CHAPTER 3: ANSWERS TO SELECTED PROBLEMS. F Se F

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1 APTER 3: ANSWERS T SELETED PRBLEMS SAMPLE PRBLEMS ( Try it yourself ) : l.. : I.. : : l.. : 3.3 Selenium needs two more electrons to satisfy the octet rule, and fluorine needs one. The simplest combination that satisfies the octet rule is one Se atom bonded to two atoms, to form a molecule of Se 2. The Lewis structure of this molecule is: Se 3.4 P 3.5 l S l In 3 l, carbon must make four bonds, while fluorine and chlorine each make one bond, based on their numbers of valence electrons. Therefore, carbon is in the center of the molecule, surrounded by the four other atoms. The structure of this molecule is: l 3.8 Structure a is the only one that has the normal numbers of bonds for each atom (three for nitrogen, four for carbon, and one for each hydrogen) and has both carbon and nitrogen satisfying the octet rule, so structure b is the only reasonable structure. 3.9 There is only one possible structure, but there are several ways to draw it. The correct answer has the carbon atom bonded to three hydrogens and the oxygen, with the remaining hydrogen bonded to the oxygen.

2 3.10 hlorine has the stronger attraction for electrons The electronegativity of nitrogen is higher than that of carbon, so the carbon atom is positively charged and the nitrogen atom is negatively charged a) phosphorus pentafluoride b) disulfur dichloride 3.13 S alcium has two valance electrons and sulfur has six. These two atoms can satisfy the octet rule by transferring two electrons from calcium to sulfur. The calcium atom becomes an calcium ion (a 2+ ), and the sulfur atom become a sulfide ion (S 2 ) a S a S 3.15 Selenium is in Group 6A, so it will gain two electrons to form an ion with a 2 charge. The symbol for this ion is Se Nitrogen needs three additional electrons to satisfy the octet rule, but lithium only has one valence electron. Therefore, we need three lithium atoms to supply the three valence electrons that nitrogen needs. The lithium atoms become Li + ions and the nitrogen atom becomes a N 3 ion. These ions then combine to form an ionic compound, Li 3 N. Li Li + Li N Li N + 3- Li 3 N Li Li We need two Na + ions to balance the charge on S 2, so the chemical formula is Na 2 S The correct formula is Mg 3 N us and u 2 S. Remember that u makes two ions, u 2+ and u +, but sulfur can only make S Sodium oxide 3.21 K 2 and Al 2 S hromium(ii) chloride or chromous chloride

3 3.23 o e() Magnesium phosphate 3.26 us a) Ionic, because g is a metal and l is a nonmetal. b) Molecular, because all of the elements in this compound are nonmetals e 2 3 is an ionic compound and is called iron(iii) oxide. N 2 3 is a molecular compound and is called dinitrogen trioxide. END SETIN PRBLEMS Section a) Br does not satisfy the octet rule. b) Kr satisfies the octet rule 3.2 a) Br b) P c) I I 3.3 Silicon has four valence electrons, so it has four empty spaces in its valence shell. As a result, silicon normally forms four covalent bonds, allowing it to complete its octet. 3.4 Any of the group 6A elements: oxygen, sulfur, selenium, tellurium, or polonium. Section a) l N b) c) Section The silicon atom is at the center, because it must make four covalent bonds. The other atoms only form one covalent bond apiece. l 3.7 a) l P l b)

4 3.8 a) N N b) S c) N 3.9 Structures a and b are reasonable, while c and d are not. In structure c, the left-hand carbon atom and the neighboring oxygen atom have the wrong numbers of bonds, and in structure d, two of the three oxygen atoms have the wrong numbers of bonds. Section rom Table 3.2, we can see that fluorine has the highest electronegativity in Group 7A and astatine (At) has the lowest, so fluorine has the strongest attraction for electrons and astatine has the weakest a) Nitrogen has a lower electronegativity than oxygen, so nitrogen is positively charged. b) ydrogen has a lower electronegativity than chlorine, so hydrogen is positively charged. c) The two nitrogen atoms have equal attractions for electrons, so neither atom has a charge hlorine has a higher electronegativity than carbon, so the chlorine atom is negatively charged. Section a) iodine pentafluoride b) nitrogen monoxide c) disulfur tetrafluoride 3.14 a) PI 3 b) Brl c) N 2 5 Section alcium has the following electron arrangement: shell 1: 2 electrons shell 2: 8 electrons shell 3: 8 electrons shell 4: 2 electrons If calcium loses the two electrons from shell 4, it becomes an ion with the following arrangement: shell 1: 2 electrons shell 2: 8 electrons shell 3: 8 electrons The atom now has eight electrons in its outermost occupied shell, so it satisfies the octet rule Sulfur has the following electron arrangement: shell 1: 2 electrons shell 2: 8 electrons shell 3: 6 electrons If sulfur gains two additional electrons, it becomes an ion with the following arrangement: shell 1: 2 electrons shell 2: 8 electrons shell 3: 8 electrons The atom now has eight electrons in its outermost occupied shell, so it satisfies the octet rule.

