Chapter 6. Ionic and Molecular Compounds. Lecture Presentation. Karen C. Timberlake Pearson Education, Inc.

Transcription

1 Lecture Presentation Chapter 6 Ionic and Molecular Compounds

2 Chapter 6 Readiness Key Math Skills Using Positive and Negative Numbers in Calculations (1.4B) Solving Equations (1.4D) Core Chemistry Skills Writing Electron Configurations (4.7) Drawing Lewis Symbols (4.8)

3 Chapter 6 Why do atoms react? Recall the chemical properties of the noble gases they are extremely unreactive What do they have in common that could be the cause of this lack of reactivity? Examination of their electron configurations reveals that the noble gases either have 1. an outermost electron energy level that is completely filled with electrons (He = 2 e- in the 1st energy level, Ne = 8 e- in the 2 nd energy level)

4 Chapter 6 Why do atoms react? Completely filled valence energy level 8 valence electrons OR: 2. 8 electrons in the outermost energy level (8 valence e-)

5 Chapter 6 Why do atoms react? It is this arrangement of electrons that imparts stability to the noble gases All other elements react in order to achieve the same electron configuration as their nearest noble gas neighbor (8 valence e- = octet rule) Atoms can gain, lose, or share electrons in order to achieve the same electronic structure as their nearest noble gas neighbor

6 6.1 Ions: Transfer of Electrons Atoms form positively charged ions when they lose electrons and negatively charged ions when they gain electrons. Ionic bonds are formed by the strong attractive forces between positive and negative ions. Learning Goal Write the symbols for the simple ions of the representative elements.

7 Ionic and Covalent Bonds Chemical bonds are formed when atoms lose, gain, or share valence electrons to acquire an octet of eight valence electrons (octet rule). Ionic bonds occur when valence electrons of a metal atom are transferred to the atom of a nonmetal. Covalent bonds occur when nonmetal atoms share electrons to attain a noble gas arrangement.

8 Ionic and Covalent Bonds

9 Positive Ions: Metals Lose Electrons In ionic bonding, ions form when atoms gain or lose their valence electrons to form a stable electron configuration. Metals in Group 1A (1), Group 2A (2), and Group 3A (13) have low ionization energies, which means it doesn t take much energy to remove an e- readily lose one or more of their valence electrons to form ions with a positive charge. lose electrons until they have the same number of valence electrons as the nearest noble gas, usually eight valence electrons.

10 Positive Ions: Loss of Electrons Sodium atoms in Group 1A (1) are neutral, with 11 electrons and 11 protons, they lose one electron to have the same number of valence electrons as neon and a filled energy level. will form an ion with 10 electrons, 11 protons, and an ionic charge of 1+: Na +.

11 Positive Ions: Loss of Electrons Magnesium atoms in Group 2A (2) are neutral, and they have 12 electrons and 12 protons. They will lose 2 electrons to have the same number of valence electrons as neon and a filled energy level. form an ion with 10 electrons, 12 protons, and an ionic charge of 2+: Mg2 +.

12 Negative Ions: Nonmetals Gain Electrons Nonmetals, Group 5A (15), Group 6A (16), and Group 7A (17) have high ionization energies, they don t tend to lose electrons but instead gain electrons readily gain one or more valence electrons to form ions with a negative charge. gain electrons until they have the same number of valence electrons as the nearest noble gas, usually eight valence electrons. Core Chemistry Skill Writing Symbols For Positive and Negative Ions

13 Negative Ions: Gain of Electrons Chlorine atoms in Group 7A (17) are neutral, and they have 17 electrons and 17 protons. They will gain one electron to have the same number of valence electrons as argon. form an ion with 18 electrons, 17 protons, and a charge of 1 : Cl.

14 Formulas, Names of Common Ions

15 Ionic Charges, Group Numbers We can use the group numbers in the periodic table to determine the charges for the ions of the representative elements.

16 Chemistry Link to Health: Important Ions in the Body Ions are important in regulating body functions. Ion Occurrence Function Source Na + K + Ca 2+ Mg 2+ Principal cation outside the cell Principal cation outside the cell Cation outside the cell; found in bones Cation outside the cell; found in bones Regulation and control of body fluids Regulation of body fluids and cellular functions Major cation in bones, needed for muscle contractions Essential for certain enzymes, muscles, and nerve control Salt, cheese, pickles Bananas, potatoes, orange juice, milk Milk, yogurt, cheese, greens, spinach Chlorophyll, nuts, grains

17 Study Check Write the formula and symbol of an ion with 16 protons and 18 electrons.

18 Study Check Consider the elements calcium and chlorine. A. Identify each as a metal or a nonmetal. B. State the number of valence electrons for each. C. State the number of electrons that must be lost or gained for each to acquire an octet. D. Write the symbol, including its ionic charge, and name of each resulting ion.

