Chemistry Reference
In-Depth Information
The horizontal rows of the periodic table are called periods. The pe-
riod that an element is in will indicate the energy level in which its valence
electrons reside. For example, sulfur is found in the third period. Using the
steps in Lesson 3-4, we can conclude that it has the electron configuration
of 1s
2
2s
2
2p
6
3s
2
3p
4
. As you can see, its valence electrons are in the third
energy level.
We can use the periodic table to check the electron configurations that
we write. The periodic table can be thought of as being made up of “s,” “p,”
“d,” and “f” sections, as shown in Figure 3-7d.
1
18
1
2
3
4
5
6
7
2
3 4 5 6 7
3456789012
s section
d section
p section
f section
Figure 3-7d
Compare Figure 3-7d to the number of electrons that each sublevel
can hold. You should recall that “s” sublevels hold 2 electrons, and the “s”
section of the periodic table is made up of 2 columns. The “p” sublevels
hold 6 electrons, and the “p” section of the periodic table is made up of 6
columns. There are 10 columns in the “d” section of the periodic table, just
as the “d” sublevels hold up to 10 electrons. The “f” sublevels hold 14 elec-
trons, and the “f” section of the periodic table has 14 columns. This con-
nection is no coincidence; rather it is a useful piece of information, which
will allow you to check your electron configurations.
Allow me to demonstrate how this information can be used to check
the electron configurations. Compare the placement of the four elements
shown on the next periodic table (Figure 3-7e) to the electron configura-
tions, which will follow.
Notice that beryllium (Be) is in the second period, which means that its
valence electrons must occupy the second energy level (n=2). Also, note
that it is in the second column of the “s” area of the periodic table. This
tells us that its valence configuration must end in “s
2
.” The combination of
these two pieces of information tells us that the valence configuration of
