that are hard to reconcile with the structures of simple ionic compounds seen previously (Figs. 4.1-4.3). For example, consider the cadmium iodide structure (Fig. 7.8). It is true that the cadmium atoms occupy octahedral holes in a hexagonal closest packed structure of iodine atoms, but in a definite layered structure that can be described accurately only in terms of covalent bonding and infinite layer molecules. 

The elements show increasing metallic character down the group (Table 14.12). Carbon has definite nonmetallic properties it forms covalent compounds with nonmetals and ionic compounds with metals. The oxides of carbon and silicon are acidic. Germanium is a typical metalloid... 

Ionic Size. The size of an ion is a somewhat hazy concept because the modern notion of atoms pictures the electron distribution to extend to infinity. Nevertheless, it is true that there are definite distances established between the centers of atoms in a compound. It is thus natural to attempt to conceive of the distance between, say, Na+ and CT in solid sodium chloride as being made up as a sum of two contributions, one from the negative ion and the other the positive ion. This amounts to defining the sizes of ions in such a manner that in each ionic compound the sum of the ionic radii equals the observed interionic distance at equilibrium. 

Solid phases of binary systems, like the liquid phases, are very commonly of variable composition. Here, as with the liquid, the stable range of composition is larger, the more similar the two components are. This of course is quite c-ontrary to the chemists notion of definite chemical composition, definite structural formulas, etc., but those notions are really of extremely limited application. It happens that the solid phases in the system water—ionic compound are often of rather definite composition, and it is largely from this rather special case that the idea of definite compositions in solids has become so firmly rooted. In such a system, there are normally two solid phases ice and the crystalline ionic compound. Ice can take up practically none of any ionic compound, so that it has practically no range of compositions. And many ionic crystals... 

Water and ionic compounds are very different types of substances, and it is not unnatural that they do not form solids of variable composition. The reason why water solutions of ionic substances exist is that the water molecules can rotate so as to be attracted to the ions this is not allowed in the solid, where the ice structure demands a fairly definite orientation of the molecule. But as soon as we think about solid phases of a mixture of similar components, we find that almost all the solid phases exist over quite a range. Such phases are often called by the chemists solid solutions, to distinguish them from chemical compounds. This distinction is valid if we mean by a chemical compound a phase which really exists at only a quite definite composition. But the chemists, and particularly the metallurgists, are not always careful about making this distinction for this reason the notation is misleading, and we shall not often use it. 

This second definition is useful when ion and electron movement is involved in chemical reactions. Chemical reactions involving ionic compounds are best interpreted by this definition. Consider the attack of dilute hydrochloric acid on metallic zinc. The zinc dissolves and forms zinc chloride solution and the hydrogen gas fizzes off ... 

We will avoid these problems by adopting an operational definition of ionic compounds Any compound that conducts an electric current when melted will be classified as ionic. Also, the generic term salt will be used interchangeably with ionic compound in this book. 

The oxidation numbers of ions are used to determine the formulas for the ionic compounds they form. Recall that in ionic compounds, oppositely charged ions combine chemically in definite ratios to form a compound that has no charge. If you add the oxidation number of each ion multiplied by the number of these ions in a formula unit, the total must be zero. 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

These properties should be treated with caution and are not definitive for ionic solids in general, since some ionic compounds do not show all these properties. For example, ammonium salts such as ammonium nitrate have low melting points ca. 150-300 °C), and some compounds containing doubly charged ions such as magnesium oxide, MgO, have very low solubility. 

Many biomolecules have acidic or basic properties. There are several possible definitions of these important classes of ionic compounds. For our purposes,... 

Another example is provided by the definition of a ionic compound ... 

Hydrates are ionic compounds that contain a definite number of water molecules associated with the formula unit of one compound, as in FeCl3-6 H20. When water evaporates from a solution of copper(II) sulfate, beautiful blue crystals remain behind that have the formula CuS04-5 H20. Five molecules of water are associated with one formula unit of copper(II) sulfate, and they are separated from the formula of CuS04, with a dot ( ). This hydrate is named copper(ll) sulfate pentahy-drate. Numerical prefixes are used to indicate the number of water molecules in one formula unit. Three other hydrates are listed and named as follows ... 

Example 3 Find the molar mass of MgS04-7 HzO. Hydrates are also ionic compounds that contain a definite amount of water in the crystalline solid. The hydrate of magnesium sulfate, MgS04-7 HzO, has 1 Mgz+ ion, 1 S04z ion, and 7 molecules of water in the formula unit. Seven moles of water are present for each 1 mole of MgS04. There are 27 atoms in the formula unit 1 Mg, 1 S, 11 O, and 14 H. The formula mass of MgS04-7 H20 is ... 

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