19 6.2 Writing Formulas for Ionic Compounds Ionic compounds consist of positive and negative charges held together by the strong electrical attractions between oppositely charged ions. Learning Goal Using charge balance, write the correct formula for an ionic compound.

20 Properties of Ionic Compounds Ionic compounds consist of positive and negative ions. have attractions called ionic bonds between positively and negatively charged ions. have high melting points. are solids at room temperature.

21 NaCl, An Ionic Compound Sodium chloride is more commonly known as table salt. The magnification of NaCl crystals shows the arrangement of Na + and Cl ions in an NaCl crystal.

22 Formulas of Ionic Compounds In a chemical formula, the symbols and subscripts are written in the lowest whole-number ratio of the atoms or ions. the sum of ion charges equals zero. the total positive charge = total negative charge.

23 Subscripts in Formulas Core Chemistry Skill Writing Ionic Formulas

24 Writing Ionic Formulas from Ion Charges To balance ionic charge in an ionic compound, total positive charge = total negative charge

25 Study Check Write the ionic formula of the compound formed with Ba 2+ and Cl ions.

26 Study Check Select the correct formula for each of the following ionic compounds. 1. Na + and O 2 A. NaO B. Na 2 O C. NaO 2 2. Al 3+ and Cl A. AlCl 3 B. AlCl C. Al 3 Cl 3. Mg 2+ and N 3 A. MgN B. Mg 2 N 3 C. Mg 3 N 2

27 6.3 Naming and Writing Ionic Compounds Iodized salt contains Kl, potassium iodide, to prevent iodine deficiency. Learning Goal Given the formula of an ionic compound, write the correct name; given the name of an ionic compound, write the correct formula.

28 Naming Ionic Compounds When naming an ionic compound, the name of the metal is written first and is the same as the name of the element. the name of the nonmetal is the first syllable of the nonmetal name + ide ending and is written second. a space is placed between the name of the metal and nonmetal ion. Core Chemistry Skill Naming Ionic Compounds

29 Names of Some Ionic Compounds

30 Guide to Naming Ionic Compounds

31 Naming Ionic Compounds, K 2 O Name the ionic compound K 2 O. STEP 1 Identify the cation and anion. The cation, K +, is from Group 1A (1), and the anion, O 2, is from Group 6A (16). STEP 2 Name the cation by its element name. The cation, K +, is potassium.

32 Naming Ionic Compounds, K 2 O Name the ionic compound K 2 O. STEP 3 Name the anion by using the first syllable of its element name followed by ide. The name of the anion is oxide. STEP 4 Write the name for the cation first and the name for the anion second. K 2 O is potassium oxide.

33 Study Check Write the names of the following compounds. A. CaO B. Al 2 O 3 C. MgCl 2

34 Solution Write the names of the following compounds. STEP 1 Identify the cation and anion. A. CaO: The cation, Ca 2+, is from Group 2A (2), and the anion, O 2, is from Group 6A (16). B. Al 2 O 3 : The cation, Al 3+, is from Group 3A (13), and the anion, O 2, is from Group 6A (16). C. MgCl 2 : The cation, Mg 2+, is from Group 2A (2), and the anion, Cl, is from Group 7A (17).

35 Solution Write the names of the following compounds. STEP 2 Name the cation by its element name. A. CaO: The cation, Ca 2+, is calcium. B. Al 2 O 3 : The cation, Al 3+, is aluminum. C. MgCl 2 : The cation, Mg 2+, is magnesium.

36 Solution Write the names of the following compounds. STEP 3 Name the anion by using the first syllable of its element name followed by ide. A. CaO: The anion, O 2, is oxide. B. Al 2 O 3 : The anion, O 2, is oxide. C. MgCl 2 : The anion, Cl, is chloride.

37 Solution Write the names of the following compounds. STEP 4 Write the name of the cation first and the name of the anion second. A. The name of CaO is calcium oxide. B. The name of Al 2 O 3 is aluminum oxide. C. The name of MgCl 2 is magnesium chloride.

38 Study Check Write the formulas and names for compounds of the following ions: Br S 2 N 3 Na + Al 3+

39 Metals with Variable Charge Transition metals except for Zn 2+, Cd 2+, and Ag + form two or more positive ions (cations). A Roman numeral equal to the ion charge is placed in parentheses immediately after the metal name. Cu 2+ copper(ii) Pb 2+ lead(ii) Cu + copper(i) Pb 4+ lead(iv) Fe 2+ iron(ii) Cr 2+ chromium(ii) Fe 3+ iron(iii) Cr 3+ chromium(iii)

40 Metals with Variable Charge

41 Determination of Variable Charge Use the charge on the anion and charge balance to calculate charge on the metal ion. MnF 2 Mn charge + 2 F charge = 0? + 2 (1 ) = = 0 Manganese(II) fluoride

