Chemistry Reference
In-Depth Information
A. If the atom is neutral, then the number of electrons is equal to the number
of protons. Because no charge was mentioned, we know that this is
a neutral atom. So, the number of electrons = 10.
Step 3. Consult the arrow diagram to determine the order in which the sublevels
should be filled. We look at the arrow diagram (Figure 3-4d) and we see
the order in which the sublevels are cut by the arrows. You start by
tracing the top arrow from tail to tip, as it passes through “1s,” which
becomes the first sublevel that we will fill in. When you get to the tip of
the first arrow you proceed to the next and trace it from tail to tip, as it
passes through “2s,” which becomes the second sublevel we will fill.
When we get to the tip of the second arrow, we move to the tail of the
third arrow and trace it through, and the order continues with 2p, 3s,
3p, 4s, 3d, and so forth. We won't need to fill all of these sublevels, but
we need to be able to read the order.
Step 4. Write the configuration, filling in up to 2 electrons in each “s” sublevel,
up to 6 electrons in each “p” sublevel, up to 10 electrons in each “d” sub-
level, and up to 14 electrons in each “f” sublevel. We have 1s
2
2s
2
2p
6
, and
that brings us up to 10 electrons.
Step 5. When you think that you are finished, add up the exponents (super-
scripts) to see if you have the correct number of electrons. Adding the ex-
ponents, 1s
2
2s
2
2p
6
or (2 + 2 + 6) we get 10 electrons, so our
configuration is probably correct.
The full electron configuration for the element neon (Ne):
1s
2
2s
2
2p
6
Neon has a total of 8 electrons in its valence shell, because both the 2s
and 2p sublevels are in the highest energy level. As you will learn, elements
with 8 valence electrons are said to have a complete octet and are consid-
ered chemically stable—that is, they are less likely to react.
For our next example, let's try an electron configuration for an ele-
ment that does not have a complete valence shell.
Example 2
Write the full electron configuration for the element aluminum (Al).
Step 1. Look up the atomic number of the element. The periodic table shows
us that the atomic number of aluminum is 13.
