The Periodic Table and
Electron Configurations
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C ONCEPT
Concept 1. The Periodic Table and Electron Configurations
1
The Periodic Table and
Electron Configurations
Lesson Objectives
• Relate an element’s position in the PT to the energy level of its valence electrons (excluding transition metals,
lanthanides, and actinides).
• Relate an element’s position in the PT to the sublevel of its highest energy valence electrons.
• Explain why there are only two elements in the first row of the PT.
Introduction
With what we have already discussed, you might realize that just as electron configurations can be used to explain
the shape and organization of the Periodic Table, the shape and organization of the Periodic Table can, in turn, be
used to predict electron configurations. In fact, if you can locate an element on the Periodic Table, you can use
the element’s position to figure out the energy level of the element’s valence electrons. Furthermore, an element’s
position on the Periodic Table tells you the sublevel of the element’s highest energy valence electrons. In this lesson,
we’ll take a close look at how the Periodic Table relates to the electron configurations.
Rows Across on the PT are Consistent With the Energy Level in An Atom
First, let’s try to figure out what we can learn from an element’s row or period in the Periodic Table.
The figure below shows how the different rows in the Periodic Table are numbered. The transition metals and the
lanthanides and actinides have been omitted. (Source: CK-12 Foundation. CC-BY-SA)
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To understand what this means in terms of an element’s electron configuration, let’s consider the Group 1A metals.
If we write the electron configuration for the Group 1A metal from each row of the Periodic Table, we have:
row
2
Li :
1s2 2s1
row
3
Na :
1s2 2s2 2p6 3s1
row
4
K:
1s2 2s2 2p6 3s2 3p6 4s1
row
5
Rb :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s1
row
6
Cs :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6 6s1
row
7
Fr :
s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6 6s2 4 f 14 5d 10 6p6 7s1
Do you see any pattern? For Group 1A metals, it seems that element’s row corresponds to the energy level of that
element’s valence electron. Lithium (Li), for instance, is found in row 2 of the Periodic Table, and its valence
electron is found in the n = 2 energy level. Cesium (Cs) is found in row 6 of the Periodic Table, and its valence
electron is found in the n = 6 energy level. Let’s see if this same pattern holds for Group 2A metals:
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Concept 1. The Periodic Table and Electron Configurations
row
2
Li :
1s2 2s2
row
3
Na :
1s2 2s2 2p6 3s2
row
4
K:
1s2 2s2 2p6 3s2 3p6 4s2
row
5
Rb :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2
row
6
Cs :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6 6s2
row
7
Fr :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6 6s2 4 f 14 5d 10 6p6 7s2
For Group 2A metals, the same rule applies! Magnesium (Mg) is found in row 3 of the Periodic Table, and its
valence electrons are found in the n = 3 energy level. Similarly, Radium (Ra) is found in row 7 of the Periodic
Table, and its valence electrons are found in the n = 7 energy level.
So far so good – but does the same pattern apply to the Group 3A - 8A elements (also known as Groups 13[U+0080][U+0093]18).
Let’s find out by writing the electron configuration for the Group 3A element in each row.
row
2
B:
1s2 2s2 2p1
row
3
Al :
1s2 2s2 2p6 3s2 3p1
row
4
Ga :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p1
row
5
In :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p1
row
6
Tl :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6 6s2 5d 10 6p1
Even though the valence electrons in Group 3A elements are found in both s and p orbitals, it turns out that an
element’s row still corresponds to the energy level of that element’s valence electrons. For example, Gallium (Ga)
is found in row 4 of the Periodic Table, and its valence electrons are found in the n = 4 energy level. Likewise,
Thallium (T l) is found in row 6 of the Periodic Table, and its valence electrons are found in the n = 6 energy level.