42 Ion Charges, Periodic Table

43 Guide to Naming Ionic Compounds with Variable Charge Metals

44 Naming Ionic Compounds with Variable Charge Metals, FeCl 2 Name the ionic compound FeCl 2. STEP 1 Determine the charge of the cation from the anion. Metal Nonmetal Formula FeCl 2 ANALYZE THE Elements iron (Fe) chloride (Cl) PROBLEM Groups transition element Group 7A (17) Ions Fe? Cl Charge Fe? + 2(1 ) = 0 Balance 1(?) + 2(1 ) = 0 Ions Fe 2+ Cl

45 Naming Ionic Compounds with Variable Charge Metals, FeCl 2 Name the ionic compound FeCl 2. STEP 2 Name the cation by its element name, and use a Roman numeral in parentheses for the charge. iron(ii) STEP 3 Name the anion by using the first syllable of its element name followed by ide. chloride STEP 4 Write the name for the cation first and the anion second. iron(ii) chloride

46 Study Check Name the following ionic compound containing a variable charge metal. SnO 2

47 Solution Name the following ionic compound containing a variable charge metal. SnO 2 STEP 1 Determine the charge of the cation from the anion. Metal Nonmetal Formula SnO 2 ANALYZE THE Elements tin (Sn) oxide (O) PROBLEM Groups transition element Group 6A (16) Ions Sn? O 2 Charge Sn? + 2(2 ) = 0 Balance 1(?) + 2(2 ) = 0 Ions Sn 4+ O 2

48 Solution Name the following ionic compound containing a variable charge metal. SnO 2 STEP 2 Name the cation by its element name and use a Roman numeral in parentheses for the charge. tin(iv) STEP 3 Name the anion by using the first syllable of its element name followed by ide. oxide STEP 4 Write the name for the cation first and the anion second. tin(iv) oxide

49 Writing Formulas from the Name of an Ionic Compound The formula for an ionic compound is written from, the first part of the name that describes the metal ion. the second part of the name that specifies the nonmetal ion. Subscripts are added to balance the charge.

50 Guide to Writing Formulas from the Name of an Ionic Compound

51 Writing Formulas from the Name of an Ionic Compound Write the formula for iron(iii) chloride. STEP 1 Identify the cation and anion. Type of Ion Cation Anion ANALYZE Name iron(iii) chloride THE Group transition element Group 7A (17) PROBLEM Symbol of Ion Fe 3+ Cl

52 Writing Formulas from the Name of an Ionic Compound Write the formula for iron(iii) chloride. STEP 2 Balance the charges. Fe 3+ Cl Cl Cl 1(3+) + 3(1 ) = 0 Becomes the subscript in the chemical formula. STEP 3 Write the formula, cation first, using subscripts from the charge balance. FeCl 3

53 Study Check Write chemical formulas for the following compounds: A. nickel(ii) sulfide B. zinc chloride C. iron(iii) oxide

54 Solution Write chemical formulas for the following compounds. STEP 1 Identify the cation and anion. A. nickel(ii) sulfide The Roman numeral (II) indicates that the charge on the nickel ion is 2+, Ni 2+. The anion sulfide S 2. B. zinc chloride Zinc is one of the transition metals with a fixed charge of 2+, Zn 2+. The anion chloride is Cl. C. iron(iii) oxide The Roman numeral (III) indicates that the charge on the iron ion is 3+, Fe 3+. The anion oxide is O 2.

55 Solution Write chemical formulas for the following compounds. STEP 2 Balance the charges. A. nickel(ii) sulfide C. iron(iii) oxide Ni 2+ S 2 Fe 3+ O 2 1(2+) + 1(2 ) = 0 Fe 3+ O 2 O 2 B. zinc chloride 2(3+) + 3(2 ) = 0 Zn 2+ Cl Cl 1(2+) + 2(1 ) = 0

56 Solution Write chemical formulas for the following compounds. STEP 3 Write the formula, cation first, using subscripts from the charge balance. A. nickel(ii) sulfide NiS B. zinc chloride ZnCl 2 C. iron(iii) oxide Fe 2 O 3

57 6.4 Polyatomic Ions Sodium chlorite, NaClO 2, is used in the processing and bleaching of pulp from wood fibers and recycled cardboard. Learning Goal Write the name and formula for an ionic compound containing a polyatomic ion.