5 3.17 The calcium atom becomes an ion with a +2 charge, and the sulfur atom becomes an ion with a 2 charge Both of these ions have the following electron arrangement: shell 1: 2 electrons shell 2: 8 electrons Since each ion has eight electrons in its outermost shell, both ions satisfy the octet rule a) Potassium loses electrons, with each atom losing one electron. b) Sulfur gains electrons, with each atom gaining two electrons c) The potassium atoms have a +1 charge and the sulfur atoms have a 2 charge. d) We need two potassium atoms to supply the two electrons needed by sulfur, so the formula of the compound is K 2 S. Section a) Na 2 Se b) Sr 2 c) ri 3 d) Mg 3 N a) KBr b) Znl 2 c) Al 2 S 3 d) o and o a) The charge on the copper ion is +2. b) The charge on the copper ion is +1. Section a) potassium oxide b) magnesium sulfide c) copper(ii) chloride or cupric chloride d) chromium(iii) oxide or chromic oxide 3.24 a) Na b) ai 2 c) MnS d) el 3 e) Zn Section a) Zn() 2 b) Ag 2 S 4 c) K 3 P 4 d) N 4 Br 3.26 a) a 3 b) Mg 3 (P 4 ) 2 c) r() 3 d) o(n 3 ) a) potassium hydrogen carbonate (or potassium bicarbonate) b) sodium carbonate c) ammonium sulfate Section I Br Na Br In IBr, the two atoms form a covalent bond by sharing a pair of electrons. NaBr contains two ions that are attracted to one another. The atoms do not share electrons.

6 3.29 a) ionic b) molecular c) ionic d) molecular 3.30 a) sulfur dichloride b) magnesium chloride UMULATIVE PRBLEMS (dd-numbered problems only) 3.31 Neon (Ne) has eight valence electrons and satisfies the octet rule elium does not have eight valence electrons. It is placed in Group 8A because its chemical properties are similar to those of the elements that have eight valence electrons, and different from the other elements that have two valence electrons hoice b is the most reasonable arrangement of atoms. xygen normally makes two covalent bonds, while hydrogen and chlorine make only one, so oxygen should be the central atom a) S b) I S I c) As d) I e) S f) Br Br I g) N 3.39 A magnesium atom has only two valence electrons, so it can only form two bonds. Each bond contributes one additional electron to the atom, so the atom ends up with four electrons in its outer shell. As a result, Mg cannot satisfy the octet rule by forming covalent bonds N

7 3.43 Structure #1 is reasonable, because each carbon atom forms four covalent bonds There are two other structures that obey the normal bonding requirements of carbon and hydrogen, and satisfy the octet rule. and Structures such as the ones below are equivalent to structure #1 in the problems Z forms two covalent bonds, so an individual atom of element Z must have six valence electrons. The only element in Period 3 that has six valence electrons is sulfur. X forms one covalent bond, so each atom of X must have seven valence electrons. The only element in Period 3 that has seven valence electrons is chlorine. Therefore, X is chlorine and Z is sulfur a) carbon b) nitrogen c) There is no positively charged atom a) 4 b) S a) chlorine trifluoride b) dinitrogen tetrafluoride c) carbon monoxide 3.55 Aluminum has the following electron arrangement: shell 1: 2 electrons shell 2: 8 electrons shell 3: 3 electrons When aluminum loses the three electrons from shell 3, it becomes an ion with a +3 charge and the following arrangement: shell 1: 2 electrons shell 2: 8 electrons The atom now has eight electrons in its outermost occupied shell, so it satisfies the octet rule a) +2 b) 1 c) 2 d) The Group 7A elements form ions that have a 1 charge Any element in Group 2A forms a +2 ion: Be, Mg, a, Sr, Ba and Ra alcium has two valence electrons and oxygen has six. These atoms can share no more than four electrons, because calcium can only contribute two electrons to the bond. If the atoms share four electrons, oxygen satisfies the octet rule, but calcium does not, as shown below. a + a

8 3.65 ormulas b and c obey the rule of charge balance, while a and d do not a) MgBr 2 b) o 3 c) (N 4 ) 2 S d) Ag 2 S a) magnesium bromide b) cobalt(iii) fluoride or cobaltic fluoride c) ammonium sulfide d) silver sulfate 3.71 Each sodium ion has a +1 charge, so the total positive charge in this compound is +2. The compound must satisfy the rule of charge balance, so the chromate ion must have a 2 charge a) +2 b) +4 c) u 2 3 copper(i) carbonate (contains u + ) u 3 copper(ii) carbonate (contains u 2+ ) 3.77 a) ZnBr 2 b) Na 3 N c) Ag 3 P 4 d) Ni() 2 e) K 3 f) r(n 3 ) 3 g) u 2 h) N 4 I 3.79 a) al 2 is an ionic compound and must be named using the rules for ionic compounds. The correct name is calcium chloride. b) e can form either a +2 or a +3 ion, and the name of the compound must tell which ion is present. The correct name is iron(ii) oxide. (errous oxide is also correct.) c) The N 3 group is a polyatomic ion, N 3. This ion is called nitrate, so the correct name of the compound is sodium nitrate a) ionic b) molecular c) molecular d) ionic 3.83 a) calcium oxide b) carbon monoxide c) nitrogen trifluoride d) iron(iii) fluoride or ferric fluoride 3.85 Nal contains Na + and l ions, which attract one another because they have opposite charges. The sodium and chlorine atoms do not share electrons in Nal. In l, the two atoms do share electrons. Na + l _ l (Nal: no electron sharing) (l: electron sharing) 3.87 Ions that the same charge (both positive or both negative) repel one another, so Na + and K + cannot come together to form a stable compound. 3.89