It really does seem as if we can predict the energy level of an element’s valence electrons using the row number
for that element in the Periodic Table. Let’s try one last example, though, just to be sure by writing the electron
configuration for the Group 7A element in each row.
row
2
B:
1s2 2s2 2p5
row
3
Al :
1s2 2s2 2p6 3s2 3p5
row
4
Ga :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p5
row
5
In :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p5
row
6
Tl :
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6 6s2 5d 10 6p5
Once again, an element’s row can be used to determine the energy level of that element’s valence electrons. Chlorine
(Cl), for instance, is found in row 3 of the Periodic Table, and its valence electrons are found in the n = 3 energy
level. Similarly, Iodine (I) is found in row 5 of the Periodic Table, and its valence electrons are found in the n = 5
energy level.
You should make note of one final point when it comes to energy levels and how they relate to the Periodic Table.
Our rule for determining the energy level of an element’s valence electrons using the element’s row in the Periodic
Table works for Group 1A – 8A elements. This rule doesn’t apply, however, to the Group 1B – 8B (also known as
Groups 3 – 12) elements. The elements in that lower portion of the Periodic Table (the middle portion of the Periodic
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Table ‘blacked out’ in Figure 1) are known as transition metals. They behave differently, and you can’t apply the
same rules to them as far as valence electrons are concerned. The same goes for the isolated lower portion of the
Periodic Table (also ‘blacked out’ in the figure of the Periodic Table above). This block contains elements known as
lanthanides and actinides. Like transition metals, lanthanides and actinides do not obey the same rules as the Group
1A - 8A elements when it comes to valence electrons and valence electron energy levels.
Hydrogen and Helium Occupy the First Period
You probably noticed that, in the last section, we didn’t mention the first row at all. Instead, we always started from
row 2. The first row in the Periodic Table is a ‘special row’ for several reasons. To begin with, the first row of
the Periodic Table contains only two elements – hydrogen and helium. Can you figure out why there are only two
elements in the first row?
According to what we learned in the last section, an element’s row number corresponds to the energy level of that
element’s valence electrons. Therefore, the first row must contain elements with valence electrons in the n = 1
energy level. By now you should know that there is only one orbital in the n = 1 energy level. That orbital, of
course, is the 1s orbital. Hydrogen has one valence electron in the 1s orbital, (its electron configuration is 1s1 ), and
helium has two valence electrons in the 1s orbital (its electron configuration is 1s2 ). Since it’s impossible to fit more
than two electrons into the 1s orbital, atoms with a total of three or more electrons must have valence electrons in an
energy level with n = 2 or greater. Clearly, then, atoms with a total of three or more electrons do not belong in the
first row of the Periodic Table.
The first row of the Periodic Table is also special because its elements have special properties. Hydrogen, for
example, is not a metal like the rest of the Group 1A elements. Instead, hydrogen atoms react with each other and
form what’s known as hydrogen gas, H2 . As was mentioned in the last lesson, some scientists will put hydrogen in
a category all by itself, rather than including it at the top of the 1A column. We won’t do that here, but you should
always keep in mind the fact that hydrogen is ‘different,’ and that you shouldn’t compare the hydrogen’s chemical
properties with the chemical properties of the other Group 1A elements.
Helium is also a special atom. You might wonder why helium appears at the far right-hand side of the Periodic Table,
rather than right next to hydrogen. Again, helium’s placement in the Periodic Table reflects its special chemical
properties. Earlier you learned that Group 8A elements were ‘inert’ and that includes helium. Even though helium
only has two valence electrons, while the rest of the Group 8A elements have eight valence electrons, helium is
placed on the top of the 8A column since helium’s chemical behavior is similar to the chemical behavior of the other
Noble Gases because it has a completed outer energy level.
The s Sublevel Block on PT
So far we know that, with the exception of transition metals, lanthanides and actinides, we can use the row in which
an element is found to determine the energy level of that element’s valence electrons. Can the organization of the
Periodic Table, and the placement of an element within the Periodic Table, tell us anything else about the elements
electron configuration? The answer is – ‘yes.’ Remember that the highest energy valence electrons in Group 1A
and Group 2A elements are always in s orbitals. In fact, the only valence electrons in Group 1A and Group 2A
elements are in s orbitals! Lithium, (Li) for instance, has the electron configuration 1s2 2s1 . Notice that lithium’s
single valence electron is in an s orbital. Similarly, magnesium (Mg) has the electron configuration 1s2 2s2 2p6 3s2 .
