Chapter 2: Atoms and Molecules
CHAPTER OUTLINE
2.1 Symbols and Formulas 2.4 Relative Masses of Atoms 2.6 Avogadro’s Number: The Mole
2.2 Inside the Atom and Molecules 2.7 The Mole and Chemical
2.3 Isotopes 2.5 Isotopes and Atomic Weights Formulas
LEARNING OBJECTIVES/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)
3. 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 2.3; Exercises 2.16 and 2.22)
4. Use atomic weights of the elements to calculate molecular weights of compounds. (Section 2.4;
Exercise 2.32)
5. Use isotope percent abundances and masses to calculate atomic weights of the elements. (Section 2.5;
Exercise 2.38)
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)
LECTURE 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 (3) 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.
3. 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.
28
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, Atoms and Molecules 29
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 CHAPTER EXERCISES
SYMBOLS AND FORMULAS (SECTION 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.
2.3 a. A diatomic molecule of chlorine gas (two chlorine atoms) Cl2; like Exercise 2.1 a
b. A diatomic molecule of hydrogen fluoride (one hydrogen HF; like Exercise 2.1 b
atom and one fluorine atom)
c. A triatomic molecule of ozone (three oxygen atoms) O3; like Exercise 2.1 c*
d. A molecule of carbon tetrachloride (one atom of carbon and CCl4; like Exercise 2.1 d*
four atoms of chlorine)
*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.
© 2018 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, 30 Chapter 2
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 C2H6O; 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. ammonia (NH3) 1 nitrogen atom; 3 hydrogen atoms
b. acetic acid (C2H4O2) 2 carbon atoms; 4 hydrogen atoms; 2 oxygen atoms
c. boric acid (H3BO3) 3 hydrogen atoms; 1 boron atom; 3 oxygen atoms
d. ethane (C2H6) 2 carbon atoms; 6 hydrogen atoms
2.6 a. Sulfur dioxide (SO2) 1 sulfur atom; 2 oxygen atoms
b. Butane (C4H10) 4 carbon atoms; 10 hydrogen atoms
c. Chlorous acid (HClO2) 1 hydrogen atom; 1 chlorine atom; 2 oxygen atoms
d. Boron trifluoride (BF3) 1 boron atom; 3 fluorine atoms
2.7 a. H3PO3 (phosphorous acid) The numbers should be subscripted: H3PO3
b. SICl4 (silicon tetrachloride) The elemental symbol for silicon is Si: SiCl4
c. SOO (sulfur dioxide) Only one O should be written and a subscript 2
should be added: SO2
d. 2HO (hydrogen peroxide—two The number 2 should be a subscript after H and
hydrogen atoms and two oxygen after O: H2O2
atoms)
2.8 a. HSH (hydrogen sulfide) More than one H is part of the compound;
a subscript should be used: H2S
b. HCLO2 (chlorous acid) The elemental symbol for chlorine is Cl (the second
letter of a symbol must be lowercase): HClO2
c. 2HN2 (hydrazine – two hydrogen The subscripts should reflect the actual number of
atoms and four nitrogen atoms) each type of atom in the compound: H2N4
d. C2H6 (ethane) The numbers should be subscripted: C2H6
INSIDE THE ATOM (SECTION 2.2)
2.9 Charge Mass (u)
a. 6 protons and 6 neutrons 6 12
b. 8 protons and 9 neutrons 8 17
c. 20 protons and 25 neutrons 20 45
d. 52 protons and 78 neutrons 52 130
2.10 Charge Mass (u)
a. 4 protons and 5 neutrons 4 9
b. 9 protons and 10 neutrons 9 19
c. 20 protons and 23 neutrons 20 43
d. 47 protons and 60 neutrons 47 107
2.11 The number of protons and electrons are equal in a neutral atom.
a. 5 electrons b. 10 electrons c. 18 electrons d. 50 electrons
© 2018 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
CHAPTER OUTLINE
2.1 Symbols and Formulas 2.4 Relative Masses of Atoms 2.6 Avogadro’s Number: The Mole
2.2 Inside the Atom and Molecules 2.7 The Mole and Chemical
2.3 Isotopes 2.5 Isotopes and Atomic Weights Formulas
LEARNING OBJECTIVES/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)
3. 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 2.3; Exercises 2.16 and 2.22)
4. Use atomic weights of the elements to calculate molecular weights of compounds. (Section 2.4;
Exercise 2.32)
5. Use isotope percent abundances and masses to calculate atomic weights of the elements. (Section 2.5;
Exercise 2.38)
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)
LECTURE 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 (3) 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.
3. 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.
28
© 2018 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Atoms and Molecules 29
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 CHAPTER EXERCISES
SYMBOLS AND FORMULAS (SECTION 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.
2.3 a. A diatomic molecule of chlorine gas (two chlorine atoms) Cl2; like Exercise 2.1 a
b. A diatomic molecule of hydrogen fluoride (one hydrogen HF; like Exercise 2.1 b
atom and one fluorine atom)
c. A triatomic molecule of ozone (three oxygen atoms) O3; like Exercise 2.1 c*
d. A molecule of carbon tetrachloride (one atom of carbon and CCl4; like Exercise 2.1 d*
four atoms of chlorine)
*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.
© 2018 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, 30 Chapter 2
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 C2H6O; 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. ammonia (NH3) 1 nitrogen atom; 3 hydrogen atoms
b. acetic acid (C2H4O2) 2 carbon atoms; 4 hydrogen atoms; 2 oxygen atoms
c. boric acid (H3BO3) 3 hydrogen atoms; 1 boron atom; 3 oxygen atoms
d. ethane (C2H6) 2 carbon atoms; 6 hydrogen atoms
2.6 a. Sulfur dioxide (SO2) 1 sulfur atom; 2 oxygen atoms
b. Butane (C4H10) 4 carbon atoms; 10 hydrogen atoms
c. Chlorous acid (HClO2) 1 hydrogen atom; 1 chlorine atom; 2 oxygen atoms
d. Boron trifluoride (BF3) 1 boron atom; 3 fluorine atoms
2.7 a. H3PO3 (phosphorous acid) The numbers should be subscripted: H3PO3
b. SICl4 (silicon tetrachloride) The elemental symbol for silicon is Si: SiCl4
c. SOO (sulfur dioxide) Only one O should be written and a subscript 2
should be added: SO2
d. 2HO (hydrogen peroxide—two The number 2 should be a subscript after H and
hydrogen atoms and two oxygen after O: H2O2
atoms)
2.8 a. HSH (hydrogen sulfide) More than one H is part of the compound;
a subscript should be used: H2S
b. HCLO2 (chlorous acid) The elemental symbol for chlorine is Cl (the second
letter of a symbol must be lowercase): HClO2
c. 2HN2 (hydrazine – two hydrogen The subscripts should reflect the actual number of
atoms and four nitrogen atoms) each type of atom in the compound: H2N4
d. C2H6 (ethane) The numbers should be subscripted: C2H6
INSIDE THE ATOM (SECTION 2.2)
2.9 Charge Mass (u)
a. 6 protons and 6 neutrons 6 12
b. 8 protons and 9 neutrons 8 17
c. 20 protons and 25 neutrons 20 45
d. 52 protons and 78 neutrons 52 130
2.10 Charge Mass (u)
a. 4 protons and 5 neutrons 4 9
b. 9 protons and 10 neutrons 9 19
c. 20 protons and 23 neutrons 20 43
d. 47 protons and 60 neutrons 47 107
2.11 The number of protons and electrons are equal in a neutral atom.
a. 5 electrons b. 10 electrons c. 18 electrons d. 50 electrons
© 2018 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
