# Chapter 2: Atoms and Molecules

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1 hapter 2: Atoms and Molecules HAPTER OUTLINE 2.1 Symbols and Formulas 2.2 Inside the Atom 2. Isotopes 2.4 Relative Masses of Atoms and Molecules 2.5 Isotopes and Atomic Weights 2.6 Avogadro s Number: The Mole 2.7 The Mole and hemical Formulas LEARNING OJETIVES/ASSESSMENT When you have completed your study of this chapter, you should be able to: 1. Use symbols for chemical elements to write formulas for chemical compounds. (Section 2.1; Exercise 2.4) 2. Identify the characteristics of protons, neutrons, and electrons. (Section 2.2; Exercises 2.10 and 2.12). Use the concepts of atomic number and mass number to determine the number of subatomic particles in isotopes and to write correct symbols for isotopes. (Section 2.; Exercises 2.16 and 2.22) 4. Use atomic weights of the elements to calculate molecular weights of compounds. (Section 2.4; Exercise 2.2) 5. Use isotope percent abundances and masses to calculate atomic weights of the elements. (Section 2.5; Exercise 2.8) 6. Use the mole concept to obtain relationships between number of moles, number of grams, and number of atoms for elements, and use those relationships to obtain factors for use in factor unit calculations. (Section 2.6; Exercises 2.44 a & b and 2.46 a & b) 7. Use the mole concept and molecular formulas to obtain relationships between number of moles, number of grams, and number of atoms or molecules for compounds, and use those relationships to obtain factors for use in factor unit calculations. (Section 2.7; Exercise 2.50 b and 2.52 b) LETURE HINTS AND SUGGESTIONS 1. The word ʺelementʺ has two usages: (1) a homoatomic, pure substance; and (2) a kind of atom. This dual usage confuses the beginning student. It often helps the beginning student for the instructor to distinguish the usage intended in a particular statement. e.g. ʺThere are 112 elements, meaning 112 kinds of atoms.ʺ or ʺEach kind of atom (element) has a name and a symbol.ʺ or ʺWater contains the element (kind of atom) oxygen.ʺ 2. Emphasize that the term ʺmoleculeʺ can mean: (1) the limit of physical subdivision of a molecular compound; (2) the smallest piece of a molecular compound; or () the basic building block of which a molecular compound is made. Do not try to differentiate at this time the differences between ionic solids, molecular compounds, or network solids.. Many students fail to make a connection that a given pure substance has only one kind of constituent particle present; i.e., pure water contains only one kind of molecule, the water molecule. The molecule of water is made up of atoms of hydrogen and oxygen, but there are no molecules of hydrogen or oxygen in pure water. 4. The student will memorize the names and symbols for approximately one third of the 112 elements to be dealt with those commonly encountered in this course or in daily living. Mentioning both the name and the symbol whenever an element is mentioned in the lecture will aid the studentʹs memorizing. 5. While memorization of the names and symbols is important, it should not become the major outcome of this class. Avoid reinforcing the mistaken notion that chemistry is merely learning formulas and equations. 29

2 0 hapter 2 6. It should be emphasized that the mole is a convenient way of measuring out needed numbers of atoms and molecules In the correct ratios for chemical reactions. Explain that the term ʺmoleʺ is the same type of term as ʺdozen,ʺ ʺpair,ʺ or ʺgross,ʺ except that it specifies a much larger number of items. SOLUTIONS FOR THE END OF HAPTER EXERISES SYMOLS AND FORMULAS (SETION 2.1) 2.1 a. A diatomic molecule of an element* b. A diatomic molecule of a compound* c. A triatomic molecule of an element d. A molecule of a compound containing one atom of one element and four atoms of another element *Note: Each of these structures could be drawn in many different ways. 2.2 a. A triatomic molecule of a compound* b. A molecule of a compound containing two atoms of one element and two atoms of a second element* c. A molecule of a compound containing two atoms of one element, one atom of a second element, and four atoms of a third element* d. A molecule containing two atoms of one element, six atoms of a second element, and one atom of a third element* *Note: Each of these structures could be drawn in many different ways. 2. a. A diatomic molecule of fluorine (two fluorine atoms) F2; like Exercise 2.1 a b. A diatomic molecule of hydrogen chloride (one hydrogen Hl; like Exercise 2.1 b atom and one chlorine atom) c. A triatomic molecule of ozone (three oxygen atoms) O; like Exercise 2.1 c* d. A molecule of methane (one carbon atom and four H4; like Exercise 2.1 d* hydrogen atoms) *The number and variety of atoms are alike. The actual structures of the molecules are different. 2.4 a. A molecule of water (two hydrogen atoms and one oxygen H2O; like Exercise 2.2 a* atom) b. A molecule of hydrogen peroxide (two hydrogen atoms and H2O2; like Exercise 2.2 b* two oxygen atoms) *The number and variety of atoms are alike. The actual structures of the molecules are different.

