Molecular and Ionic Compounds

The formulas of ionic compounds and molecular compounds are reported differently. Crystals of sodium chloride have different total numbers of cations and anions. We cannot simply specify the numbers of ions present as the formula of this ionic compound, for each crystal would have a different formula and the subscripts would be enormous. However, the ratio of the number of cations to the number of anions is the same in all the crystals, and the chemical formula shows this ratio. In sodium chloride, there is one Na+ ion for each Cl ion, so its formula is NaCl. Sodium chloride is an example of a binary ionic compound, a compound formed from the ions of two elements. 

Ionic Compounds To name an ionic compound, you just name the cation and then the anion. There is a crucial difference between naming ionic compounds and molecular compounds. In molecular compounds you must include prefix multipliers (di, tri, etc.) to indicate the number of each kind of atom in the molecule. In ionic compounds you must not include prefix multipliers, because the number of each ion in the formula unit is controlled by the charges on the ions. If the cation is a representative element, it is not necessary to indicate the charge, because (with few exceptions) these metals form cations with an ionic charge equal to the group number. 

Students who are just learning chemistry usually have a great deal of trouble distinguishing ionic compounds from molecular compounds, which is understandable, considering how similar their formulas can look. For example, KC1 is an ionic compound, yet HC1 is a molecular compound. It is hard to see and remember this difference, and students often become frustrated when they are corrected by their instructor after they incorrectly refer to an ionic compound as a molecule. How can you quickly tell ionic compounds from molecular ones without looking up the electronegativities, so you can avoid the embarrassment of being corrected in class More importantly, what is the big deal about the distinction between ionic compounds and molecular compounds anyway ... 

The chemist also needs to be able to convert between mass and moles for ionic compounds. The calculations are the same as for molecular compounds, but some of the terminology is different. Remember that solid ionic compounds and molecular Objective 8 compounds differ in the way their particles are organized and held together (Figure 9.2). 

As you have iearned, solid compounds can be ionic or moiecular. In an ionic soiid, a crystal structure is made up of charged particles held together by ionic attractions. In a molecular solid, molecules are composed of covalently bonded atoms. The solid is held together by noncovalent, intermolecular forces. When they dissolve in water, ionic compounds and molecular compounds behave differently. 

In Chapter 3, the formulas and names of compounds were introduced and, in order to establish naming schemes, a distinction was made between ionic compounds and molecular compounds. Ionic compounds were... 

New art comparing the particles and bonding of ionic compounds and molecular compounds has been added. 

Elements at the right of the p block have characteristically high electron affinities they tend to gain electrons to complete closed shells. Except for the metalloids tellurium and polonium, the members of Groups 16/VI and 17/VII are nonmetals (Fig. 1.62). They typically form molecular compounds with one another. They react with metals to form the anions in ionic compounds, and hence many of the minerals that surround us, such as limestone and granite, contain anions formed from non-metals, such as S2-, CO,2-, and S042-. Much of the metals industry is concerned with the problem of extracting metals from their combinations with nonmetals. 

C.l What is meant by (a) an ionic compound and (b) a molecular compound What are the typical properties of the two classes of compounds ... 

KEY CONCEPT PROBLEM 2.13 Which of the following drawings is most likely to represent an ionic compound, and which a molecular compound Explain. 

The above characterizations primarily concern the interactions between molecular solutes and ILs. However, ILs are also good solvents for ionic compounds, and have been studied extensively as media for transition metal catalysis [4, 38, 219] and for the extraction of heavy metals [23]. ILs are capable of solvating even simple salts, such as NaCl, to some degree [219], and in fact the removal of halide impurities resulting from synthesis can be a considerable challenge [68]. However, ionic complexes are generally far more soluble than simple salts [220], and we focus our attention on these systems as they have received greater study and are more relevant to the processes noted above. 

Table 13.2 shows some of the properties of the trihalides of the Group VA elements. Several trends in the data shown in Table 13.2 are of interest. For example, the trihalides of phosphorus and arsenic can be considered as covalent molecules. As a result, the intermolecular forces are dipole-dipole and London forces that are weak. Therefore, the melting and boiling points increase with molecular weight as expected. The trifluorides of antimony and bismuth are essentially ionic compounds and the melting points are much higher than those of the halogen derivatives that are more covalent. 

Elements in Groups 1 and 2 reacting with elements in Groups 16 and 17 will almost always form ionic compounds and not molecular compounds. Therefore, the formula CaO likely indicates an ionic compound because Ca is a Group 2 metal and O is a Group 16 nonmetal. In contrast, the formula ICl likely indicates a molecular compound because both I and Cl are members of Group 17. 

Like ionic compounds, some molecular compounds can form crystals, in which the basic unit is a molecule. Often you can see the pattern of the units in the shape of ionic and molecular crystals, as shown in Figure 20. 

You know that ionic compounds share many properties. The properties of a molecular substance—a substance that has atoms held together by covalent rather than ionic bonds—are more variable than the properties of ionic compounds. Some molecular substances, such as polyethylene plastic and the fats in butter, are soft rubber is elastic and diamond and quartz are hard. 

Although molecular substances have varied properties, some generalities can be made to distinguish them from ionic compounds. Molecular substances usually have lower melting points, and most are not as hard as ionic compounds. Figure 5.11. In addition, most molecular substances are less soluble in water than ionic compounds and are not electrolytes. The properties of most ionic and molecular substances are different enough that their differences can be used to classify and separate them from one... 

How did the melting points of the ionic compounds and the molecular compormds compare What factors affect melting point ... 

It is more difficult to predict the solubility of polar molecular substances than to predict the solubility of ionic compounds and nonpolar molecular substances. Many polar molecular substances are soluble in both water and hexane. For example, ethanol is miscible with both water and hexane. The following generalization is helpfiil ... 

NH3, BF3, and C2H2 are molecular compounds, and their formulas represent the composition of the molecules. BaBr2 and Na20 are ionic compounds, and their formulas represent one formula unit of the compound. 

In this chapter we shall discuss the structures of a number of simple compounds of composition AmXz, and also of some complex oxides and sulphides of composition AmBnXz. We shall find among these compounds representatives of ionic, covalent and molecular structures. 

The mechanisms of the usual organic reactions are now clearly established, and the reactions are classified as ionic, radical, and molecular. More detailed classifications have also been made. The mechanisms of many reactions involving non-transition metal compounds are clear enough for example, in the Grig-nard or Reformatsky reaction, the first step is the irreversible oxidative addition of alkyl halides to form Mg-carbon or Zn-carbon bonds, in which the carbon is considered to be a nucleophilic center or carbanion which reacts with various electrophiles. 

Distinguish between an ionic compound and a molecular compound dissolved in water. Do all molecular compounds ionize when dissolved in water Explain. 

The explanations of such solid-state phenomena involve many factors. Nevertheless, it is known that covalent energy terms are of underlying importance in determining many of these properties of ionic solids as well as the behavior of molecular complexes. Although, therefore, we cannot draw a neat boundary between typically ionic compounds and, on the one hand, complex compounds nor, on the other, covalent solids and metals, many properties of compounds to which the ionic model may be applied indicate that a more rigorous explanation of the bonding is necessary. 

Which of the following diagrams most likely represents an ionic compound, and which represents a molecular one Explain your choice. [Sections 2.6 and 2.7]... 

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