CHAPTER 8
Electron Configurations and Periodicity
■ 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.
8.1. a. Possible orbital diagram.
b. Possible orbital diagram.
c. Impossible orbital diagram; there are two electrons in a 2p orbital with the same spin.
d. Possible electron configuration.
e. Impossible electron configuration; only two electrons are allowed in an s subshell.
f. Impossible electron configuration; only six electrons are allowed in a p subshell.
8.2. Look at the periodic table. Start with hydrogen and go through the periods, writing down the
subshells being filled, stopping with manganese (Z = 25). You obtain the following order:
Order: 1s 2s2p 3s3p 4s3d4p
Period: first second third fourth
Now fill the subshells with electrons, remembering that you have a total of twenty-five electrons
to distribute. You obtain
1s22s22p63s23p64s23d5, or 1s22s22p63s23p63d54s2
8.3. Arsenic is a main-group element in Period 4, Group 5A, of the periodic table. The five outer
electrons should occupy the 4s and 4p subshells; the five valence electrons have the configuration
4s24p3.
8.4. Because the sum of the 6s2 and 6p2 electrons gives four outer (valence) electrons, lead should be
in Group 4A, which it is. Looking at the table, you find lead in Period 6. From its position, it
would be classified as a main-group element.
8.5. The electron configuration of phosphorus is 1s22s22p63s23p3. The orbital diagram is
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, 272 Chapter 8: Electron Configurations and Periodicity
8.6. The radius tends to decrease across a row of the periodic table from left to right, and it tends to
increase from the top of a column to the bottom. Therefore, in order of increasing radius,
Be < Mg < Na
8.7. It is more likely that answer a, 1000 kJ/mol, is the ionization energy for iodine because ionization
energies tend to decrease with atomic number in a group (I is below Cl in Group 7A).
8.8. Fluorine should have a more negative electron affinity because (1) carbon has only two electrons
in the p subshell, (2) the 1− fluoride ion has a stable noble-gas configuration, and (3) the electron
can approach the fluorine nucleus more closely than the carbon nucleus. This follows the general
trend, which is toward more negative electron affinities from left to right in any period.
■ ANSWERS TO CONCEPT CHECKS
8.1. The second-period elements are those in which the 2s and 2p orbitals fill. Each orbital can hold
only one electron, so all four orbitals will be filled after four electrons. Therefore, the second
period will have four elements.
8.2. The s orbital fills in the first two elements of the period (Groups 1A and 2A); then the p orbital
starts to fill (Group 3A). Thus, the first element is in Group 2A (Mg), and the next element is in
Group 3A (Al).
8.3. From the information given, the element must be in Group 2A. These elements have positive
electron affinities and also have large third ionization energies.
8.4. A metalloid is an element near the staircase line in the periodic table (the green elements in the
periodic table on the inside front cover of the text). The formula R2O5 suggests a Group 5A
element. There are two metalloids in Group 5A, arsenic and antimony. That this is an acidic oxide
indicates this metalloid has considerable nonmetal character. So, of the two metalloids, the one
nearer the top of the column, arsenic, seems more likely. This agrees with the text, which notes
that arsenic(V) oxide is acidic, whereas antimony(V) oxide is amphoteric.
■ ANSWERS TO SELF-ASSESSMENT AND REVIEW QUESTIONS
8.1. In the original Stern-Gerlach experiment, a beam of silver atoms is directed into the field of a
specially designed magnet. (The same can be done with hydrogen atoms.) The beam of atoms is
split into two by the magnetic field; half are bent toward one magnetic pole face and the other
half toward the other magnetic pole face. This effect shows that the atoms themselves act as
magnets with a positive or a negative component, as indicated by the positive or negative spin
quantum numbers.
8.2. In effect, the electron acts as though it were a sphere of spinning charge (Figure 8.3). Like any
circulating electric charge, it creates a magnetic field with a spin axis that has more than one
possible direction relative to a magnetic field. Electron spin is subject to a quantum restriction to
one of two directions corresponding to the ms quantum numbers +1/2 and −1/2.
© 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 8: Electron Configurations and Periodicity 273
8.3. The Pauli exclusion principle limits the configurations of an atom by excluding configurations in
which two or more electrons have the same four quantum numbers. For example, each electron in
the same orbital must have different ms values. This also implies that only two electrons occupy
one orbital.
8.4. According to the principles discussed in Section 7.5, the number of orbitals in the f subshell
(l = 3) is given by 2l + 1 and is thus equal to 7. Because each orbital can hold a maximum of two
electrons, the f subshell can hold a maximum of fourteen electrons.