58 Polyatomic Ions Polyatomic ions are a group of atoms with an overall charge. often consist of a nonmetal such as phosphorus, sulfur, carbon, or nitrogen and oxygen. usually have a 1, 2, or 3 charge. have a negative charge, except for NH 4+, ammonium, which has a positive charge,

59 Polyatomic Ions Many products contain polyatomic ions, which are groups of ions that have an ionic charge.

60 Polyatomic Ions, Names, and Formulas

61 Names of Polyatomic Ions 1. Names of most common polyatomic ions end in ate. SO 2 4 sulfate PO 3 4 phosphate NO 3 nitrate 2. When a related ion has one less oxygen, its name ends in ite. SO 3 2 sulfite PO 3 3 phosphite NO 2 nitrite

62 Names of Polyatomic Ions 3. Exceptions to these rules are the following: CN cyanide OH hydroxide 4. Add an H + to the polyatomic ion, and add +1 to its charge. CO H + = HCO 3 carbonate + H + = bicarbonate or hydrogen carbonate SO H + = HSO 4 sulfate + H + = bisulfate or hydrogen sulfate

63 Names of Polyatomic Ions 5. Halogens form 4 polyatomic ions with oxygen. Each has a 1 charge. ClO 4 ClO 3 ClO 2 perchlorate chlorate chlorite ClO hypochlorite Sodium chlorite is used in the processing and bleaching of pulp from wood fibers and recycled cardboard.

64 Writing Formulas for Compounds Containing Polyatomic Ions When writing formulas for ionic compounds containing polyatomic ions, we use the same rules of charge balance as those for simple ionic compounds. Consider the formula for magnesium nitrate: Magnesium ion Nitrate ion Mg 2+ + NO 3 NO 3 (2+) + 2(1 ) = 0 Mg NO 3 = Mg(NO 3 ) 2 Parentheses are placed around the polyatomic ion, and the subscript is placed just outside the parentheses.

65 Guide to Writing Formulas with Polyatomic Ions

66 Writing Formulas with Polyatomic Ions, Aluminum Bicarbonate Write the formula for aluminum bicarbonate. STEP 1 Identify the cation and polyatomic ion (anion). Cation Al 3+ Polyatomic ion HCO 3 STEP 2 Balance the charges. Al 3+ HCO 3 HCO 3 HCO 3 1(3+) + 3(1 ) = 0

67 Writing Formulas with Polyatomic Ions, Aluminum Bicarbonate Write the formula for aluminum bicarbonate. STEP 3 Write the formula, cation first, using the subscripts from charge balance. Al(HCO 3 ) 3

68 Study Check Write the formula for sodium phosphate.

69 Solution Write the formula for sodium phosphate. STEP 1 Identify the cation and polyatomic ion (anion). Cation Polyatomic ion Na + STEP 2 Balance the charges. Na + Na + Na + PO 4 3 PO 4 3 3(1+) + 1(3 ) = 0

70 Solution Write the formula for sodium phosphate. STEP 3 Write the formula, cation first, using the subscripts from charge balance. Na 3 PO 4

71 Naming Compounds with Polyatomic Ions When naming ionic compounds containing polyatomic ions, first write the positive ion, usually a metal. write the name of the polyatomic ion second. Recognizing polyatomic ions in a chemical formula helps to name it correctly. As with other ionic compounds, no prefixes are used.

72 Compounds with Polyatomic Ions

73 Study Check Select the correct formula for each. 1. aluminum nitrate A. AlNO 3 B. Al(NO) 3 C. Al(NO 3 ) 3 2. copper(ii) nitrate A. CuNO 3 B. Cu(NO 3 ) 2 C. Cu 2 (NO 3 ) 3. iron(iii) hydroxide A. FeOH B. Fe 3 OH C. Fe(OH) 3 4. tin(iv) hydroxide A. Sn(OH) 4 B. Sn(OH) 2 C. Sn 4 (OH)

74 Guide to Naming Compounds with Polyatomic Ions

75 Flow Chart, Naming Ionic Compounds

76 Study Check Name the following ionic compounds: A. Ca(NO 3 ) 2 B. FePO 4

77 Solution Name the following ionic compounds: STEP 1 STEP 2 STEP 3 STEP 4 Identify Name Name Name cation/anion cation anion compound A. Ca(NO 3 ) 2 Ca 2+ NO 3 calcium nitrate calcium nitrate B. FePO 4 Fe 3+ PO 4 3 iron(iii) phosphate iron(iii) phosphate

78 Study Check Name each of the following compounds containing polyatomic ions. A. MgSO 3 B. MgSO 4 C. Pb 3 (PO 3 ) 2

79 Study Check Name each of the following compounds: A. Fe 2 (SO 4 ) 3 B. Ba 3 (PO 3 ) 2 C. NiCO 3

80 6.5 Molecular Compounds: Sharing Electrons The names of molecular compounds need prefixes because several different compounds can be formed from the same two nonmetals. Learning Goal Given the formula of a molecular compound, write its correct name; given the name of a molecular compound, write its formula.

81 Covalent Bonds Molecular compounds form when atoms of two or more nonmetals share electrons and form a covalent bond. valence electrons are shared by nonmetal atoms to achieve stability. A molecule forms when two or more atoms share electrons. Core Chemistry Skill Writing the Names and Formulas for Molecular Compounds

82 Names and Formulas, Molecular Compounds When naming a molecular compound, the first nonmetal in the formula is named by its element name. second nonmetal is named using the first syllable of the name followed by ide. When a subscript indicates two or more atoms of an element, a prefix is shown in front of its name.