3 Atoms and Molecules 1 c. A molecule of sulfuric acid (two hydrogen atoms, one sulfur H2SO4; like Exercise 2.2 c* atom, and four oxygen atoms) d. A molecule of ethyl alcohol (two carbon atoms, six 2H6O; like Exercise 2.2 d* hydrogen atoms, and one oxygen atom) *The number and variety of atoms are alike. The actual structures of the molecules are different. 2.5 a. nitrous acid (HNO2) 1 hydrogen atom; 1 nitrogen atom; 2 oxygen atoms b. chlorine dioxide (lo2) 1 chlorine atom; 2 oxygen atoms c. ethyl alcohol (2H6O) 2 carbon atoms; 6 hydrogen atoms; 1 oxygen atom d. chloroform (Hl) 1 carbon atom; 1 hydrogen atom; chlorine atoms 2.6 a. sulfur dioxide (SO2) 1 sulfur atom; 2 oxygen atoms b. butane (4H10) 4 carbon atoms; 10 hydrogen atoms c. chlorous acid (HlO2) 1 hydrogen atom; 1 chlorine atom; 2 oxygen atoms d. boron trifluoride (F) 1 boron atom; fluorine atoms 2.7 a. HPO (phosphorous acid) The numbers should be subscripted: HPO b. SIl4 (silicon tetrachloride) The elemental symbol for silicon is Si: Sil4 c. SOO (sulfur dioxide) Only one O should be written and a subscript 2 should be added: SO2 d. 2HO (hydrogen peroxide two hydrogen atoms and two oxygen atoms) The number 2 should be a subscript after H and after O: H2O2 2.8 a. HSH (hydrogen sulfide) More than one H is part of the compound; a subscript should be used: H2S b. HLO2 (chlorous acid) The elemental symbol for chlorine is l (the second letter of a symbol must be lowercase): HlO2 c. 2HN2 (hydrazine two hydrogen atoms and four nitrogen atoms) The subscripts should reflect the actual number of each type of atom in the compound: H2N4 d. 2H6 (ethane) The numbers should be subscripted: 2H6 INSIDE THE ATOM (SETION 2.2) 2.9 harge Mass (u) a. protons and 4 neutrons 7 b. 10 protons and 12 neutrons c. 5 protons and 46 neutrons 5 81 d. 56 protons and 81 neutrons harge Mass (u) a. 5 protons and 6 neutrons 5 11 b. 11 protons and 10 neutrons c. 6 protons and 50 neutrons 6 86 d. 50 protons and 68 neutrons The number of protons and electrons are equal in a neutral atom. a. electrons b. 10 electrons c. 5 electrons d. 56 electrons

4 2 hapter The number of protons and electrons are equal in a neutral atom. a. 5 electrons b. 11 electrons c. 6 electrons d. 50 electrons ISOTOPES (SETION 2.) 2.1 Electrons Protons a. sulfur b. As c. element number Electrons Protons a. silicon b. Sn c. element number Protons Neutrons Electrons a. Mg b c. 19 K Protons Neutrons Electrons 7 a b. 10 Ne c. 20 a a. cadmium 110 d b. cobalt o c. uranium a. silicon U 28 Si 14 b. argon Ar c. strontium Sr 2.19 Mass Number Atomic Number Symbol 7 a. protons and 4 neutrons 7 Li 22 b. 10 protons and 12 neutrons Ne 81 c. 5 protons and 46 neutrons r d. 56 protons and 81 neutrons a 2.20 Mass Number Atomic Number Symbol 11 a. 5 protons and 6 neutrons b. 11 protons and 10 neutrons Na c. 6 protons and 50 neutrons Kr 6 d. 50 protons and 68 neutrons Sn

5 Atoms and Molecules 2.21 a. contains 15 electrons and 16 neutrons 1 15P b. a radon atom with a mass number of Rn c. an oxygen atom that contains 10 neutrons 18 8O a. contains 17 electrons and 20 neutrons 17 l b. a copper atom with a mass number of u 29 c. a zinc atom that contains 6 neutrons 66 0Zn RELATIVE MASSES OF ATOMS AND MOLEULES (SETION 2.4) Two element pairs whose average atoms have masses that are within 0. u of each other are 2. argon (Ar 9.95 u) and calcium (40.08 u) as well as cobalt (o 58.9u) and nickel (Ni 58.69u) atomHe 12 u = atoms He 4uHe 1atomLi 14 u = 2 atoms Li 7uLi % u = u = 40.1 u; a; calcium In the first 6 elements, the elements with atoms whose average mass is within 0.2 u of being twice the atomic number of the element are: Atom Atomic Number Relative Mass Ratio helium (He) carbon () nitrogen (N) oxygen (O) neon (Ne) silicon (Si) sulfur (S) calcium (a) u = 14.05u;N;nitrogen a. oxygen (O2) ( u) = 2.00 u b. carbon monoxide (O) ( u) + ( u) = u c. chloric acid (HlO) ( u) + ( u) + ( u) = u d. glycerine (H8O) ( u) + ( u) + ( u) = u e. sulfur dioxide (SO2) ( u) + ( u) = u 2.0 a. nitrogen dioxide (NO2) ( u) + ( u) = u b. ammonia (NH) ( u) + ( u) = 17.0 u c. glucose (6H12O6) ( u) + ( u) + ( u) = u d. ozone (O) u = u