Again, notice that magnesium’s two valence electrons are in an s orbital. Since all of the valence electrons in Group
1A and Group 2A elements exist in s orbitals, the first two columns of the Periodic Table (columns 1A and 2A) are
known as the ‘s sublevel block.’ The s sublevel block is shown in the figure below. Notice that the s sublevel block
consists of all of the metals from Li down to Fr in column 1A, and all of the metals from Be down to Ra in column
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Concept 1. The Periodic Table and Electron Configurations
2A. Hydrogen is not included in the s sublevel block, again, because of its special properties.
As pictured below, the s sublevel block of the Periodic Table includes the Group 1A metals and the Group 2A metals.
(Source: CK-12 Foundation. CC-BY-SA)
The p Sublevel Block on PT
What can we say about the valence electrons in Group 3A- Group 8A elements? In particular, what can we say
about the highest energy valence electrons? If you look carefully, you’ll notice that for Group 3A Group 8A (or
if you prefer, Groups 13[U+0080][U+0093]18) elements, the highest energy valence electrons are always in p
orbitals. Boron, (B) for instance, has the electron configuration 1s2 2s2 2p1 . While boron has both 2s and 2p valence
electrons, the 2p valence electrons are higher in energy. Similarly, Krypton (Kr) has the electron configuration
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 . Again, while krypton has both 4s and 4p valence electrons, the 4p valence electrons
are higher in energy. Since the highest energy valence electrons in Group 3A Group 8A elements exist in p orbitals,
the final six columns of the Periodic Table (columns 3A through 8A) are known as the ‘p sublevel block.’ The
p sublevel block is shown in the figure below. Additionally, as illustrated in the figure below the p sublevel block
consists of all of the elements from B down to T l in column 3A, all of the elements from C down to Pb in column
4A, all of the elements from N down to Bi in column 5A, all of the elements from O down to Po in column 6A, all of
the elements from F down to At in column 7A and, finally, all of the elements from Ne down to Rn in column 8A.
Helium is not included in the p sublevel block, which should make sense, since helium has no p electrons!
As illustrated below, the p sublevel block of the Periodic Table includes the Group 3A – Group 8A elements. (Source:
CK-12 Foundation. CC-BY-SA)
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Just as the Periodic Table has an s sublevel block and a p sublevel block, it also has a d sublevel block and an f
sublevel block. Defining valence electrons in the d and f sublevel blocks can be more difficult but, in general, most
of the high energy valence electrons in the d sublevel block are in d orbitals while most of the high energy valence
electrons in the f sublevel block are in f orbitals.
As illustrated in the figure below, the Periodic Table is divided into the s sublevel block, the p sublevel block, the d
sublevel block, and the f sublevel block. (Source: CK-12 Foundation. CC-BY-SA)
A complete summary of the Periodic Table, and the information it contains about an element’s valence electrons is
shown in the figure below. (Source: http://en.wikipedia.org/wiki/File:PTable
_structure.png. GNU-FDL)
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Concept 1. The Periodic Table and Electron Configurations
Earlier you learned that the Periodic Table was a convenient way to summarize all of the information that scientists
know about the different elements found in our world. The Periodic Table probably even looked funny to you,
because you had no way of understanding what its shape and organization meant. Now that we’ve discussed electron
orbitals and electron energy levels, though, the Periodic Table shouldn’t seem so strange anymore. In fact, the shape
of the Periodic Table actually reflects the way in which electrons are organized in atoms of the different elements.
The following webpage, http://www.iun.edu/ cpanhd/periodictable.html contains an interactive periodic table that
allows you to click on the element and the web page then shows a number of physical constants for that element.
The data includes, name, density, electron configuration, heat of vaporization, heat of fusion, melting point, boiling
point, specific heat, and electronegativity.