83 Names and Formulas, Molecular Compounds Several compounds may be formed from the same two nonmetals: CO 2 CO = carbon dioxide = carbon monoxide The number of oxygen atoms present is indicated by the prefix. When the vowels o and o or a and o appear together, the first vowel is omitted. NO = nitrogen monoxide,

84 Common Molecular Compounds

85 Guide to Naming Molecular Compounds

86 Naming Molecular Compounds, NCl 3 Name the molecular compound NCl 3. Symbol of the Element N Cl ANALYZE THE Name nitrogen chloride PROBLEM Subscript 1 3 Prefix none tri STEP 1 Name the first nonmetal by its element name. In NCl 3, the first nonmetal (N) is nitrogen.

87 Naming Molecular Compounds, NCl 3 Name the molecular compound NCl 3. STEP 2 Name the second nonmetal by using the first syllable of the element name followed by ide. In NCl 3, the second nonmetal (Cl) is chloride. STEP 3 Add prefixes to indicate the number of atoms (subscripts). Because there is one nitrogen atom, no prefix is needed. The subscript 3 for the Cl atoms is shown as the prefix tri. The name of NCl 3 is nitrogen trichloride.

88 Study Check Name the molecular compound B 2 O 3.

89 Solution Name the molecular compound B 2 O 3. Symbol of the Element B O ANALYZE THE Name boron oxide PROBLEM Subscript 2 3 Prefix di tri STEP 1 Name the first nonmetal by its element name. In B 2 O 3, the first nonmetal (B) is boron. STEP 2 Name the second nonmetal by using the first name followed by ide. The name of the second nonmetal (O) is oxide.

90 Solution Name the molecular compound B 2 O 3. STEP 3 Add prefixes to indicate the number of atoms (subscripts). In B 2 O 3, the first nonmetal (B) has the subscript of 2, which is shown as the prefix di. The subscript of 3 for the O atoms is shown as the prefix tri. The name of B 2 O 3 is diboron trioxide.

91 Guide to Writing Formulas from the Names of Molecular Compounds

92 Study Check Write the molecular formula for diphosphorus pentoxide.

93 Solution Write the molecular formula for diphosphorus pentoxide. STEP 1 Write the symbols in the order of the elements in the name. STEP 2 Write any prefixes as subscripts. The prefix di in diphosphorus indicates there are two phosphorus atoms. The prefix penta in pentoxide indicates there are five oxygen atoms. P 2 O 5

94 Compounds, Ionic or Covalent? A compound is usually ionic if the first element in the formula or the name is a metal or the polyatomic ion NH 4+. K 2 O: K is a metal; the compound is ionic. potassium oxide covalent if the first element in the formula is a nonmetal. N 2 O: N is a nonmetal; the compound is covalent. dinitrogen oxide

95 Flowchart, Naming Compounds

96 Study Check Select the correct name for each compound. 1. SiCl 4 A. silicon chloride B. tetrasilicon chloride C. silicon tetrachloride 2. P 2 O 5 A. phosphorus oxide B. phosphorus pentoxide C. diphosphorus pentoxide 3. Cl 2 O 7 A dichlorine heptoxide B. dichlorine oxide C. chlorine heptoxide

97 Study Check Identify each compound as ionic or covalent and give its correct name. A. SO 3 B. BaCl 2 C. (NH 4 ) 3 PO 3 D. Cu 2 CO 3 E. N 2 O 4

98 6.6 Lewis Structures for Molecules and Polyatomic Ions A molecule is represented by a Lewis structure in which the valence electrons of all the atoms are arranged to give octets. Learning Goal Draw the Lewis structures for molecular compounds or polyatomic ions.

99 The Simplest Molecule, H 2 A hydrogen molecule, H 2, forms as the atoms move closer and the positive charge of the nucleus attracts the electron of the other atom. has a shared pair of electrons, a covalent bond, to give a noble gas arrangement of He to each H atom. forms when the molecule formed is more stable than the two individual H atoms.

100 Formation of H 2 Molecule A covalent bond forms as H atoms move close together to share electrons.

101 Lewis Structures, Molecules A molecule is represented by a Lewis structure in which the valence electrons of all the atoms are arranged to give octets. The shared electrons, or bonding pairs, are shown as two dots or a single line between atoms. The nonbonding pairs, or lone pairs, are placed on the outside of the atoms.

102 Lewis Structures, Molecules A molecule is represented by a Lewis structure in which the valence electrons of all the atoms are arranged to give octets. To draw the electron-dot formula for a fluorine molecule, F 2, we start with the electron-dot symbols for each fluorine atom. each fluorine atom shares one electron to form a covalent bond, giving each fluorine an octet.

103 Elements, Diatomic Molecules The elements hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist as diatomic molecules.

