CHAPTER 2
Atoms, Molecules, and Ions
■ SOLUTIONS TO EXERCISES
Note on significant figures: If the final answer to a solution needs to be rounded off, it is given first with
one nonsignificant figure, and the last significant figure is underlined. The final answer is then rounded to
the correct number of significant figures. In multistep problems, intermediate answers are given with at
least one nonsignificant figure; however, only the final answer has been rounded off.
2.1. The element with atomic number 17 (the number of protons in the nucleus) is chlorine, symbol
Cl. The mass number is 17 + 18 = 35. The symbol is 1735 Cl.
2.2. Multiply each isotopic mass by its fractional abundance; then sum:
34.96885 amu 0.75771 = 26.496247
36.96590 amu 0.24229 = 8.956467
35.452714 = 35.453 amu
The atomic weight of chlorine is 35.453 amu.
2.3. a. Se: Group 6A, Period 4; nonmetal
b. Cs: Group 1A, Period 6; metal
c. Fe: Group 8B, Period 4; metal
d. Cu: Group 1B, Period 4; metal
e. Br: Group 7A, Period 4; nonmetal
2.4. Take as many cations as there are units of charge on the anion and as many anions as there are
units of charge on the cation. Two K+ ions have a total charge of 2+, and one CrO42 ion has a
charge of 2, giving a net charge of zero. The simplest ratio of K+ to CrO42 is 2:1, and the
formula is K2CrO4.
2.5. a. CaO: Calcium, a Group 2A metal, is expected to form only a 2+ ion (Ca2+, the calcium ion).
Oxygen (Group 6A) is expected to form an anion of charge equal to the group number
minus 8 (O2−, the oxide ion). The name of the compound is calcium oxide.
b. PbCrO4: Lead has more than one monatomic ion. You can find the charge on the Pb ion if
you know the formula of the anion. From Table 2.5, the CrO4 refers to the anion CrO42 (the
chromate ion). Therefore, the Pb cation must be Pb2+ to give electrical neutrality. The name of
Pb2+ is lead(II) ion, so the name of the compound is lead(II) chromate.
2.6. Thallium(III) nitrate contains the thallium(III) ion, Tl3+, and the nitrate ion, NO3. The formula is
Tl(NO3)3.
34
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 2: Atoms, Molecules, and Ions 35
2.7. a. Dichlorine hexoxide
b. Phosphorus trichloride
c. Phosphorus pentachloride
2.8. a. CS2 b. SO3
2.9. a. Boron trifluoride b. Hydrogen selenide
2.10. When you remove one H+ ion from HBrO4, you obtain the BrO4 ion. You name the ion from the
acid by replacing -ic with -ate. The anion is called the perbromate ion.
2.11. Sodium carbonate decahydrate
2.12. Sodium thiosulfate is composed of sodium ions (Na+) and thiosulfate ions (S2O32), so the
formula of the anhydrous compound is Na2S2O3. Since the material is a pentahydrate, the formula
of the compound is Na2S2O35H2O.
2.13. Balance O first in parts (a) and (b) because it occurs in only one product. Balance S first in part
(c) because it appears in only one product. Balance H first in part (d) because it appears in just
one reactant as well as in the product.
a. Write a 2 in front of POCl3 for O; this requires a 2 in front of PCl3 for final balance:
O2 + 2PCl3 2POCl3
b. Write a 6 in front of N2O to balance O; this requires a 6 in front of N2 for final balance:
P4 + 6N2O P4O6 + 6N2
c. Write 2As2S3 and 6SO2 to achieve an even number of oxygens on the right to balance what
will always be an even number of oxygens on the left. The 2As2S3 then requires 2As2O3.
Finally, to balance (6 + 12) O's on the right, write 9O2.
2As2S3 + 9O2 2As2O3 + 6SO2
d. Write a 4 in front of H3PO4; this requires a 3 in front of Ca(H2PO4)2 for twelve H's.
Ca3(PO4)2 + 4H3PO4 3Ca(H2PO4)2
■ ANSWERS TO CONCEPT CHECKS
2.1. CO2 is a compound that is a combination of 1 carbon atom and 2 oxygen atoms. Therefore, the
chemical model must contain a chemical combination of 3 atoms stuck together with 2 of the
atoms being the same (oxygen). Since each "ball" represents an individual atom, the three models
on the left can be eliminated since they don't contain the correct number of atoms. Keeping in
mind that balls of the same color represent the same element, only the model on the far right
contains two elements with the correct ratio of atoms, 1:2; therefore, it must be CO2.