Lesson Summary
• For Group 1A, 2A, 3A, 4A, 5A, 6A, 7A and 8A elements, the row in which an element is found corresponds
to the energy level of that element’s valence electrons. For example, lithium (Li) is in row 2, and its valence
electrons are in the n = 2 energy level.
• You can predict the energy level of an element’s valence electrons using the element’s row number in the
Periodic Table.
• Elements in the first row have special properties.
• Hydrogen is not an alkali metal, and is usually found as a gas.
• Helium is a Noble Gas, and exhibits chemical properties similar to the other Noble Gases found in Group 8A.
• The Periodic Table can be divided into s, d, p and f sublevel blocks.
• For elements in the s sublevel block, all valence electrons are found in s orbitals.
• For elements in the p sublevel block, the highest energy valence electrons are found in p orbitals.
Review Questions
1. Use the Periodic Table to determine the energy level of the valence electrons in each of the following elements.
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a.
b.
c.
d.
e.
B
Ga
Rb
At
He
2. Fill in the blanks:
a.
b.
c.
d.
e.
B is in the __ level block of the Periodic Table
Sr is in the __ level block of the Periodic Table
Fe is in the __ level block of the Periodic Table
Cs is in the __ level block of the Periodic Table
O is in the __ level block of the Periodic Table
3. Use the Periodic Table to determine the energy level and sublevel of the highest energy electrons in each of
the following elements:
a.
b.
c.
d.
e.
N
Ca
Rb
P
In
4. Decide whether each of the following statements is true or false.
a.
b.
c.
d.
e.
Li has valence electrons in the n = 1 energy level.
Si has valence electrons in the n = 3 energy level.
Ga has valence electrons in the n = 3 energy level.
Xe has valence electrons in the n = 5 energy level.
P has valence electrons in the n = 2 energy level.
5. Match the element to the sublevel block it is found in:
a.
b.
c.
d.
C - i. s sublevel block
Cs - ii. p sublevel block
Ce - iii. d sublevel block
Cr - iv. f sublevel block
6. The first row of the Periodic Table has:
a.
b.
c.
d.
e.
1 element
2 elements
3 elements
4 elements
5 elements
7. Use the Periodic Table to determine which of the following elements has the highest energy valence electrons.
a.
b.
c.
d.
e.
Sr
As
H
At
Na
8. Use the Periodic Table to determine which of the following elements has the lowest energy valence electrons.
a.
b.
c.
d.
e.
8
Ga
B
Cs
Bi
Cl
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Concept 1. The Periodic Table and Electron Configurations
9. Which energy level does the first row in the d sublevel block correspond to?
Further Reading / Supplemental Links
• http://learner.org/resources/series61.html
The learner.org website allows users to view streaming videos of the Annenberg series of chemistry videos. You are
required to register before you can watch the videos but there is no charge. The website has one video that applies
to this lesson called The Periodic Table.
• http://dl.clackamas.cc.or.us/ch104-06/valence
_electrons.htm
• http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/index.php
• http://www.cem.msu.edu/ reusch/VirtTxtJml/intro2.htm
• http://en.wikipedia.org/wiki/Valence
_electrons
Vocabulary
transition metals Elements in the d sublevel block (columns 1-B through 8B) of the Periodic Table. The highest
energy electrons in transition metals are found in d orbitals.
lanthanides and actinides Elements in the f sublevel block of the Periodic Table. The highest energy electrons
in lanthanides and actinides are found in f orbitals.
The elements in the Periodic Table in columns 1A and 2A (excluding hydrogen). All valence electrons for elements
in the s sublevel block are in s orbitals.
The elements in the Periodic Table in columns 3A through 8A (excluding helium). The highest energy valence
electrons for elements in the p sublevel block are in p orbitals.
The elements in the Periodic Table in columns 1B through 8B (also known as transition metals).
The elements in the lanthanide and actinide rows of the Periodic Table.
noble gases Group 8A elements. These are elements found in the eight column of the Periodic Table. They are
inert, which means that they are very non-reactive.
inert Non-reactive.
9