8.5. The orbitals, in order of increasing energy up to and including the 3p orbitals (but not including
the 3d orbitals), are as follows: 1s, 2s, 2p, 3s, and 3p (Figure 8.7).
8.6. The noble-gas core is an inner-shell configuration corresponding to one of the noble gases. The
pseudo-noble-gas core is an inner-shell configuration corresponding to one of the noble gases
together with (n − 1)d10 electrons. Like the noble-gas core electrons, the d10 electrons are not
involved in chemical reactions. The valence electron is an electron (of an atom) located outside
the noble-gas core or pseudo-noble-gas core. It is an electron primarily involved in chemical
reactions.
8.7. The orbital diagram for the 1s22s22p4 ground state of oxygen is
Another possible oxygen orbital diagram, but not a ground state, is
8.8. A diamagnetic substance is a substance that is not attracted by a magnetic field or is very slightly
repelled by such a field. This property generally indicates the substance has only paired electrons.
A paramagnetic substance is a substance that is weakly attracted by a magnetic field. This
property generally indicates the substance has one or more unpaired electrons. Ground-state
oxygen has two unpaired 2p electrons and is therefore paramagnetic.
8.9. In Groups 1A and 2A, the outer s subshell is being filled: s1 for Group 1A and s2 for Group 2A. In
Groups 3A to 8A, the outer p subshell is being filled: p1 for 3A, p2 for 4A, p3 for 5A, p4 for 6A, p5
for 7A, and p6 for 8A. In the transition elements, the (n − 1)d subshell is being filled from d1 to
d10 electrons. In the lanthanides and actinides, the f subshell is being filled from f1 to f14 electrons.
8.10. Mendeleev arranged the elements in order of increasing atomic weight, an arrangement that was
later changed to atomic numbers. His periodic table was divided into rows (periods) and columns
(groups). In his first attempt, he left spaces for what he believed to be undiscovered elements. In
his row 5, under aluminum and above indium in Group III, he left a blank space. This Group III
element he called eka-aluminum, and he predicted its properties from those of aluminum and
indium. Later, the French chemist de Boisbaudran discovered this element and named it gallium.
© 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.
Electron Configurations and Periodicity
■ 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.
8.1. a. Possible orbital diagram.
b. Possible orbital diagram.
c. Impossible orbital diagram; there are two electrons in a 2p orbital with the same spin.
d. Possible electron configuration.
e. Impossible electron configuration; only two electrons are allowed in an s subshell.
f. Impossible electron configuration; only six electrons are allowed in a p subshell.
8.2. Look at the periodic table. Start with hydrogen and go through the periods, writing down the
subshells being filled, stopping with manganese (Z = 25). You obtain the following order:
Order: 1s 2s2p 3s3p 4s3d4p
Period: first second third fourth
Now fill the subshells with electrons, remembering that you have a total of twenty-five electrons
to distribute. You obtain
1s22s22p63s23p64s23d5, or 1s22s22p63s23p63d54s2
8.3. Arsenic is a main-group element in Period 4, Group 5A, of the periodic table. The five outer
electrons should occupy the 4s and 4p subshells; the five valence electrons have the configuration
4s24p3.
8.4. Because the sum of the 6s2 and 6p2 electrons gives four outer (valence) electrons, lead should be
in Group 4A, which it is. Looking at the table, you find lead in Period 6. From its position, it
would be classified as a main-group element.
8.5. The electron configuration of phosphorus is 1s22s22p63s23p3. The orbital diagram is
© 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.
, 272 Chapter 8: Electron Configurations and Periodicity
8.6. The radius tends to decrease across a row of the periodic table from left to right, and it tends to
increase from the top of a column to the bottom. Therefore, in order of increasing radius,
Be < Mg < Na
8.7. It is more likely that answer a, 1000 kJ/mol, is the ionization energy for iodine because ionization
energies tend to decrease with atomic number in a group (I is below Cl in Group 7A).
8.8. Fluorine should have a more negative electron affinity because (1) carbon has only two electrons
in the p subshell, (2) the 1− fluoride ion has a stable noble-gas configuration, and (3) the electron
can approach the fluorine nucleus more closely than the carbon nucleus. This follows the general
trend, which is toward more negative electron affinities from left to right in any period.
■ ANSWERS TO CONCEPT CHECKS
8.1. The second-period elements are those in which the 2s and 2p orbitals fill. Each orbital can hold
only one electron, so all four orbitals will be filled after four electrons. Therefore, the second
period will have four elements.
