Ionic compounds composition

A solid that contains cations and anions in balanced whole-number ratios is called an ionic compound. Sodium chloride, commonly known as table salt, is a simple example. Sodium chloride can form through the vigorous chemical reaction of elemental sodium and elemental chlorine. The appearance and composition of these substances are very different, as Figure 2-24 shows. Sodium is a soft, silver-colored metal that is an array of Na atoms packed closely together. Chlorine is a faintly yellow-green toxic gas made up of diatomic, neutral CI2 molecules. When these two elements react, they form colorless ciystals of NaCl that contain Na and Cl" ions in a 1 1 ratio. 

The three most important structure types for ionic compounds of composition MX. Compared to their effective sizes, the ions have been drawn to a smaller scale... 

As in ionic compounds, the atoms in a binary intermetallic compound show a tendency, albeit less pronounced, to be surrounded by atoms of the other kind as far as possible. However, it is not possible to fulfill this condition simultaneously for both kinds of atoms if they form a closest-packed arrangement. For compositions MXn with n < 3 it cannot be fulfilled for either the M or the X atoms in every case every atom has to have some adjacent atoms of the same kind. Only with a higher content of X atoms, beginning with MX3 (n > 3), are atomic arrangements possible in which every M atom is surrounded solely by X atoms the X atoms, however, must continue to have other X atoms as neighbors. 

In crystals of more complex formula, such as titanium dioxide, TiC>2, a Schottky defect will consist of two anion vacancies and one cation vacancy. This is because it is necessary to counterbalance the loss of one Ti4+ ion from the crystal by the absence of two O2- ions in order to maintain composition and electroneutrality. This ratio of two anion vacancies per one cation vacancy will hold in all ionic compounds of formula MX2. In crystals like A1203, two Al3+ vacancies must be balanced by three O2- vacancies. Thus, in crystals with a formula M2X3, a Schottky defect will consist of two vacancies on the cation sublattice and three vacancies on the anion sublattice. These vacancies are not considered to be clustered together but are distributed... 

An ionic compound has a fixed composition. It may consist of several cations (A, B,. ..) and several anions (X, Y,. ..) so that its general formula is given by ... 

In this section, you learned why solutions of different salts have different pH values. You learned how to analyze the composition of a salt to predict whether the salt forms an acidic, basic, or neutral solution. Finally, you learned how to apply your understanding of the properties of salts to calculate the pH at the equivalence point of a titration. You used the pH to determine a suitable indicator for the titration. In section 9.2, you will further investigate the equilibria of solutions and learn how to predict the solubility of ionic compounds in solution. 

Nitric oxide reacts with sodium in liquid ammonia forming sodium hyponi-trite, an ionic compound of composition Na22+(N002 ... 

The structure analysis of nitrites, however, shows that they contain isolated NO " groups in complexes of trivalent metals, e.g. in the alkali aluminates (KA102) that have the same bruto composition as the nitrites, the coordination, indeed, is higher. Here the AI3 ions are surrounded by six oxygen ions, a coordination number that is as to be expected for a really ionic compound. Hypochlorites ACIO, chlorites AC102, chlorates AG103, sulphites A2S03, etc., all have coordination numbers that are smaller than those to be expected in ionic compounds. 

Liquid thioxanthones, for example, 631 and 632, have been synthesized in order to improve formulation of photopolymerizable compositions <1998W09842697> and thioxanthones possessing either cationic 633 <1998W09853369> or anionic 634 <2000MI212> hydrophilic substituents have been prepared and successfully employed for curing water-based ink formulations. The mode of action of the latter ionic compounds appears to involve both the lowest excited triplet and singlet states . 

For all these reasons, it will be understandable that LLC systems have been virtually replaced by chemically bonded phases (section 3.2.2) in current LC practice. Consequently, the various parameters of interest for the optimization of these systems will not be discussed extensively. With regard to the influence of temperature and mobile phase composition on retention and selectivity, it is suggested that the same relationships may be used for insoluble LLC stationary phases as are used for LBPC. LLC systems have been used extensively for the separation of ionic compounds by means of ion-pairing techniques. Such systems will be discussed in section 3.3.2. 

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. 

Since this is an ionic compound, the empirical formula is the only formula that is possible. However, for a molecular compound, it is possible to have an empirical formula that does not represent the molecular formula. Here is a look at such an example. In the first problem, we will also show the procedure for beginning with a percentage composition, which is quite typical for these problems. 

Groups of atoms of certain elements combine in a rather stable state, making up ionic compounds. An example is CaC03, the compositional formula of limestone, composed of Ca2+ + CO2-. A second example is CaS04, the compositional formula of anhydrite, composed of Ca2+ + S04. ... 

Many ionic compounds exist as hydrates. Often you can convert hydrates to anhydrous ionic compounds by heating them. Thus, hydrates are well suited to determining percentage composition experimentally. 

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