6 4 hapter 2 e. ethylene glycol (2H6O2) ( u) + ( u) + ( u) = u 2.1 The gas is most likely to be N2O based on the following calculations: NO : ( u) + ( u) = 0.01 u N2O : ( u) + ( u) = u NO : u u = u 2 ( ) ( ) The experimental value for the molecular weight of an oxide of nitrogen was 4.98 u, which is closest to the theoretical value of u, which was calculated for N2O. 2.2 The gas is most likely to be ethylene based on the following calculations: acetylene : ( u) + ( u) = u ethylene : ( u) + ( u) = u ethane : u u = 0.07 u ( ) ( ) The experimental value for the molecular weight of a flammable gas known to contain only carbon and hydrogen is u, which is identical to the theoretical value of u, which was calculated for ethylene. 2. The x in the formula for glycine stands for 5, the number of hydrogen atoms in the chemical formula. ( u) + ( x u) + ( u) + ( u) = u x u u = u x u = 5.04 u x = The y in the formula for serine stands for, the number of carbon atoms in the chemical formula. ( ) ( ) ( ) ( ) y u u u u = u y u u = u y u = 6.0 u y = ISOTOPES AND ATOMI WEIGHTS (SETION 2.5) 2.5 a. The number of neutrons in the nucleus = neutrons b. The mass (in u) of the nucleus (to three significant figures) 9.01 u 2.6 a. The number of neutrons in the nucleus = neutrons b. The mass (in u) of the nucleus (to three 27.0 u significant figures) % u % u = u u = u; u with SF or ( u) + ( u) = u; u with SF 100

7 Atoms and Molecules 5 The atomic weight listed for lithium in the periodic table is u. The two values are the very close % u % u = u u = u; u with SF or ( u) + ( u) = u; u with SF 100 The atomic weight listed for gallium in the periodic table is u. The two values are the same % u % u +.09% u = u u u = u; u with SF or ( u) + ( u) + ( u) = u; u with SF 100 The atomic weight listed for silicon in the periodic table is u. The two values are the same % u % u = u u = u; 6.55 u with SF or ( u) + ( u) = u; 6.55 u with SF 100 The atomic weight listed for copper in the periodic table is 6.55 u. The two values are the same. AVOGADRO S NUMER: THE MOLE (SETION 2.6) atoms g = atoms g atoms P 1.0 g P = 0.1 g P 10 atoms P atoms O g O = 10 atoms O g O atoms F 19.0 g F = 1.90 g F 10 atoms F 2.4 a. potassium 1 mol K atoms = 10 K atoms 10 K atoms = 9.1 g K 1 mol K atoms = 9.1 g K

8 6 hapter 2 b. magnesium 1 mol Mg atoms = 10 Mg atoms 10 Mg atoms = 24. g Mg 1 mol Mg atoms = 24. g Mg c. tin 1 mol Sn atoms = 10 Sn atoms 10 Sn atoms = 119 g Sn 1 mol Sn atoms = 119 g Sn 2.44 a. phosphorus 1 mol P atoms = 10 P atoms 10 P atoms = 1.0 g P 1 mol P atoms = 1.0 g P b. aluminum 1 mol Al atoms = 10 Al atoms 10 Al atoms = 27.0 g Al 1 mol Al atoms = 27.0 g Al c. krypton 1 mol Kr atoms = 10 Kr atoms 10 Kr atoms = 8.8 g Kr 1 mol Kr atoms = 8.8 g Kr 2.45 a. The number of moles of potassium atoms in a 50.0 g sample of potassium b. The number of magnesium atoms in a 1.82 mol sample of magnesium c. The number of tin atoms in a 200 g sample of tin 2.46 a. The mass in grams of one phosphorus atom b. The number of grams of aluminum in 1.65 mol of aluminum 1 mol K atoms 1 mol K atoms = 9.1 g K; 9.1 g K 1 mol K atoms 50.0 g K = 1.28 mol K atoms 9.1 g K 10 Mg atoms 1 mol Mg atoms = 10 Mg atoms; 1 mol Mg atoms 10 Mg atoms mol Mg = Mg atoms 1 mol Mg atoms 10 Sn atoms 10 Sn atoms = 119 g Sn; 119 g Sn 10 Sn atoms g Sn = Sn atoms 119 g Sn 1.0 g P P atoms = 1.0 g P; P atoms 1.0 g P 1atomP = g P 10 P atoms (Note: One atom is assumed to be an exact number.) 27.0 g Al 1 mol Al atoms = 27.0 g Al; 1 mol Al atoms 27.0 g Al 1.65 mol Al = 44.6 g Al 1molAl

