Chemistry Reference
In-Depth Information
If you trace the path of the arrows
correctly, you see that after you fill in
the “4s” electrons, you start to fill in the
electrons in the “3d” sublevel. This
means that, as you go across the columns
of the “d” section of the periodic table,
you are filling in electrons underneath
the higher-level “s” section.
The way this applies to iron (Fe) is
that it is found in the fourth period, in
the sixth column of the “d” section. The
fact that it is in the fourth period tells us
that its valence electrons must be in the
fourth energy level. However, the “d”
section that is being filled in last is in the
next lowest energy level. I combine this
information to predict that the configu-
ration of iron should end in “4s
2
3d
6
,”
where the “4s
2
” is actually the valence shell, because it has the highest value
for the principal quantum number (
n
).
Constructing the full electron configuration, as I reviewed in Lesson 3-4,
I get 1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
6
. We filled in the “3d” section last, but the “4s
2
”
actually represents the valence shell, so it is not uncommon to rewrite this
configuration as 1s
2
2s
2
2p
6
3s
2
3p
6
3d
6
4s
2
, or with the shorthand notation:
[Ar] 3d
6
4s
2
.
Finally, let's try the element silicon (Si), which has 14 electrons in its
cloud. Silicon is located in the second column of the “p” section of the third
period. This tells us that the electron configuration of silicon must end in
3p
2
. Checking the arrow diagram once again will remind you that before a
“p” sublevel is filled in, the “s” sublevel of the same energy level is already
filled in. In other words, to have an electron configuration that ends in 3p
2
,
silicon must have already filled in the “3s” sublevel, so we can predict that
the configuration should end in 3s
2
3p
2
. As predicted, when we construct the
full electron configuration for silicon, we get 1s
2
2s
2
2p
6
3s
2
3p
2
.
Using the periodic table to check your electron configurations will give
you a huge advantage. Anytime you are asked to construct an electron con-
figuration, find the period and column that the element is in, and predict
the end of the configuration. If your complete configuration doesn't end as
you predicted, you know to go back and check your work. There are some
The Arrow Diagram for the
Order of Filling Sublevels
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
7s
7p
Figure 3-7g