104 Sharing Electrons Between Atoms of Different Elements The number of electrons a nonmetal atom shares and the number of covalent bonds it forms are usually equal to the number of electrons it needs to achieve a stable electron configuration. Typical Bonding Patterns of Some Nonmetals Core Chemistry Skill Drawing Lewis Structures

105 Drawing Lewis Structures To draw the electron-dot formula for methane, CH 4, draw the Lewis symbols for carbon and hydrogen. carbon needs four hydrogen atoms to have an octet. two bonding electrons between carbon and hydrogen can be drawn as a line. the central atom is the atom in the formula with the least number of atoms.

106 Molecules, Lewis Structures Electron-dot formulas for common molecules such as methane, ammonia, and water are shown in Table 6.13.

107 Guide to Drawing Electron-Dot Formulas

108 Study Check Draw the Lewis structure for PCl 3, phosphorus trichloride, which is used in the preparation of pesticides and flame retardants.

109 Solution Draw the Lewis structure for PCl 3, phosphorus trichloride, which is used in the preparation of pesticides and flame retardants. STEP 1 Determine the arrangement of atoms. In PCl3, P is the central atom; there is only one phosphorus atom in the formula.

110 Solution Draw the Lewis structure for PCl 3, phosphorus trichloride, which is used in the preparation of pesticides and flame retardants. STEP 2 Determine the total number of valence electrons. Use the group number to determine the number of valence electrons for each element.

111 Solution Draw the Lewis structure for PCl 3, phosphorus trichloride, which is used in the preparation of pesticides and flame retardants. STEP 3 Attach each bonded atom to the central atom with a pair of electrons. Each bonding pair can be represented with a single line.

112 Solution Draw the Lewis structure for PCl 3, phosphorus trichloride, which is used in the preparation of pesticides and flame retardants. STEP 4 Place the remaining electrons using single or multiple bonds to complete octets (two for H). Six electrons (3 2 e ) are used to bond the central P atom to three Cl atoms. Twenty valence electrons are left.

113 Double and Triple Bonds A double bond occurs when atoms share two pairs of electrons. forms when there are not enough electrons to complete octets. A triple bond occurs when atoms share three pairs of electrons. forms when there are not enough electrons to complete octets.

114 Exceptions to the Octet Rule While the octet rule is useful, there are exceptions. Hydrogen requires just 2 electrons to form a noble gas arrangement. Nonmetals P, S, Cl, Br, and I can form compounds with 10 or 12 valence electrons. The S atom has an octet in many compounds, but in SF 6, there are 12 valence electrons or 6 bonds to the sulfur atom.

115 6.7 Electronegativity and Bond Polarity The electronegativity values of representative elements in Group 1A (1) to Group 7A (17). Learning Goal Use electronegativity to determine the polarity of a bond.

116 Electronegativity The electronegativity of an atom is its ability to attract the shared electrons in a bond. It increases from left to right going across a period on the periodic table. increases from the bottom to the top of the periodic table. is high for the nonmetals, with fluorine as the highest. is low for the metals. Core Chemistry Skill Using Electronegativity

117 Electronegativity and the Periodic Table

118 Polarity of Bonds The difference in electronegativity of bonding atoms can be used to predict the polarity of the bond. In the H 2 molecule, electrons are shared equally and the bond is nonpolar. In the HCl molecule, electrons are shared unequally, and the bond is polar.

119 Nonpolar Covalent Bonds A nonpolar covalent bond occurs between nonmetals. It is an equal or almost equal sharing of electrons by the two bonding atoms. has a very small electronegativity difference between atoms. Examples Electronegativity Atoms Difference Type of Bond N-N = 0.0 Nonpolar covalent Cl-Br = 0.2 Nonpolar covalent H-Si = 0.3 Nonpolar covalent

120 Polar Covalent Bonds A polar covalent bond occurs between nonmetal atoms. It is an unequal sharing of electrons. has a moderate electronegativity difference. Examples Electronegativity Atoms Difference Type of Bond O-Cl = 0.5 Polar covalent Cl-C = 0.5 Polar covalent O-S = 1.0 Polar covalent

121 Dipoles and Bond Polarity A polar covalent bond becomes more polar as the difference in electronegativity increases. The separation of charges in a polar bond is called a dipole. The positive and negative ends of the dipole are located by using the lowercase Greek letter delta with a positive or negative charge. an arrow that points from the positive to the negative end of the dipole.

122 Ionic Bonds An ionic bond occurs between metal and nonmetal ions. is a result of electron transfer. has a large electronegativity difference (1.8 or more). Examples Electronegativity Atoms Difference Type of Bond Cl-K = 2.2 Ionic N-Na = 2.1 Ionic S-Cs = 1.8 Ionic

123 Variations in Bonding The difference in electronegativity values for two atoms can be used to predict the type of chemical bond. If the electronegativity difference is between 0 and 0.4, the bond is nonpolar covalent. between 0.5 and 1.8, the bond is polar covalent. greater than 1.8, the bond is ionic and the electrons are considered transferred.