9 c. The total mass in grams of onefourth Avogadro s number of krypton atoms Atoms and Molecules g Kr 1 mol Kr atoms = 8.8 g Kr; 1 mol Kr atoms 8.8 g Kr 1 4 mol Kr = g Kr 1mol Kr (Note: One fourth is assumed to be an exact number.) THE MOLE AND HEMIAL FORMULAS (SETION 2.7) 2.47 ( u) + ( 1.01 u) = 4.0 u; 1 mole PH = 4.0 g PH ( u) + ( u) = 64.1 u; 1 mole SO2 = 64.1 g SO2 10 molecules SO g SO 2 = 10 molecules SO g SO molecules PH 2.48 ( ) ( ) ( ) ( ) 4.0 g PH = 10 molecules PH u u = 67.8 u; 1 mole F = 67.8 g F u u = 4.1u; 1moleH S= 4.1gH S 0.4 g H2S g PH 10 molecules H2S 21 = molecules H2S 4.1 g H2S molecules F 67.8 g F = 10 molecules F 0.68 g F 2.49 a. methane (H4) 1. 2 H4 molecules contain 2 atoms and 8 H atoms H4 molecules contain 10 atoms and 40 H atoms H4 molecules contain 100 atoms and 400 H atoms H molecules contain 10 atoms and H atoms mol of H4 molecules contain 1 mole of atoms and 4 moles of H atoms g of methane contains 12.0 g of and 4.04 g of H. b. ammonia (NH) 1. 2 NH molecules contain 2 N atoms and 6 H atoms NH molecules contain 10 N atoms and 0 H atoms NH molecules contain 100 N atoms and 00 H atoms NH molecules contain 10 N atoms and H atoms mol of NH molecules contain 1 mole of N atoms and moles of H atoms g of ammonia contains 14.0 g of N and.0 g of H.

10 8 hapter 2 c. chloroform (Hl) 2.50 a. ethyl ether (4H10O) 1. 2 Hl molecules contain 2 atoms, 2 H atoms,and 6 l atoms Hl molecules contain 10 atoms, 10 H atoms,and 0 l atoms Hl molecules contain 100 atoms, 100 H atoms, and 00 l atoms Hl molecules contain 10 H atoms,and l atoms atoms, 5. 1 mol of Hl molecules contain 1 mole of atoms, 1 mole of H atoms, and moles l atoms g of chloroform contains 12.0 g of, 1.01 g of H, and 106 g of l H O molecules contain 8 atoms, 20 H atoms,and 2 O atoms H O molecules contain 40 atoms, 100 H atoms,and 10 O atoms H O molecules contain 400 atoms, 1000 H atoms, 4 10 and 100 O atoms H10O molecules contain atoms, H atoms,and O atoms mol of H O molecules contain 4 moles of atoms, 10 moles 4 10 of H atoms, and 1 mole O atoms g of ethyl ether contains 48.0 g of, 10.1 g of H, and 16.0 g of O. b. fluoroacetic acid (2HO2F) 1. 2 H O F molecules contain 4 atoms, 6 H atoms, O atoms, and 2 F atoms H O F molecules contain 20 atoms, 0 H atoms, O atoms, and 10 F atoms H O F molecules contain 200 atoms, 00 H atoms, O atoms, and 100 F atoms. 10 H O F molecules contain atoms, H atoms, O atoms, and F atoms mol of H O F molecules contain 2 moles of atoms, moles 2 2 of H atoms, 2 moles of O atoms, and 1 mole of F atoms g of fluoroacetic acid contains 24.0 g of,.0 g of H, 2.0 g of O, and 19.0 g of F. c. aniline (6H7N) 1. 2 H N molecules contain 12 atoms, 14 H atoms,and 2 N atoms H N molecules contain 60 atoms, 70 H atoms,and 10 N atoms H N molecules contain 600 atoms, 700 H atoms, 6 7 and 100 N atoms H7N molecules contain atoms, H atoms,and N atoms mol of H N molecules contain 6 moles of atoms, 7 moles 6 7 of H atoms, and 1 mole N atoms g of aniline contains 72.0 g of, 7.07 g of H, and 14.0 g of N.

