Dot Diagram and Electron Configuration
Electron configuration and dot diagram (also called Lewis Dot diagrams) will serve two purposes
for us. Electron configurations will help in understanding some basic rules about electrons and atoms.
Dot Diagrams will be useful in understanding how chemical compounds form and chemical reactions.
Dot diagrams are made up of 2 things, the symbol for the element and dots. The dots represent
the valence electrons or the electrons in the outer energy level. The valence electrons are the same as
the “A” group number. Each element in a group has the same number of valence electrons- but there is
one exception. You will figure out later which element it is and why. Once the number of valence
electrons is determined, you will begin placing dots around the symbol. Different text/websites will
show this done in different ways. PLACE THEM WHERE THIS PAGE INSTRUCTS YOU. There is a reason
for the pattern. If you choose to do the extra credit, you will find out why. Imagine the symbol in a box.
Place the dots on the lines that represent the sides of the box. The first 2 dots go on the right side.
Notice the word side- not corner! If there are 3 dots, place the third on top. The fourth dot goes on the
left side. The fifth dot goes on the bottom. If there is a sixth dot, it will go next to the third. The
seventh will go next to the fourth. The eight will go next to the fifth. That’s it. Answer the questions
below, the do the practice problems.
1. What do dot diagrams show?
2. What are the two parts of a dot diagram?
3. Define valence electrons.
4. Draw the dot diagram for Magnesium
5. Draw the dot diagram for chlorine.
6. Draw the dot diagram for aluminum.
Electron Configuration
In order to understand electron configuration, several basic concepts about atoms and the
periodic table need to be understood. These include:
1. How is the total number of electrons determined?
2. How are the valence electrons determined?
3. How is the number of energy levels an element has determined?
4. How many electrons can each energy level holdFirst level
Second level
Third level
Fourth level
The fifth concept that needs to be understood is called the Octet Rule (oct = 8). The octet rule
states that the maximum number of electrons the outer level can hold is eight. It doesn’t matter what
level it is- eight is the limit. Ok- stop for a second and think about that. Remember there are patterns
on the periodic table. So…
5. Of what significance is 8 on the periodic table?
Now back to a previous question in the dot diagram section. There is an exception to our dot
diagram. It is Helium.
6. How many valence electrons does helium have?
7. Check you answer to #6- how many total electrons does helium have?
8. Why is Helium placed in Group 8A?
9. Write the dot diagram for helium.
The pattern you are going to see for the electron configuration works for the first 20 elements.
There is more to the pattern than you have seen at this point. The correct way to do the electron
configuration will be shown for extra credit at a later time. Read the two examples that tell how to do
this simplified way of doing the electron configuration. Then do the next problem.
Here is how it works. Lithium has 3 electrons (atomic number 3), 1 valence electron (group A)
and 2 energy levels (period 2). The electron configuration for lithium is 2,1. The 2 numbers represent
the 2 energy levels. The 1 represents the valence electron. Added together, 2 + 1 =3, the total number
of electrons. This is the only way the configuration can work. Remember the first level cannot hold over
2 electrons and the outer level will have 1 electron (group number).
The electron configuration for Magnesium is 2,8,and 2. Here is how we get that. Magnesium is
in the 3rd period, so it has 3 energy levels. The electron configuration needs 3 numbers. The 3 numbers
added together equal 12, its atomic number. The last number in the configuration is 2, magnesium’s
group number. Other combinations mathematically may equal 12, but will not work here because of the
how many electrons each energy level can hold. For example, 1,9,2 will not work since the 2nd level
cannot hold more than 8 electrons. 3,7,2 will not work because the 1st level cannot hold more than 2
electrons.
7. Write the electron configuration for Potassium.