We can show how many electrons are in each 
sublevel by drawing an energy level diagram 
and filling in the arrows to represent the 
electrons.  But we’d really like to have a 
shorthand method to list the electrons and 
their sublevels.  That shorthand notation is 
called an electron configuration.  There are 
different kinds of electron configurations.  
We’ll start with the simplest, and move toward 
the most complicated.
 
 
We saw this energy level diagram for silicon 
earlier.  We could say that silicon has a 3p two 
configuration.  In this language, the 
superscript two indicates that there are two 
electrons in the 3p sublevel.  
 
We can also find this configuration from the 
periodic table.  Silicon is in the p block, and 
we can determine that the highest occupied 
orbital is 3p.  Notice that it is the second 
element into the p block in that period, so 
there would be two electrons in the 3p 
sublevel.  That gives us the configuration 3p 
two.  
 
The other item of interest here is the number 
of unpaired electrons in the highest occupied 
orbitals.  Going back to our energy level 
diagram for silicon, notice that all of the 
electrons in the levels below 3p are paired.  In 
other words, for every up arrow, there is a 
down arrow paired with it.  More technically, 
for every electron with a plus one half spin 
quantum number, there is an electron with a 
minus one half spin quantum number.  In the 
3p sublevel, the two electrons are unpaired.  
They have the same spin.  We would say that 
a silicon atom has two unpaired electrons.
 
 
We notice that to determine the number of 
unpaired electrons, we only need to look at 
the highest occupied orbitals, since all the 
electrons below those on the diagram are 
paired.