11 2.51 a. b. c. Statement 5. 4 Atoms and Molecules 9 1 mol of H molecules contain 1 mole of atoms and 4 moles of H atoms. 4molesHatoms 1moleH4 4molesHatoms 1molH4 = 4molesHatoms 1moleH 4 Statement g of ammonia contains 14.0 g of N and.0 g of H g N 1moleNH 14.0 g N 1.00 mole NH = 14.0 g N 1moleNH Statement g of chloroform contains 12.0 g of, 1.01 g of H, and 106 g of l. 106 g l 119 g Hl 106 g l 100 = 89.1% l in Hl 119 g Hl 2.52 a. Statement 5. 1 mol of 4H10O molecules contain 4 moles of atoms, 10 moles of H atoms, and 1 mole O atoms. 10 moles H atoms 1mole4H10O 10 moles H atoms 0.50 mol 4H10O = 5.0 moles H atoms 1moleH 4 10O b. Statement H O F molecules contain atoms, H atoms, O atoms, and 10 F atoms atoms 1mole2HO2F atoms 0.25 mole 2HO2F =.0 10 atoms 1moleHOF 2 2 c. Statement g of aniline contains 72.0 g of, 7.07 g of H, and 14.0 g of N g H 1mole6H7N 7.07 g H 2.00 mol 6H7N = 14.1 g H 1moleHN moleHOF 2molesatoms 10 atoms = atoms 1moleHOF mole atoms 1 mole atoms 24 1mole4H10O atoms = moles 4H10O 10 atoms 4moles atoms Note: The 1 mol assumed to be an exact number.

12 40 hapter mole O atoms 10 O atoms 0.75 mole H2O = O atoms 1moleHO 2 1moleOatoms 1mole Oatoms 1moleHO g 2H6O O atoms O atoms 1mole Oatoms 1moleHO 2 6 = g H O 5 g with SF g N 14.0 g N 100 = 6.6% N in N O 100 = 0.4% N in NO 44.0 g N O 46.0 g NO gO 2.0gO 100 = 57.1% O in O 100 = 72.7% O in O 28.0 g O 44.0 g O Statement H NO molecules contain atoms, H atoms, N atoms, and O atoms. Statement 5. 1 mol H NO molecules contain 6 moles of atoms, 5 moles of H atoms, 6 5 1moleofNatoms,andmoles of O atoms. Statement g of nitrophenol contains 72.0 g of, 5.05 g of H, 14.0 g of N, and 48.0 g of O. a. Statement g of nitrophenol contains 72.0 g of, 5.05 g of H, 14.0 g of N, and 48.0 g of O. b g N 19 g H NO g N 70.0 g 6H5NO = 7.05 g N 19 g 6H5NO Statement 5. 1 mol H NO molecules contain 6 moles of atoms, 5 moles of H atoms, mole of N atoms, and moles of O atoms. moles of O atoms 1mole6H5NO moles of O atoms 1.50 moles 6H5NO = 4.50 moles of O atoms 1moleHNO 6 5 c. Statement H NO molecules contain atoms, H atoms, N atoms, and O atoms atoms H5NOmolecules molecules H NO atoms = atoms 10 6H5NOmolecules

13 2.58 Statement and O atoms Atoms and Molecules H O molecules contain atoms, H atoms, Statement 5. 1 mol H O molecules contain 6 moles of atoms, 12 moles of H atoms, and 6 moles of O atoms. Statement g of fructose contains 72.0 g of, 12.1 g of H, and 96.0 g of O. a. Statement g of fructose contains 72.0 g of, 12.1 g of H, and 96.0 g of O. b g O 180 g H O g O 4.5 g 6H12O6 =.2 g O 180 g 6H12O 6 Statement 5. 1 mol H O molecules contain 6 moles of atoms, 12 moles of H atoms, and moles of O atoms. 12 moles of H atoms 1mole6H12O moles H O moles of H atoms = 18.0 moles of H atoms 1moleH 6 12O 6 c. Statement H O molecules contain atoms, H atoms, and O atoms atoms 10 6H12O6molecules molecules 6H12O atoms = 10 6H12O6 molecules atoms 2.59 Urea (H4N2O) contains the higher mass percentage of nitrogen as shown in the calculation below: 28.0 g N 28.0 g N 100 = 46.7% N in H4N2O 100 = 21.2% N in N2H8SO g H N O 12 g N H SO Magnetite (FeO4) contains the higher mass percentage of iron as shown in the calculation below: 167 g Fe 112 g Fe 100 = 72.% Fe in FeO4 100 = 70.0% Fe in Fe2O 1 g Fe O 160 g Fe O alcite (ao) contains the higher mass percentage of nitrogen as shown in the calculation below: 40.1 g a 100 = 40.1% a in ao 100 g ao 40.1 g a 100 = 21.8% a in amg2o6 184 g amg O 2 6 ADDITIONAL EXERISES 2.62 U 8 contains more neutrons in its nucleus than U 5. U 8 and U 5 have the same volume because the extra neutrons in U 8 do not change the size of the electron cloud.

