The Formation of Ionic Compounds

The strength of the ionic bonding depends to a great extent on the net strength of these attractions and repulsions and is described by Coulomb s law, which can be expressed as follows the energy of attraction (or repulsion) between two particles Is directly proportional to the product of the charges and inversely proportional to the distance between them. 

Ionic compounds are neutral that is, they possess no net charge. For this to occur, they must contain equal numbers of positive and negative charges—not necessarily equal numbers of positive and negative ions. Because Na and Cl each bear a unit charge (1+ or 1—), equal numbers of these ions are present in sodium chloride but in sodium oxide, for example, there are two Na ions needed to balance the 2— charge of each oxide ion, 

Problem What monatomic ions do the following elements form  

Plan We use the given Z value to find the element in the periodic table and see where its group lies relative to the noble gases. Elements in Groups 1 A, 2A, and 3A lose electrons to attain the same number as the nearest noble gas and become positive ions those in Groups 5A, 6A, and 7A gain electrons and become negative ions. 

Solution (a) I Iodine (53I) is a nonmetal in Group 7A(17), one of the halogens. Like any member of this group, it gains 1 electron to have the same number as the nearest Group 8A(18) member, in this case 54Xe. 

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We can use Lewis dot formulas to represent the transfer of electrons in the formation of ionic compounds. For example, the formation of the ionic compound sodium fluoride, NaF, can be represented using Lewis dot formulas and valence electron conflgurations ... 

Lewis and many other chemists had recognized the shortcomings of the ionic bond. When diatomic molecules, such as or Cl, were considered, there was no reason why one atom should lose an electron and an identical atom should gain an electron. There had to be another explanation for how diatomic molecules formed. We have seen how the octet rule applies to the formation of ionic compounds by the transfer of electrons. This rule also helps explain the formation of covalent bonds when molecules (covalent compounds) form. Covalent bonds result when atoms share electrons. Using fluorine, F, as a representative halogen, we can see how the octet rule applies to the formation of the molecule. Each fluorine atom has seven valence electrons and needs one more electron to achieve the stable octet valence configuration. If two fluorines share a pair of electrons, then the stable octet configuration is achieved ... 

Clearly, U is the biggest number in the cycle and is the main driving force for the formation of ionic compounds. Nevertheless, the other factors can tip the balance one way or another. For example, AHSub is particularly large for the transition metals niobium, tantalum, molybdenum, tungsten, and rhenium, with the result that, in their lower oxidation states, they do not form simple ionic compounds such as ReCl3 but rather form compounds that contain clusters of bonded metal atoms (in this example, Re3 clusters are involved, so the formula is better written ResClg). 

The formation of ionic compounds depends on a balance of energies and energy input required to form ions must be compensated by the lattice energy of the compound. For ions in solution, a similar cycle could be drawn, including the solvation energy rather than the lattice energy. 

Redox reactions are not limited to single replacement reactions. They really describe a wide variety of reactions, but each shares the common theme of involving an oxidation and a reduction. An oxidation occurs when a substance loses electrons and becomes more positively charged. Earlier in the book we discussed a similar phenomenon in the formation of ionic compounds. Substances don t just lose electrons for no reason. They lose electrons because another substance takes them. When a substance acquires additional electrons and becomes more negatively charged, it is called a reduction. An oxidation cannot take place without a reduction, so these processes must occur simultaneously. These reactions describe the simultaneous oxidation and reduction of materials, which has earned them the name oxidation-reduction reactions. 

The formation of ionic compounds in the reactions of NF30 with strong Lewis acids was reported in the initial publications on the compound and studied in greater detail subsequently (47,288). Loss of a fluoride ion occurs as shown below, as in the reactions of NOF and N02F with Lewis acids. 

To write octet rule electronic structures for the formation of ionic compounds and to deduce the formulas of compounds of main group metals with nonmetals... 

With regard to the formation of ionic compounds, it is not too relevant whether the 8p or 7d shell is occupied in the neutral atom, as studied in extenso by Mann and Wdber (50). Instead, the significant question for more ionic compounds is whether in the ions, after all outer s, p and d electrons are removed, some g or f electrons will be in frontier orbitals or whether they might be easily excited to an outer electron shell so that they can be removed as well. Prince and Waber (103) showed that even in the divalent state of element 126 one g electron has changed to an / electronic state. However, the 8s electrons are not the first to be removed. Thus, the divalent ions will be expected to act as soft Lewis acids and possibly form covalent complex ions readily. Crystal or ligand fields influence the nature of the hybridization. Details such as directionality of bonds... 

At the same time, interaction of the neutral base 1 with polyboranes or boron trifluoride does occur even at low temperatures (Scheme 17)179. NMR monitoring showed that while 1 with diborane or BF3 gave salts of similar type, 144 and 145, interaction between 1 and decaborane resulted in proton transfer, and reaction with pentaborane led to the formation of ionic compound X+ B9II 4, whose cationic part (X+) remained unidentified. 

MiniLab 4.2 The Formation of Ionic Compounds ChemLab The Formation and Decomposition of Zinc Iodide... 

The force of attraction between two species is called a bond. It might be a strong attraction or a weak one, but it is a bond nonetheless. Most ionic compounds are composed of metals and nonmetals, and most covalent compounds are composed of only nonmetals. Let s start the discussion by looking at the formation of ionic compounds. 

The following three examples show the formation of ionic compounds by the complete transfer of one or more electrons from the metal to the nonmetal. In each case, the total number of electrons lost equals the total number gained. Note that each ion has the electronic configuration of a noble gas. As you inspect each example, notice how the number of electrons lost by the metal and gained by the nonmetal determines the formula of the ionic product. The formula of the product can be predicted from the electronic configurations of the reactants. 

The definitions of oxidation and reduction in terms of loss and gain of electrons apply to the formation of ionic compounds such as CaO and the reduction of ions by Zn. However, these definitions do not accurately characterize the formation of hydrogen chloride (HCl) and sulfur dioxide (SO2) ... 

These reactions involve the formation of ionic compounds from neutral metal carbonyls in the presence of a Lewis base with a nitrogen or oxygen donor atom. The reaction of Fe3(CO)i2 with pyridine typifies... 

Writing equations for the formation of ionic compounds using Lewis symbois... 

The formation of sodium chloride from sodium and chlorine is veiy exothermic, as indicated by the large negative enthalpy of formation value given in Equation 8.1, AHj = —410.9 kj. Appendix C shows that the heat of formation of other ionic substances is also quite negative. What factors make the formation of ionic compounds so exothermic ... 

Low ionization energy for one of the elements, high electron affinity for the other element and high lattice energy favour the formation of ionic compounds. 

Use the octet rule to correctly predict the ions formed during the formation of ionic compounds, and write correct formulas for binary ionic compounds containing a representative metal and a representative nonmetal. (Section 4.3)... 

A A Why do atoms of two nonmetals generally form a compound by sharing valence electrons, rather than by transferring electrons, as in the formation of ionic compounds ... 

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