2.2. If 7999 out of 8000 alpha particles deflected back at the alpha-particle source, this would imply
that the atom was a solid, impenetrable mass. Keep in mind that this is in direct contrast to what
was observed in the actual experiments, where the majority of the alpha particles passed through
without being deflected.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, 36 Chapter 2: Atoms, Molecules, and Ions
2.3. Elements are listed together in groups because they have similar chemical and/or physical
properties.
2.4. Statement (a) is the best statement regarding molecular compounds. Although you may have
wanted to classify Br2 as a molecular compound, it is an element and not a compound. Regarding
statement (b), quite a few molecular compounds exit that don’t contain carbon. Water and the
nitrogen oxides associated with smog are prime examples. Statement (c) is false; ionic
compounds consist of anions and cations. Statement (d) is very close to the right selection but it is
too restrictive. Some molecular compounds containing both metal and nonmetal atoms are known
to exist, e.g., cisplatin, Ni(CO)4, etc. Because numerous molecular compounds are either solids or
liquids at room temperature, statement (e) is false.
2.5. a. This compound is an ether because it has a functional group of an oxygen atom between
two carbon atoms (–O–).
b. This compound is an alcohol because it has an –OH functional group.
c. This compound is a carboxylic acid because it has the –COOH functional group.
d. This compound is a hydrocarbon because it contains only carbon and hydrogen atoms.
2.6. The SO42-, NO2-, and I3- are considered to be polyatomic ions. Statement (a) is true based on the
prefix poly. By definition, any ion must have a negative or positive charge; thus, statement (b) is
true. Bring that the triiodide ion has only iodine atoms bonded together, and no other elements
present, statement (c) is false. There are numerous examples to show that statement (d) is true,
e.g., chromate, dichromate, permanganate to name a few. Oxoanions are polyatomic ions
containing a central characteristic element surrounded by a number of oxygen atoms, e.g., sulfate
and nitrite given in this concept check’s. Statement (e) is true.
2.7. A bottle containing a compound with the formula Al2Q3 would have an anion, Q, with a charge of
2. The total positive charge in the compound due to the Al3+ is 6+ (2 x 3+), so the total negative
charge must be 6; therefore, each Q ion must have a charge of 2. Thus, Q would probably be
an element from Group 6A on the periodic table.
■ ANSWERS TO SELF-ASSESSMENT AND REVIEW QUESTIONS
2.1. Atomic theory is an explanation of the structure of matter in terms of different combinations of
very small particles called atoms. Since compounds are composed of atoms of two or more
elements, there is no limit to the number of ways in which the elements can be combined. Each
compound has its own unique properties. A chemical reaction consists of the rearrangement of
the atoms present in the reacting substances to give new chemical combinations present in the
substances formed by the reaction.
2.2. Divide each amount of chlorine, 1.270 g and 1.904 g, by the lower amount, 1.270 g. This gives
1.000 and 1.499, respectively. Convert these to whole numbers by multiplying by 2, giving 2.000
and 2.998. The ratio of these amounts of chlorine is essentially 2:3. This is consistent with the law
of multiple proportions because, for a fixed mass of iron (1 gram), the masses of chlorine in the
other two compounds are in a ratio of small whole numbers.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 2: Atoms, Molecules, and Ions 37
2.3. A cathode-ray tube consists of a negative electrode, or cathode, and a positive electrode, or
anode, in an evacuated tube. Cathode rays travel from the cathode to the anode when a high
voltage is turned on. Some of the rays pass through the hole in the anode to form a beam, which is
then bent toward positively charged electric plates in the tube. This implies that a cathode ray
consists of a beam of negatively charged particles (or electrons) and that electrons are
constituents of all matter.
2.4. Millikan performed a series of experiments in which he obtained the charge on the electron by
observing how a charged drop of oil falls in the presence and in the absence of an electric field.
An atomizer introduces a fine mist of oil drops into the top chamber (Figure 2.6). Several drops
happen to fall through a small hole into the lower chamber, where the experimenter follows the
motion of one drop with a microscope. Some of these drops have picked up one or more electrons
as a result of friction in the atomizer and have become negatively charged. A negatively charged
drop will be attracted upward when the experimenter turns on a current to the electric plates. The
drop’s upward speed (obtained by timing its rise) is related to its mass-to-charge ratio, from
which you can calculate the charge on the electron.