124 Electronegativity and Bond Types

125 Predicting Bond Type Using Electronegativity

126 Study Check Use the electronegativity difference to identify the type of bond (nonpolar covalent [NP], polar covalent [P], or ionic [I]) between the following: A. K N B. N O C.Cl Cl D.H Cl

127 6.8 Shapes and Polarity of Molecules In a molecule of methane, CH 4, the central C atom is bonded to four H atoms. The best geometry for minimal repulsion is tetrahedral, with bond angles of 109. Learning Goal Predict the three-dimensional structure of a molecule, and classify it as polar or nonpolar.

128 VSEPR Theory Valence Shell Electron-Pair Repulsion Theory (VSEPR) describes the orientation of electron groups around the central atom. states that electron groups are arranged as far apart as possible around the central atom. states that the specific shape of a molecule is determined by the number of atoms attached to the central atom. Core Chemistry Skill Predicting Shape

129 Central Atoms with Two Electron Groups In a molecule of CO 2 two electron groups are placed around the central atom, carbon. the repulsion is minimized by placing the two groups on opposite sides of the carbon atom, giving this a linear arrangement with bond angles of 180. the shape with two electron groups around the central atom is linear.

130 Central Atoms with Three Electron Groups In a molecule of formaldehyde, H 2 CO 3, three electron groups are placed around the central atom, carbon. the repulsion is minimized by placing the three groups as far apart as possible at bond angles of 120. the shape with three electron groups around the central atom is trigonal planar.

131 Central Atoms with Three Electron Groups In a molecule of SO 2, there are three electron groups around the central atom S: 1. a single-bonded O atom 2. a double-bonded O atom 3. a lone pair of electrons The repulsion is minimized by placing the three electron groups as far apart as possible. The shape with two bonds and a lone pair on the central atom is bent with a bond angle of 120.

132 Central Atoms with Four Electron Groups In a molecule of CH 4 four electron groups are attached to H atoms around the central atom, carbon. the repulsion is minimized by placing the four electron groups at corners of a tetrahedron with bond angles of 109. the shape with four bonds on the central atom is called tetrahedral.

133 Central Atoms with Four Electron Groups In a molecule of NH 3 four electron groups, three bonds to H atoms and one lone pair, are around the central atom, N. the repulsion is minimized by placing the four electron groups at corners of a tetrahedron with bond angles of 109. the shape with three bonds and a lone pair on the central atom is called trigonal pyrimidal.

134 Central Atoms with Four Electron Groups In a molecule of H 2 O four electron groups, two bonds to H atoms and two lone pairs, are around the central atom, O. the repulsion is minimized by placing the four electron groups at corners of a tetrahedron with bond angles of 109. the shape with two bonds and two lone pairs on the central atom is called bent.

135 Molecular Shapes, Electron-Groups

136 Guide to Predicting Molecular Shape

137 Predict Molecular Shape of H 2 S Predict the shape of a molecule of H 2 S. STEP 1 Draw the Lewis structure. STEP 2 Arrange electron groups around the central atom to minimize repulsion. To minimize repulsion, electron groups have a tetrahedral arrangement.

138 Predict Molecular Shape of H 2 S Predict the shape of a molecule of H 2 S. STEP 3 Use the atoms bonded to the central atom to determine the shape. The central atom S has two bonds and two lone pairs. The shape is bent, 109.

139 Study Check State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. A. PF 3 B. H 2 O C. CCl 4

140 Solution State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. STEP 1 Draw the Lewis structure. A. PF 3 B. H 2 O C. CCl 4 O H H

141 Solution State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. STEP 2 Arrange electron groups around the central atom to minimize repulsion. A. PF 3 : To minimize repulsion, the electron groups would have a tetrahedral arrangement. B. H 2 O: To minimize repulsion, the electron groups would have a tetrahedral arrangement. C. CCl 4 : To minimize repulsion, the electron groups would have a tetrahedral arrangement.

142 Solution State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. STEP 3 Use the atoms bonded to the central atom to determine the molecular shape. A. PF 3 : With three bonds and one lone pair on the central atom, the shape is trigonal pyrimidal. B. H 2 O: With two bonds and two lone pairs on the central atom, the shape is bent. C. CCl 4 : With four bonds on the central atom, the shape is tetrahedral.

143 Polarity of Molecules Nonpolar molecules such as H 2, Cl 2, and O 2 are nonpolar because they contain nonpolar bonds. with polar bonds can be nonpolar if the polar bonds (dipoles) cancel in a symmetrical arrangement, such as in CO 2 and CF 4.

