Chemistry
Section
Name:
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ENERGY LEVEL DIAGRAMS
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Date:
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Introduction: If you know how to read the periodic table there is a lot of information that can
be gathered about the electrons in an atom. We have already learned that the number of
electrons in the outermost part of an atom (the valence shell) can be read from the older
group notations (the Roman numeral ones). For example, oxygen is in group VI A and
therefore has 6 electrons in its outermost energy level (or, oxygen has 6 valence electrons).
Later, we'll use this information as a quick double-check.
To place electrons around atoms, you must know that electrons are defined by layers
called energy levels and sub layers called sublevels. Energy levels are identified by numbers
and you only need numbers 1 through 7 (look at the numbers on the left side of the Periodic
Table) to list the energy levels of every atom on the periodic table. Only 4 sublevels are
required for the periodic table and the sublevels are identified by the letters s, p, d or f. Let's
start with the periodic table on page 183 in your text book and we will learn how to read the
sublevel right off the chart. Note that this table shows four blocks, the s-block, the p-block,
the d-block, and the f-block. You must memorize these blocks and you must be able to
identify them on your reference tables. These blocks tell you the sublevel.
Now look at the periodic table in your reference tables. Every new element on the table
adds one new electron in the atom so that in neutral atoms the total number of electrons will
be equal to the atomic number for that atom. The period numbers along the left edge of that
table tell you the highest numbered energy level of all the atoms in that row or period. Let's
pick cobalt (Co, atomic number 27) as an example. The cobalt atom will have 4 energy
levels (because Co is in period 4) and the last 7 electrons (the 21st through 27th electrons)
will go into a d sublevel. Let's start out easy and practice identifying energy level and
sublevel for a few atoms.
Write the number of energy levels you expect to see for each of the following atoms.
1. 3Li
2
2. 8O
2
3.
24Cr
4
4.
50Sn
5
Write the block (sublevel) in which you find each of the following atoms.
5.
11Na
s
6.
20Ca
s
7.
63Eu
f
8.
80Hg
d
Probing further: Now that you know how to find the number of energy levels and to identify
blocks, let's learn how to list the energy levels for electrons in atoms. While this information
can be read electron by electron off any periodic table, your 2011 reference tables give it
directly. Look at 20Ca as our first example. We have already learned that Ca should have 4
energy levels (it is in period 4) and there should be 2 e– in the valence shell (outermost
energy level). However, if you look at the very bottom of the box for Ca, you will see the
numbers 2-8-8-2. The regents board calls this the electron configuration. More strictly, there
are several methods of writing the electron configuration and this particular method is called
Created 11 November 2015
an energy level diagram, but since energy level diagrams are the only full electron
configuration required by the board of regents, they just use the term electron configuration.
Do note that 57La has an electron configuration of 2-8-18-18-9-2 while the element next door,
72Hf, has the numbers -18-32-10-2. The diagram for Hf has been shortened by leaving out
the first two energy levels in order to fit in the small boxes. When you write it out, you
should include the first two energy levels like this: 72Hg: 2-8-18-32-10-2. Use the same
technique for all the elements past 72Hf.
Do note that no configurations are given for elements in the f block. Look at 41Nb and
29Cu. A theoretical application of the aufbau principle would predict the following electron
configurations:
41Nb 2-8-18-11-2
29Cu: 2-8-17-2
The actual experimentally determined configurations are listed on the table:
41Nb 2-8-18-12-1
29Cu: 2-8-18-1
There are several examples like this in the d-block but the f-block has many more shifts
across multiple sublevels (s, d, and f) instead of just the s and d blocks. Such shifts are a
result of the fact that the e– probability function shapes (sublevels) are pulled toward the
nucleus as more protons are added to the nucleus. Because the s, p, d, and f sublevels all
have differing shapes, they are pulled toward the nucleus at differing rates and therefore
some crossovers occur.
Let's try a few examples of electron configurations.
Write electron configurations (energy level diagrams) for each of the following atoms. Identify
elements in which sublevel shifting of e– has occurred by placing an asterisk (*) in front of the
problem number.
9. 3Li
2-1
10. 8O
2-6
12.
20Ca
2-8-8-2
11.
11Na
2-8-1
13.
21Sc
2-8-9-2
* 14.
24Cr
2-8-13-1
15.
26Fe
2-8-14-2
* 16.
47Ag
2-8-18-18-1