Some Questions to Consider

1. Some Questions to Consider • Why are so few elements (such as Au, S, Ar, N, O, Ag) found free in Nature as atoms? • Why do atoms of different elements combine (react) to form compounds? • What is happening in this process? • How can we explain the tremendous number of compounds that are known today? • Many of the answers will be found in Chapter 6 (“Chemical Formulas and Bonding”), and they are the result of a ‘tug of war’ between elements. (Example)

2. Chapter 7: Chemical Formulas and Bonding PPowell 05

3. 7-1 Objectives • Describe the distinguishing characteristics of an ionic bond. • Describe some properties of ionic compounds. • Explain the “octet rule.” • Draw Lewis dot diagrams to show the valence electrons of an atom. • Distinguish among anions, cations, and polyatomic ions. • Name binary ionic compounds. • Write the empirical formula for binary ionic compounds.

4. Ions: A Review • Cation: a positively charged ion. • Metals tend to form cations. (How?) • Anion: a negatively charged ion. • Nonmetals tend to form anions. (How?) • How do you think cations and anions will interact? OBJ: Describe the distinguishing characteristics of an ionic bond.

5. Ionic Compounds • Ionic bond: results from the attraction between a positive ion and a negative ion. • Most metals will form ionic bonds with most nonmetals. • Ionic compound: consists of cations and anions. • Electrically neutral; the total charges of the cations and anions must balance. OBJ: Describe the distinguishing characteristics of an ionic bond.

6. Video Clip: Ionic Bonds In Action • Reaction of magnesium and oxygen releases light energy. • Product (MgO) is more stable than the reactants alone. OBJ: Describe the distinguishing characteristics of an ionic bond.

7. Why is magnesium oxide more stable than elemental magnesium and oxygen? • Compare electron configurations of the atoms to their ions: • Mg: [Ne]3s2 • Mg  Mg2+ + 2e- • Mg2+ = [Ne]2+ • O:[He]2s22p4 • O + 2e- O2- • O2-:[He]2s22p6, or [Ne]2- • Noble Gas electron configurations are more stable! 7-1A: Describe the distinguishing characteristics of an ionic bond.

8. The Octet Rule • Atoms tend to gain, lose, or share electrons in order to get a full set of valence electrons. • Most atoms have eight valence electrons in a full set. • Exceptions: H, He (Why?) • The representative elements act like “noble gas impersonators.” OBJ: Explain the octet rule.

9. The Octet Rule • Metals will lose e- to “impersonate” a noble gas. • Metals form cations. • Nonmetals will gain e- to “impersonate” a noble gas. • Nonmetals form anions. OBJ: Explain the octet rule.

10. The Role of Valence Electrons • Note that the valence electrons were involved in these changes, NOT the core electrons. • Why? (Which orbitals & electrons are encountered first when two atoms interact?) • Chemists focus on the valence electrons (s and p outer electrons) to understand the chemistry of atoms. • To aid us, we use shorthand diagrams, called Lewis Dot Diagrams, where dots represent the valence electrons around an atom.

11. LEWIS DOT DIAGRAMS

12. Lewis Dot diagrams • A way of keeping track of valence electrons. • How to write them: • Write the symbol. • Put one dot for each valence electron. • Don’t pair electrons until you have to. X OBJ: Draw Lewis dot diagrams to show the valence electrons of an atom.

13. Lewis Dot Structure: Nitrogen • Nitrogen has 5 valence electrons. • Write the symbol. • Draw one dot per side of the symbol: top, right, bottom, left. • Don’t pair electrons until you have to. N OBJ: Draw Lewis dot diagrams to show the valence electrons of an atom.

14. Dot Diagram: Ionic Bond Formation Na Cl  Na Cl  Na+ Cl - This is sodium chloride. In solid form, it would have the lattice structure. Is the octet rule still satisfied for Na+? OBJ: Draw Lewis dot diagrams to show the valence electrons of an atom.

15. With a partner, decide what IONS those elements are likely to form, based on their Lewis dot structure. • Na  Na+ + 1e- gives Na+ = [Ne]1+ • Mg  Mg2+ + 2e- gives Mg2+ = [Ne]2+ • N + 3e-  N3- gives N3- = [Ne]3- • F + e-  F- gives F- = [Ne]1- • Most ions form in order to get the electron configuration of the nearest noble gas. + Na 2+ Mg 3- N F - OBJ:Explain the octet rule.

16. Lewis Dot Diagram Practice:

17. Important Note • When an anion is formed, the electron(s) come(s) from some other atom. • When a cation is formed, the original atom has donated its electron(s) to some other atom. • In other words, the previous slide only showed you only half the story. OBJ: Describe the distinguishing characteristics of an ionic bond.

18. What about Argon? • Ar is a noble gas. • Ar has an octet. • Ar is INERT, or unreactive. Ar OBJ:Explain the octet rule.

19. Types of Ions • Monatomic ion: has one atom. • May be cations or anions. • Polyatomic ion: has >1 atom. • May be cations or anions. OBJ: Distinguish among anions, cations, and polyatomic ions.

20. Monatomic Cations • See p. 231 Figure 7-10 • See p. 232 Figure 7-11 • Can you find them in Figures 7-10 and 7-11? Monatomic Anions Polyatomic Ions OBJ: Distinguish among anions, cations, and polyatomic ions.

21. Structure of Ionic Compounds • The ions within an ionic compound arrange to maximize their electrical attraction. • This results in a repetitive pattern called a “lattice network.” Cl - Na+ 7-1B: Describe some properties of ionic compounds.

22. - + - + + - + - - - + + - - + + + + - - + + - - - - + + - - + + Properties of Ionic Compounds • High bond strength means high melting points. • Often dissolve in water (soluble). • Ions in solution move freely and allow the solution to conduct electricity. • Ionic compounds are not good conductors as a solid, but they are when melted. • Ionic compounds are brittle. 7-1B: Describe some properties of ionic compounds.

23. 1 Ca2+ for every 2 F- Binary Ionic Compounds • Have ions of two different elements. • Do not always have 1:1 ratio of the two ions, because charges have to balance (electrical neutrality). • Ratio of ions in a compound is given by the empirical formula: CaF2 OBJ: Name binary ionic compounds.

24. Try naming these: • KCl • FeO • ZnF2 • NH4Cl Naming Binary Ionic Compounds CaF2 • Name the cation, then the anion. • Change the ending of the anion to “-ide.” CalciumFluoride • Potassium Chloride • Iron Oxide • Zinc Fluoride • Ammonium Chloride OBJ: Name binary ionic compounds.

25. Writing Formulas for Binary Ionic Compounds Given: Potassium Nitride • Write the element symbols and the charges for both the cation and the anion. K P • Criss-cross the two charges. • Reduce (simplify) the formula if necessary. 1+ 3- 3 1 OBJ: Write the formula for binary ionic compounds.

26. 7-1 Practice Problems • Do problems 1-5 with a partner. • Do problems 6-10 on your own. OBJ: Write the formula for binary ionic compounds.

27. Using ion cards, write the correct formulas for the following:

28. Can you now meet the objectives for 7-1? • Describe the distinguishing characteristics of an ionic bond. • Describe some properties of ionic compounds. • Explain the “octet rule.” • Draw Lewis dot diagrams to show the valence electrons of an atom. • Distinguish among anions, cations, and polyatomic ions. • Name binary ionic comounds. • Write the formula for binary ionic compounds.