2.5. The nuclear model of the atom is based on experiments of Geiger, Marsden, and Rutherford.
Rutherford stated that most of the mass of an atom is concentrated in a positively charged center
called the nucleus around which negatively charged electrons move. The nucleus, although it
contains most of the mass, occupies only a very small portion of the space of the atom. Most of
the alpha particles passed through the metal atoms of the foil undeflected by the lightweight
electrons. When an alpha particle does happen to hit a metal-atom nucleus, it is scattered at a
wide angle because it is deflected by the massive, positively charged nucleus (Figure 2.8).
2.6. The atomic nucleus consists of two kinds of particles, protons and neutrons. The mass of each is
about the same, on the order of 1.67 x 1027 kg, and about 1800 times that of the electron. An
electron has a much smaller mass, on the order of 9.11 x 1031 kg. The neutron is electrically
neutral, but the proton is positively charged. An electron is negatively charged. The charges on
the proton and the electron are equal in magnitude.
2.7. Protons (hydrogen nuclei) were discovered as products of experiments involving the collision of
alpha particles with nitrogen atoms that resulted in a proton being knocked out of the nitrogen
nucleus. Neutrons were discovered as the radiation product of collisions of alpha particles with
beryllium atoms. The resulting radiation was discovered to consist of particles having a mass
approximately equal to that of a proton and having no charge (neutral).
2.8. Oxygen consists of three different isotopes, each having 8 protons but a different number of
neutrons.
2.9. The percentages of the different isotopes in most naturally occurring elements have remained
essentially constant over time and in most cases are independent of the origin of the element.
Thus, what Dalton actually calculated were average atomic weights (relative weights). He could
not weigh individual atoms, but he could find the average mass of one atom relative to the
average mass of another.
2.10. A mass spectrometer measures the mass-to-charge ratio of positively charged atoms (and
molecules). It produces a mass spectrum, which shows the relative numbers of atoms (fractional
abundances) of various masses (isotopic masses). The mass spectrum gives us all the information
needed to calculate the atomic weight.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
Atoms, Molecules, and Ions
■ SOLUTIONS TO EXERCISES
Note on significant figures: If the final answer to a solution needs to be rounded off, it is given first with
one nonsignificant figure, and the last significant figure is underlined. The final answer is then rounded to
the correct number of significant figures. In multistep problems, intermediate answers are given with at
least one nonsignificant figure; however, only the final answer has been rounded off.
2.1. The element with atomic number 17 (the number of protons in the nucleus) is chlorine, symbol
Cl. The mass number is 17 + 18 = 35. The symbol is 1735 Cl.
2.2. Multiply each isotopic mass by its fractional abundance; then sum:
34.96885 amu 0.75771 = 26.496247
36.96590 amu 0.24229 = 8.956467
35.452714 = 35.453 amu
The atomic weight of chlorine is 35.453 amu.
2.3. a. Se: Group 6A, Period 4; nonmetal
b. Cs: Group 1A, Period 6; metal
c. Fe: Group 8B, Period 4; metal
d. Cu: Group 1B, Period 4; metal
e. Br: Group 7A, Period 4; nonmetal
2.4. Take as many cations as there are units of charge on the anion and as many anions as there are
units of charge on the cation. Two K+ ions have a total charge of 2+, and one CrO42 ion has a
charge of 2, giving a net charge of zero. The simplest ratio of K+ to CrO42 is 2:1, and the
formula is K2CrO4.
2.5. a. CaO: Calcium, a Group 2A metal, is expected to form only a 2+ ion (Ca2+, the calcium ion).
Oxygen (Group 6A) is expected to form an anion of charge equal to the group number
minus 8 (O2−, the oxide ion). The name of the compound is calcium oxide.
b. PbCrO4: Lead has more than one monatomic ion. You can find the charge on the Pb ion if
you know the formula of the anion. From Table 2.5, the CrO4 refers to the anion CrO42 (the
chromate ion). Therefore, the Pb cation must be Pb2+ to give electrical neutrality. The name of
Pb2+ is lead(II) ion, so the name of the compound is lead(II) chromate.
2.6. Thallium(III) nitrate contains the thallium(III) ion, Tl3+, and the nitrate ion, NO3. The formula is
Tl(NO3)3.