14 42 hapter 2 U 8 is u heavier than U 5 because of the extra neutrons. Density is a ratio of mass to volume; therefore, U 8 is more dense than U 5 because it has a larger mass divided by the same volume = % g 1 12atom 14 protons + neutrons g = 1 12 atom 12 protons + neutrons 1 14atom 1 14atom ( ) ( ) D O : 2 2 u u = 20 u 2.66 In Figure 2.2, the electrons are much closer to the nucleus than they would be in a properly scaled drawing. onsequently, the volume of the atom represented in Figure 2.2 is much less than it should be. Density is calculated as a ratio of mass to volume. The mass of this atom has not changed; however, the volume has decreased. Therefore, the atom in Figure 2.2 is much more dense than an atom that is % empty. ALLIED HEALTH EXAM ONNETION 2.67 The symbol K on the periodic table stands for (a) potassium (b) Water is a chemical compound. (a) lood and (d) air are mixtures, while (c) oxygen is an element (c) ompounds are pure substances that are composed of two or more elements in a fixed proportion. ompounds can be broken down chemically to produce their constituent elements or other compounds l has (a) 17 protons, 17 neutrons (4 17=17), and 17 electrons (electrons = protons in neutral atom) If two atoms are isotopes, they will (c) have the same number of protons, but different numbers of neutrons opper has (b) 29 protons because the atomic number is the number of protons. 2.7 Atoms are electrically neutral. This means that an atom will contain (c) an equal number of protons and electrons The negative charged particle found within the atom is the (b) electron Two atoms, L and M are isotopes; therefore, they would not have (b) atomic weight in common The major portion of an atom s mass consists of (a) neutrons and protons The mass of an atom is almost entirely contributed by its (a) nucleus.

15 Atoms and Molecules (d) 2 16S has 16 protons, 17 neutrons, and 18 electrons An atom with an atomic number of 58 and an atomic mass of 118 has (c) 60 neutrons The mass number of an atom with 60 protons, 60 electrons, and 75 neutrons is (b) Avogadro s number is (c) x (c) 1.0 mol NO2 has the greatest number of atoms (1.8 x atoms). 1.0 mol N has 6.0 x 10 atoms, 1.0 g N has 4. x atoms, and 0.5 mol NH has 1.2 x atoms. 2.8 A sample of 11 grams of O2 contains (c).0 grams of carbon g 11 g O 2 =.0 g 44.0 g O The molar mass of calcium oxide, ao, is (a) 56 g (40 g a + 16 g O) The mass of mol of calcium phosphate is (b) 62.0 g. 10 g a( PO ) mol a( PO4) 2 2 = 62.0 g a PO 1mola( PO4) (b) 2.0 moles Al are contained in a 54.0 g sample of Al. 1moleAl 54.0 g Al = 2.00 mole Al 27.0 g Al ( ) 4 2 HEMISTRY FOR THOUGHT 2.87 a. Atoms of different elements contain different numbers of protons. b. Atoms of different isotopes contain different numbers of neutrons, but the same number of protons Aluminum exists as one isotope; therefore, all atoms have the same number of protons and neutrons as well as the same mass. Nickel exists as several isotopes; therefore, the individual atoms do not have the weighted average atomic mass of u g g = oranges orange None of the oranges in the bowl is likely to have the exact mass calculated as an average. Some oranges will weigh more than the average and some will weigh less dry bean mass 1 = jelly bean mass gdrybeans 472 g jelly beans = 295 g dry beans 1.60 g jelly beans

16 44 hapter 2 1jellybean 472 g jelly beans = 400 jelly beans Each jar contains 400 beans g jelly bean molSatoms 1.5 mol S 2 =.0 mol S atoms 1molS 2 10 S2 molecules mol S = S2 molecules 2molS 2.92 If the atomic mass unit were redefined as being equal to 1/24 th the mass of a carbon 12 atom, then the atomic weight of a carbon 12 atom would be 24 u. hanging the definition for an atomic mass unit does not change the relative mass ratio of carbon to magnesium. Magnesium atoms are approximately times as heavy as carbon 12 atoms; therefore, the atomic weight of magnesium would be approximately 48.6 u. 2.9 The ratio of the atomic weight of magnesium divided by the atomic weight of hydrogen would not change, even if the atomic mass unit was redefined The value of Avogadro s number would not change even if the atomic mass unit were redefined. Avogadro s number is the number of particles in one mole and has a constant value of x 10. EXAM QUESTIONS MULTIPLE HOIE 1. Why is ao the symbol for calcium oxide instead of AO? a. They both can be the symbols for calcium oxide. b. They are both incorrect as the symbol should be cao. c. A capital letter means a new symbol. d. They are both incorrect as the symbol should be aox. 2. What is the meaning of the two in ethyl alcohol, 2H5OH? a. All alcohol molecules contain two carbon atoms. b. There are two carbon atoms per molecule of ethyl alcohol. c. arbon is diatomic. d. All of these are correct statements.. The symbols for elements with accepted names: a. consist of a single capital letter. b. consist of a capital letter and a small letter. c. consist of either a single capital letter or a capital letter and a small letter. d. no answer is correct

