chem_TE_ch07.fm Page 194 Monday, April 17, 2006 9:30 AM 7.2 7.2 Ionic Bonds and Ionic Compounds 1 FOCUS You have heard of harvesting Objectives Guide for Reading crops such as wheat or rice—but salt? In many coastal countries that have 7.2.1 Explain the electrical charge of warm, relatively dry climates, salt is produced by the Key Concepts evaporation of seawater. The salty water is chan- an ionic compound. • What is the electrical charge of neled into a series of shallow ponds, where it an ionic compound? 7.2.2 Describe three properties of • What are three properties of becomes more concentrated as the water ionic compounds. ionic compounds? evaporates by exposure to the sun. When the Vocabulary saltwater is concentrated enough, it is Guide for Reading ionic compounds diverted into a pan, on which the sodium ionic bonds chloride crystals deposit. Salt farmers then chemical formula drain the pans and collect the salt into piles to L2 Build Vocabulary formula unit dry. In this section, you will learn how cations Paraphase Have students skim coordination number and anions combine to form stable compounds through the section to locate the mean- Reading Strategy such as sodium chloride. Previewing Before you read ings of the vocabulary terms. Then have this section, rewrite the headings them paraphrase each definition. as how, why, and what questions about ionic compounds. As Formation of Ionic Compounds Reading Strategy L2 you read, write answers to Compounds composed of cations and anions are called ionic compounds. the questions. Ionic compounds are usually composed of metal cations and nonmetal Identify Main Ideas/Details Have anions. For example, sodium chloride, or table salt, is composed of sodium students identify the main idea of each cations and chloride anions. Although they are composed of ions, ionic paragraph of this section. Have them compounds are electrically neutral. The total positive charge of the cations include these ideas in an outline of the equals the total negative charge of the anions. section. Ionic Bonds Anions and cations have opposite charges and attract one another by means of electrostatic forces. The electrostatic forces that hold ions together in ionic compounds are called ionic bonds. 2 INSTRUCT Sodium chloride provides a simple example of how ionic bonds are formed. Consider the reaction between a sodium atom and a chlorine atom. Sodium has a single valence electron that it can easily lose. (If the sodium atom loses its valence electron, it achieves the stable electron con- figuration of neon.) Chlorine has seven valence electrons and can easily Have students examine the section- gain one. (If the chlorine atom gains a valence electron, it achieves the sta- opening photograph and Figure 7.8. Dis- ble electron configuration of argon.) When sodium and chlorine react to cuss how the reactive (and poisonous) form a compound, the sodium atom gives its one valence electron to a elements sodium metal and chlorine gas chlorine atom. Thus sodium and chlorine atoms combine in a one-to-one can combine to form harmless table salt. ratio and both ions have stable octets. .. .. Ask, What characteristics of sodium Na Cl.. Na .Cl.. s2 s2 p6 s1 sp2 s2 6 s2 p5 s22s p6 sp2 s2 6 s26p and chlorine atoms allow them to 1 2 2 3 1 2 2 3 3 1 (12 )21* 1 2 2 (13 )31* form the stable compound sodium octet octet Animation 8 Take an chloride, also known as table salt? atomic-level look at the Ne Ar (Sodium atoms can lose an electron easily, formation of KCl. 22 6 sp2 s2 6 s26p 1s (12s)21*p 1 2 2 (13 )31* and chlorine atoms can accept an electron withChemASAP octet octet easily. The resulting ions can combine with the other oppositely charged ions.) 194 Chapter 7 Remind students that NaCl is an example of an ionic compound. Formation of Ionic Section Resources Compounds Print Technology Discuss L2 •Guided Reading and Study Workbook, •Interactive Textbook with ChemASAP, Section 7.2 Animation 8, Simulation 5, Problem- Explain that the formation of positive •Core Teaching Resources, Section 7.2 Solving 7.12, Assessment 7.2 ions and of negative ions are simulta- Review, Interpreting Graphics •Go Online, Section 7.2 neous and interdependent processes. •Transparencies, T79–T81 •Virtual Chemistry Lab, Lab 2 An ionic compound is the result of the •Probeware Laboratory Manual, transfer of electrons from one set of Section 7.2 atoms to another set of atoms. An ionic compound consists entirely of ions. 194 Chapter 7 chem_TE_ch07.fm Page 195 Thursday, April 14, 2005 11:45 AM Figure 7.7 shows aluminum and bromine reacting to form the com- Discuss L2 pound aluminum bromide. Each aluminum atom has three valence elec- Ask, Why are crystalline ionic com- trons to lose. Each bromine atom has seven valence electrons and readily gains one additional