34
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 2: Atoms, Molecules, and Ions 35
2.7. a. Dichlorine hexoxide
b. Phosphorus trichloride
c. Phosphorus pentachloride
2.8. a. CS2 b. SO3
2.9. a. Boron trifluoride b. Hydrogen selenide
2.10. When you remove one H+ ion from HBrO4, you obtain the BrO4 ion. You name the ion from the
acid by replacing -ic with -ate. The anion is called the perbromate ion.
2.11. Sodium carbonate decahydrate
2.12. Sodium thiosulfate is composed of sodium ions (Na+) and thiosulfate ions (S2O32), so the
formula of the anhydrous compound is Na2S2O3. Since the material is a pentahydrate, the formula
of the compound is Na2S2O35H2O.
2.13. Balance O first in parts (a) and (b) because it occurs in only one product. Balance S first in part
(c) because it appears in only one product. Balance H first in part (d) because it appears in just
one reactant as well as in the product.
a. Write a 2 in front of POCl3 for O; this requires a 2 in front of PCl3 for final balance:
O2 + 2PCl3 2POCl3
b. Write a 6 in front of N2O to balance O; this requires a 6 in front of N2 for final balance:
P4 + 6N2O P4O6 + 6N2
c. Write 2As2S3 and 6SO2 to achieve an even number of oxygens on the right to balance what
will always be an even number of oxygens on the left. The 2As2S3 then requires 2As2O3.
Finally, to balance (6 + 12) O's on the right, write 9O2.
2As2S3 + 9O2 2As2O3 + 6SO2
d. Write a 4 in front of H3PO4; this requires a 3 in front of Ca(H2PO4)2 for twelve H's.
Ca3(PO4)2 + 4H3PO4 3Ca(H2PO4)2
■ ANSWERS TO CONCEPT CHECKS
2.1. CO2 is a compound that is a combination of 1 carbon atom and 2 oxygen atoms. Therefore, the
chemical model must contain a chemical combination of 3 atoms stuck together with 2 of the
atoms being the same (oxygen). Since each "ball" represents an individual atom, the three models
on the left can be eliminated since they don't contain the correct number of atoms. Keeping in
mind that balls of the same color represent the same element, only the model on the far right
contains two elements with the correct ratio of atoms, 1:2; therefore, it must be CO2.
2.2. If 7999 out of 8000 alpha particles deflected back at the alpha-particle source, this would imply
that the atom was a solid, impenetrable mass. Keep in mind that this is in direct contrast to what
was observed in the actual experiments, where the majority of the alpha particles passed through
without being deflected.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, 36 Chapter 2: Atoms, Molecules, and Ions
2.3. Elements are listed together in groups because they have similar chemical and/or physical
properties.
2.4. Statement (a) is the best statement regarding molecular compounds. Although you may have
wanted to classify Br2 as a molecular compound, it is an element and not a compound. Regarding
statement (b), quite a few molecular compounds exit that don’t contain carbon. Water and the
nitrogen oxides associated with smog are prime examples. Statement (c) is false; ionic
compounds consist of anions and cations. Statement (d) is very close to the right selection but it is
too restrictive. Some molecular compounds containing both metal and nonmetal atoms are known
to exist, e.g., cisplatin, Ni(CO)4, etc. Because numerous molecular compounds are either solids or
liquids at room temperature, statement (e) is false.
2.5. a. This compound is an ether because it has a functional group of an oxygen atom between
two carbon atoms (–O–).
b. This compound is an alcohol because it has an –OH functional group.
c. This compound is a carboxylic acid because it has the –COOH functional group.
d. This compound is a hydrocarbon because it contains only carbon and hydrogen atoms.
2.6. The SO42-, NO2-, and I3- are considered to be polyatomic ions. Statement (a) is true based on the
prefix poly. By definition, any ion must have a negative or positive charge; thus, statement (b) is
true. Bring that the triiodide ion has only iodine atoms bonded together, and no other elements
present, statement (c) is false. There are numerous examples to show that statement (d) is true,
e.g., chromate, dichromate, permanganate to name a few. Oxoanions are polyatomic ions
containing a central characteristic element surrounded by a number of oxygen atoms, e.g., sulfate
and nitrite given in this concept check’s. Statement (e) is true.