17 4. A molecular formula: a. is represented using the symbols of the elements in the formula. b. is represented using a system of circles that contain different symbols. c. cannot be represented conveniently using symbols for the elements. d. is represented using words rather than symbols. A Atoms and Molecules Which of the following uses the unit of ʺuʺ? a. atomic weights of atoms c. molecular weights of molecules b. relative masses of atoms d. more than one response is correct D 6. What is meant by carbon 12? a. The carbon atom has a relative mass of approximately 12 grams. b. The carbon atom has a relative mass of approximately 12 pounds. c. The carbon atom has a relative mass of approximately 12 amu. d. The melting point of carbon is Refer to a periodic table and tell how many helium atoms (He) would be needed to get close to the same mass as an average oxygen atom (O). a. six b. four c. twelve d. one fourth 8. Determine the molecular weight of hydrogen peroxide, H2O2 in u. a b c d Using whole numbers, determine the molecular weight of calcium hydroxide, a(oh)2. a. 56 b. 57 c. 58 d. 74 D 10. The average relative mass of an ozone molecule is 48.0 u. An ozone molecule contains only oxygen atoms. What does this molecular weight indicate about the formula of the ozone molecule? a. It contains a single oxygen atom. b. It contains two oxygen atoms. c. It contains three oxygen atoms. d. The data tell nothing about the formula of an ozone molecule. 11. Which of the following pairs are about equal in mass? a. proton and electron c. proton and neutron b. electron and neutron d. nucleus and surrounding electron 12. Which of the following particles is the smallest? a. proton b. electron c. neutron d. they are all the same size

18 46 hapter 2 1. How many electrons are in a neutral atom of carbon 1, 1? a. 6 b. 18 c. 12 d. no way to tell A 14. Which of the following carries a negative charge? a. a proton c. an electron b. a neutron d. both proton and neutron 15. Which of the following is located in the nucleus of an atom? a. protons c. electrons b. neutrons d. protons and neutrons D 16. Atoms are neutral. How can they have no charge? a. equal numbers of protons and neutrons b. equal numbers of protons and electrons c. equal numbers of neutrons and electrons d. any charge has been drained out of the atom 17. Isotopes differ from each other in what way? a. They have different numbers of protons in the nucleus. b. They have different numbers of neutrons in the nucleus. c. They have different numbers of electrons outside the nucleus. d. More than one response is correct 18. In what way is U 8 different from U 5? a. three more electrons c. three more neutrons b. three more protons d. there is no difference 19. How many protons are found in the nucleus of a boron 11 () atom? a. 11 b. 6 c. 5 d How many neutrons are found in the nucleus of a boron 11 () atom? a. 11 b. 6 c. 5 d What is the mass number of a carbon 1 () atom? a. 1 b. 12 c. 6 d. 7 A

19 Atoms and Molecules Naturally occurring neon (Ne) has the following isotopic composition (the mass of each isotope is given in parenthesis). alculate the atomic weight of neon in u from these data. neon 20, 90.92% (19.99 u); neon 21, 0.257% (20.99 u); neon 22, 8.82% (21.99 u) a b c d D. Naturally occurring lithium (Li) consists of only two isotopes, Li 6 (6.02 u) and Li 7 (7.02 u), where the isotopic masses are given in parentheses. Use the periodic table and determine which isotope is present in the larger percentage in the natural element. a. Li 6 b. Li 7 c. each is present at 50% d. cannot be determined from the information available 24. What mass of arsenic (As) in grams contains the same number of atoms as 9.95 g of argon (Ar)? a..0 b c d Which is greater: the number of r atoms in a 26.0 g sample of chromium or the number of Al atoms in a g sample of aluminum? a. The number of r atoms is greater than the number of Al atoms. b. The number of Al atoms is greater than the number of Al atoms. c. The number of r atoms and Al atoms are the same. d. The number of r atoms and Al atoms cannot be determined from the provided data. 26. The mass of mercury (Hg), a liquid at room temperature, is g/mol. A gram sample of mercury is heated until it boils. What is the mass of one mole of mercury vapor (gas)? a. less than or it would not be a gas b. the same as Avogadroʹs number c. the same as when it is a liquid d. none of the answers is correct 27. The formula for dinitrogen monoxide is N2O. If a sample of the oxide was found to contain g of oxygen, how many grams of nitrogen would it contain? a b c d A 28. Avogadroʹs number of iron (Fe) atoms would weigh a g b g c x 10 g d x 10 g A 29. How many atoms are contained in a sample of krypton, Kr, that weighs 8.8 g? a. Avogadroʹs number c. one b. one tenth Avogadroʹs number d. one tenth