144 Polarity of Molecules Polar molecules such as HCl are polar because one end of the molecule is more negatively charged than the other. the polar bonds in the molecule do not cancel each other. the electrons are shared unequally in the polar covalent bond. Core Chemistry Skill Identifying Polarity of Molecules

145 Polarity of Molecules In a polar molecule such as H 2 O, there are two lone pairs and two bonds around the central atom. are dipoles that do not cancel since the shape is bent, making the molecule positive at one end and negative at the other end. is a partial negative charge on the central atom.

146 Polarity of Molecules In a polar molecule such as NH 3, there is one lone pair and three bonds around the central atom. are dipoles that do not cancel since the shape is trigonal pyramidal, making the molecule positive at one end and negative at the other end. is a partial negative charge on the central atom.

147 Guide to Determining the Polarity of a Molecule

148 Determination of Polarity Determine if the molecule OF 2 is polar or nonpolar. STEP 1 Determine if the bonds are polar covalent or nonpolar covalent. O (3.5) and F (4.0) give an electronegativity difference of 0.5, which makes the bonds polar covalent. STEP 2 If the bonds are polar covalent, draw the Lewis structure and determine if the dipoles cancel. OF 2 is a polar molecule. F O F

149 Study Check Identify each of the following molecules as polar or nonpolar. A. PBr 3 B. HBr C. CH 4

150 Solution Identify each of the following molecules as polar or nonpolar. STEP 1 STEP 2 Bonds, polar or Draw molecule to see if nonpolar any polar bonds cancel A. PBr 3 P = 2.1 Br = 2.8, polar polar P B. HBr H = 2.1 Br = 2.8, polar polar Br H Br Br Br C. CH 4 C = 2.5 H = 2.1, nonpolar nonpolar H H C H H

151 6.9 Attractive Forces in Compounds The protein shape is stabilized by attractive forces between functional groups of side chains (R groups) on the amino acids, causing it to twist and bend into a specific three-dimensional shape. Learning Goal Describe the attractive forces between ions, polar covalent molecules, and nonpolar covalent molecules.

152 Dipole-Dipole, Hydrogen Bonds In covalent compounds, polar molecules exert attractive forces called dipole-dipole attractions. form strong dipole attractions called hydrogen bonds between hydrogen atoms bonded to F, O, or N, and a lone pair on F, O, or N. Hydrogen bonds are the strongest force between molecules and play a major role in the shape of DNA. Core Chemistry Skill Identifying Attractive Forces

153 Dispersion Forces Dispersion forces are weak attractions between nonpolar molecules. caused by temporary dipoles that develop when molecules bump into each other. weak but make it possible for nonpolar molecules to form liquids and solids.

154 Melting Points and Attractive Forces Melting points of compounds are related to the strength of attractive forces between molecules or compounds. are lower due to weak forces such as dispersion forces. are higher due to stronger attractive forces such as hydrogen bonding. are highest in ionic compounds due to the strong attractive forces between ions in the compound.

155 Melting Points and Attractive Forces

156 Comparison of Bonding and Attractive Forces

157 Study Check Identify the main type of attractive forces that are present in liquids of the following compounds: ionic bonds, dipole-dipole, hydrogen bonds, or dispersion forces. A. NCl 3 B. H 2 O C. Br-Br D. KCl E. NH 3

158 Chemistry Link to Health: Attractive Forces, Proteins Proteins are biological molecules with many different functions. They are needed for structural components such as cartilage, muscles, hair. the formation of enzymes that regulate biological reactions. myoglobin and hemoglobin, which transport oxygen in blood and muscle.

159 Chemistry Link to Health: Attractive Forces, Proteins Proteins are composed of building blocks called amino acids. Every amino acid has a central carbon atom bonded to an NH 3+, from an amine. a COO, from a carboxylic acid. an H atom. an R group, which is unique for each amino acid.

160 Chemistry Link to Health: Attractive Forces, Proteins Several amino acids have R groups or side chains that contain an amide group, CONH 2. a hydroxyl, OH, group. a carboxyl, COOH, group ionized as carboxylate, COO. an amine, NH 2, group ionized as ammonium, NH 3+.

161 Chemistry Link to Health: Attractive Forces, Proteins The primary structure of a protein is its sequence of amino acids. It is the sequence of amino acids that determines the protein s function. Amino acids in a protein are linked by a peptide bond between the COO of one amino acid and the NH 3 + of the next amino acid.

162 Chemistry Link to Health: Attractive Forces, Proteins Proteins have a higher level of structure that is determined by the attractive forces between the amino acids. When hydrogen bonds form between a hydrogen atom in an N H group and the oxygen of the C O group, the protein forms an alpha helix. H-bonds can form between the OH of serine and the NH 2 of asparagine.

163 Chemistry Link to Health: Attractive Forces, Proteins Hydrogen bonds can also form between the polar side chains of the amino acids on the outside of the protein and the OH and H of polar water molecules in the external aqueous environment. Attractive forces hold the protein in a specific shape.

164 Concept Map