2.7. A bottle containing a compound with the formula Al2Q3 would have an anion, Q, with a charge of
2. The total positive charge in the compound due to the Al3+ is 6+ (2 x 3+), so the total negative
charge must be 6; therefore, each Q ion must have a charge of 2. Thus, Q would probably be
an element from Group 6A on the periodic table.
■ ANSWERS TO SELF-ASSESSMENT AND REVIEW QUESTIONS
2.1. Atomic theory is an explanation of the structure of matter in terms of different combinations of
very small particles called atoms. Since compounds are composed of atoms of two or more
elements, there is no limit to the number of ways in which the elements can be combined. Each
compound has its own unique properties. A chemical reaction consists of the rearrangement of
the atoms present in the reacting substances to give new chemical combinations present in the
substances formed by the reaction.
2.2. Divide each amount of chlorine, 1.270 g and 1.904 g, by the lower amount, 1.270 g. This gives
1.000 and 1.499, respectively. Convert these to whole numbers by multiplying by 2, giving 2.000
and 2.998. The ratio of these amounts of chlorine is essentially 2:3. This is consistent with the law
of multiple proportions because, for a fixed mass of iron (1 gram), the masses of chlorine in the
other two compounds are in a ratio of small whole numbers.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
, Chapter 2: Atoms, Molecules, and Ions 37
2.3. A cathode-ray tube consists of a negative electrode, or cathode, and a positive electrode, or
anode, in an evacuated tube. Cathode rays travel from the cathode to the anode when a high
voltage is turned on. Some of the rays pass through the hole in the anode to form a beam, which is
then bent toward positively charged electric plates in the tube. This implies that a cathode ray
consists of a beam of negatively charged particles (or electrons) and that electrons are
constituents of all matter.
2.4. Millikan performed a series of experiments in which he obtained the charge on the electron by
observing how a charged drop of oil falls in the presence and in the absence of an electric field.
An atomizer introduces a fine mist of oil drops into the top chamber (Figure 2.6). Several drops
happen to fall through a small hole into the lower chamber, where the experimenter follows the
motion of one drop with a microscope. Some of these drops have picked up one or more electrons
as a result of friction in the atomizer and have become negatively charged. A negatively charged
drop will be attracted upward when the experimenter turns on a current to the electric plates. The
drop’s upward speed (obtained by timing its rise) is related to its mass-to-charge ratio, from
which you can calculate the charge on the electron.
2.5. The nuclear model of the atom is based on experiments of Geiger, Marsden, and Rutherford.
Rutherford stated that most of the mass of an atom is concentrated in a positively charged center
called the nucleus around which negatively charged electrons move. The nucleus, although it
contains most of the mass, occupies only a very small portion of the space of the atom. Most of
the alpha particles passed through the metal atoms of the foil undeflected by the lightweight
electrons. When an alpha particle does happen to hit a metal-atom nucleus, it is scattered at a
wide angle because it is deflected by the massive, positively charged nucleus (Figure 2.8).
2.6. The atomic nucleus consists of two kinds of particles, protons and neutrons. The mass of each is
about the same, on the order of 1.67 x 1027 kg, and about 1800 times that of the electron. An
electron has a much smaller mass, on the order of 9.11 x 1031 kg. The neutron is electrically
neutral, but the proton is positively charged. An electron is negatively charged. The charges on
the proton and the electron are equal in magnitude.
2.7. Protons (hydrogen nuclei) were discovered as products of experiments involving the collision of
alpha particles with nitrogen atoms that resulted in a proton being knocked out of the nitrogen
nucleus. Neutrons were discovered as the radiation product of collisions of alpha particles with
beryllium atoms. The resulting radiation was discovered to consist of particles having a mass
approximately equal to that of a proton and having no charge (neutral).
2.8. Oxygen consists of three different isotopes, each having 8 protons but a different number of
neutrons.
2.9. The percentages of the different isotopes in most naturally occurring elements have remained
essentially constant over time and in most cases are independent of the origin of the element.
Thus, what Dalton actually calculated were average atomic weights (relative weights). He could
not weigh individual atoms, but he could find the average mass of one atom relative to the
average mass of another.
2.10. A mass spectrometer measures the mass-to-charge ratio of positively charged atoms (and
molecules). It produces a mass spectrum, which shows the relative numbers of atoms (fractional
abundances) of various masses (isotopic masses). The mass spectrum gives us all the information
needed to calculate the atomic weight.
© 2017 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