20 48 hapter 2 0. Which of the following has the largest mass? a. 5.0 mol H2O b..5 mol NH c. 8.0 mol d. 6.0 mol 2H2 D 1. How many silicon atoms (Si) are contained in a 12.5 g sample of silicon? a x 10 b. 5.8 x c. 1.5 x d x A 2. What is the number of hydrogen atoms in a gram sample of water? a b x 10 c d x D. How many moles of oxygen atoms are in one mole of O2? a. 1 b. 2 c x 10 d x How many hydrogen atoms are in 1.00 mole of NH? a..00 b x 10 c x 10 d x 10 D 5. How many moles of hydrogen molecules (H2) contain the same number of hydrogen atoms as two moles of hydrogen peroxide (H2O2)? a. 1 c. b. 2 d alculate the weight percentage of hydrogen in water, rounded to significant figures. a.. b c d D 7. What is the weight percentage of nitrogen in urea, N2H4O, rounded to significant figures? a b. 0.4 c. 2.6 d. 16. A 8. How many carbon atoms are contained in 5.50 g of ethane, 2H6? a x b..29 x c x 10 d x 10 D 9. Which element is approximately 65 percent of sulfuric acid (H2SO4) by weight? a. hydrogen b. sulfur c. oxygen d. any of these 40. How many moles of N2O contain the same number of nitrogen atoms as 4.60 g of NO2? a b c d How many grams of iron (Fe) are contained in 15.8 g of Fe(OH)? a b c d. 5.21

21 Atoms and Molecules What is the symbol for bromine? a. b. r c. e d. none of these 4. What is the weight percent of sulfur in K2SO4, rounded to significant figures? a. 14.2% b. 18.4% c. 54.4% d. 22.4% 44. What is the number of moles of water in one liter of water if one gram of water takes up one milliliter of space? a. 1 b. 18 c d How many neutrons are in an atom that has a mass number of 75 and contains 5 protons? a. 40 b. 5 c. 75 d. no way to know A 46. Atoms that have the same atomic number but differ by mass number are called? a. protons b. neutrons c. isotopes d. positrons 47. If you have.011x10 atoms of carbon, what would you expect their combined mass to be? a g b g c..00 g d g 48. What is wrong with the following molecular formula: SOO (sulfur dioxide) a. OSO is the correct form c. OO should be written as O2 b. SO should be So d. OO should be written as O2 D 49. Determine the number of electrons and protons in element 4, technetium, Tc. a. 4 protons, 4 electrons c. 56 protons, 4 electrons b. 4 protons, 56 electrons d. 99 protons, 4 electrons A 50. Upon which of the following is the system of atomic mass units based? a. Assigning 12 as weighing exactly 12 u and comparing other elements to it. b. Measuring the true mass of each subatomic particle. c. omparing the differences in protons and electrons. d. Viewing how atoms are affected by electromagnetic fields. A TRUE FALSE 1. The symbols for all of the elements are derived from the Latin names. F 2. The symbols for all of the elements always begin with a capital letter. T

22 50 hapter 2. The first letter of the symbol for each of the elements is the first letter of its English name. F 4. The most accurate way to determine atomic mass is with a mass spectrometer. T 5. H2O2 contains equal parts by weight of hydrogen and oxygen. F 6. Electrons do not make an important contribution to the mass of an atom. T 7. The charge of the nucleus depends only on the atomic number. T 8. Isotopes of the same element always have the same number of neutrons. F 9. Isotopes of the same element always have the same atomic number. T 10. Isotopes of the same element always have the same atomic mass. F 11. A mole of copper contains the same number of atoms as a mole of zinc. T 12. One mole of average atoms of an element would have the same mass as a mole of one isotope of the same element. F 1. One mole of silver has the same mass as a mole of gold. F 14. One mole of H2O contains two moles of hydrogen atoms. T 15. One mole of H2O contains 2.0 grams of hydrogen. 16. One mole of O weighs 16 grams. T F 17. The pure substance, water, contains both hydrogen molecules and oxygen molecules. F 18. A diet is planned for a trip on a space ship and is lacking in milk, but is rich in turnips and broccoli. Such a diet could provide a sufficient amount of calcium for adults. T

23 19. alcium supplements can be taken in 1,000 mg increments. F Atoms and Molecules Protons and neutrons have approximately the same mass. T

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