electron. Therefore, when aluminum and bromine pounds generally so rigid and brit- react, three bromine atoms combine with each aluminum atom. tle? (The crystal is rigid because it is held together by a specific three-dimensional Formula Units The ionic compound sodium chloride is composed of equal numbers of sodium cations (Naϩ) and chloride anions (ClϪ). As you array of relatively strong attractive forces can see in Figure 7.8, the ions in solid sodium chloride are arranged in between anions and cations, which is an orderly pattern. There are no single discrete units, only a continuous accompanied by minimal charge repul- array of ions. sion of like ions. The crystal is brittle Chemists represent the composition of substances by writing chemical Bromine (Br ) Aluminum (Al) because the attractive interactions are formulas. A chemical formula shows the kinds and numbers of atoms in the 2 smallest representative unit of a substance. NaCl, for example, is the chem- specifically arranged within the crystal ical formula for sodium chloride. Note, however, that the formula NaCl structure. If this arrangement is dis- does not represent a single discrete unit. Because an ionic compound exists turbed, as it would be if the crystal were Aluminum bromide (AlBr ) as a collection of positively and negatively charged ions arranged in repeat- 3 hit with a hammer, charge repulsion ing patterns, its chemical formula refers to a ratio known as a formula unit. Figure 7.7 Aluminum metal and between ions of the same charge can formula unit A is the lowest whole-number ratio of ions in an ionic com- the nonmetal bromine react to force the crystal to fragment.) pound. For sodium chloride, the lowest whole-number ratio of the ions is form an ionic solid, aluminum ϩ Ϫ bromide. 1:1 (one Na to each Cl ). Thus the formula unit for sodium chloride is Discuss L2 NaCl. Although ionic charges are used to derive the correct formula, they are not shown when you write the formula unit of the compound. To assess students’ prior knowledge The ionic compound magnesium chloride contains magnesium cat- about ionic bonds and crystals, ask, ϩ Ϫ ions (Mg2 ) and chloride anions (Cl ). In magnesium chloride, the ratio of What is an ionic bond? (an electical 2ϩ Ϫ magnesium cations to chloride anions is 1:2 (one Mg to two Cl ). So its attraction between ions of opposite formula unit is MgCl2. Because there are twice as many chloride anions (each with a 1Ϫ charge) as magnesium cations (each with a 2ϩ charge), the charge) How do a polyatomic ion and compound is electrically neutral. In aluminum bromide, described earlier, Figure 7.8 Sodium cations and a monatomic ion differ? (A mona- the ratio of aluminum cations to bromide ions is 1:3 (one Al3ϩ to three BrϪ chloride anions form a repeating tomic ion is an ion formed from a single ions), so the formula unit is AlBr . three-dimensional array in 3 sodium chloride (NaCl). atom; a polyatomic ion is a stable unit of Checkpoint What is the formula unit for magnesium chloride? Inferring How does the two or more tightly bound atoms that arrangement of ions in a sodium carries a charge.) Why are crystals of chloride crystal help explain why the compound is so stable? different ionic compounds different shapes? (The shapes reflect different ؊ Chloride ion (Cl ) geometric arrangements of anions and ؊ 18e cations with different sizes and charges.) 17p؉ 18n0 (Sodium ion (Na؉ 10e؊ 11p؉ 12n0 Structures of sodium Arrangement of Na ions Crystals of sodium chloride ion and chloride ion and Cl؊ ions in a crystal of sodium chloride Section 7.2 Ionic Bonds and Ionic Compounds 195 Differentiated Instruction English Learners L1 Encourage students to look up and define terms used to describe ionic compounds. Answers to... Students should define terms such as crystal Figure 7.8 Each ion in the arrange- and formula unit in English and their native ment is strongly attracted to its language. Emphasize how their strong neighbors and repulsions are bonding arrangement accounts for crystals minimized. having unique properties. Checkpoint MgCl2 Ionic and Metallic Bonding 195 chem_TE_ch07_PPL.fm Page 196 Wednesday, August 4, 2004 3:16 AM Section 7.2 (continued) CONCEPTUAL PROBLEM 7.2 CONCEPTUAL PROBLEM 7.2 Predicting Formulas of Ionic Compounds The ionic compound formed from potassium and oxygen is used in ceramic glazes. Use electron dot structures to predict Answers the formulas of the ionic compounds formed from the 12. a. KI following elements. a. potassium and oxygen b. magnesium and nitrogen b. Al2O3 13. CaCl2 Analyze Identify the relevant concepts. Electrons lost now equals electrons gained. L2 Practice Problems Plus Atoms of metals lose their valence electrons The formula of the compound formed (potas- Use electron dot structures to deter- when forming an ionic compound.