8.2. The s orbital fills in the first two elements of the period (Groups 1A and 2A); then the p orbital
starts to fill (Group 3A). Thus, the first element is in Group 2A (Mg), and the next element is in
Group 3A (Al).
8.3. From the information given, the element must be in Group 2A. These elements have positive
electron affinities and also have large third ionization energies.
8.4. A metalloid is an element near the staircase line in the periodic table (the green elements in the
periodic table on the inside front cover of the text). The formula R2O5 suggests a Group 5A
element. There are two metalloids in Group 5A, arsenic and antimony. That this is an acidic oxide
indicates this metalloid has considerable nonmetal character. So, of the two metalloids, the one
nearer the top of the column, arsenic, seems more likely. This agrees with the text, which notes
that arsenic(V) oxide is acidic, whereas antimony(V) oxide is amphoteric.
■ ANSWERS TO SELF-ASSESSMENT AND REVIEW QUESTIONS
8.1. In the original Stern-Gerlach experiment, a beam of silver atoms is directed into the field of a
specially designed magnet. (The same can be done with hydrogen atoms.) The beam of atoms is
split into two by the magnetic field; half are bent toward one magnetic pole face and the other
half toward the other magnetic pole face. This effect shows that the atoms themselves act as
magnets with a positive or a negative component, as indicated by the positive or negative spin
quantum numbers.
8.2. In effect, the electron acts as though it were a sphere of spinning charge (Figure 8.3). Like any
circulating electric charge, it creates a magnetic field with a spin axis that has more than one
possible direction relative to a magnetic field. Electron spin is subject to a quantum restriction to
one of two directions corresponding to the ms quantum numbers +1/2 and −1/2.
© 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 8: Electron Configurations and Periodicity 273
8.3. The Pauli exclusion principle limits the configurations of an atom by excluding configurations in
which two or more electrons have the same four quantum numbers. For example, each electron in
the same orbital must have different ms values. This also implies that only two electrons occupy
one orbital.
8.4. According to the principles discussed in Section 7.5, the number of orbitals in the f subshell
(l = 3) is given by 2l + 1 and is thus equal to 7. Because each orbital can hold a maximum of two
electrons, the f subshell can hold a maximum of fourteen electrons.
8.5. The orbitals, in order of increasing energy up to and including the 3p orbitals (but not including
the 3d orbitals), are as follows: 1s, 2s, 2p, 3s, and 3p (Figure 8.7).
8.6. The noble-gas core is an inner-shell configuration corresponding to one of the noble gases. The
pseudo-noble-gas core is an inner-shell configuration corresponding to one of the noble gases
together with (n − 1)d10 electrons. Like the noble-gas core electrons, the d10 electrons are not
involved in chemical reactions. The valence electron is an electron (of an atom) located outside
the noble-gas core or pseudo-noble-gas core. It is an electron primarily involved in chemical
reactions.
8.7. The orbital diagram for the 1s22s22p4 ground state of oxygen is
Another possible oxygen orbital diagram, but not a ground state, is
8.8. A diamagnetic substance is a substance that is not attracted by a magnetic field or is very slightly
repelled by such a field. This property generally indicates the substance has only paired electrons.
A paramagnetic substance is a substance that is weakly attracted by a magnetic field. This
property generally indicates the substance has one or more unpaired electrons. Ground-state
oxygen has two unpaired 2p electrons and is therefore paramagnetic.
8.9. In Groups 1A and 2A, the outer s subshell is being filled: s1 for Group 1A and s2 for Group 2A. In
Groups 3A to 8A, the outer p subshell is being filled: p1 for 3A, p2 for 4A, p3 for 5A, p4 for 6A, p5
for 7A, and p6 for 8A. In the transition elements, the (n − 1)d subshell is being filled from d1 to
d10 electrons. In the lanthanides and actinides, the f subshell is being filled from f1 to f14 electrons.
8.10. Mendeleev arranged the elements in order of increasing atomic weight, an arrangement that was
later changed to atomic numbers. His periodic table was divided into rows (periods) and columns
(groups). In his first attempt, he left spaces for what he believed to be undiscovered elements. In
his row 5, under aluminum and above indium in Group III, he left a blank space. This Group III
element he called eka-aluminum, and he predicted its properties from those of aluminum and
indium. Later, the French chemist de Boisbaudran discovered this element and named it gallium.
© 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.
