Electron Configurations
In the ground state electrons occupy the lowest energy orbitals possible. This is essentially what
is referred to as the Aufbau Principle. This also infers that orbitals farther away from the positive
nucleus (higher value of n) have higher energy.
In depicting the configuration—or where the electrons are distributed throughout an atom in the
ground state—you need to be aware of four different methods: 1) orbital energy diagram, 2) orbital
diagram, 3) electron configuration, and 4) noble gas notation. You will be tested on each one of these
methods. Number 1 is the most comprehensive look at where all the electrons in a given atom/element
reside. It incorporates the Aufbau Principle in an energy hierarchy, and accounts for all of the electrons
in a given atom/element. Number 2 is usually an abbreviation of number 1 since it is usually interested in
the valence electrons, or at most will include the previous energy level electrons. Number 3 lists all of the
electrons in a given atom/element, but does not display them in an energy hierarchy. Number 4 is an
abbreviation of number 3. The mechanics of these four methods are discussed in more detail below.
Orbital Energy Diagram
3p
The diagram to the left is a representation of
where all the electrons in a sodium atom “live” in
3s
ground state. Notice that the electrons that tend to
exist farther from the nucleus are depicted in a
2p
higher energy level and sublevel. When a sublevel
has multiple orbitals (called “degenerate orbitals”)
they occur on the same energy level. Thus all the
2s
electrons in a 2p orbital have the same energy.
Orbital Diagram
This diagram (shown below) is more
1s
interested in the electrons that exist in the higher
energy orbitals; in this case, the 2s, 2p, and the 3s
orbitals. As you can see, the boxes depicting the orbitals are all on the same level. We are not interested
in showing an energy relationship. The focus
is on the lone valence electron in the 3s
orbital, and those core electrons in the next
2s
2p
3s
lowest energy level.
Electron Configuration
If you are asked on a test to give the electron configuration for an element this is what you would
write. Again, we will use the sodium as our example. The electron configuration would be written:
1s22s22p63s1. Notice that this method accounts for all eleven electrons. The main energy level (principle
quantum number) is listed first, followed by the sublevel, and the superscript number is the number of
electrons in that sublevel. This is useful up to about the 4th energy level, but beyond that it starts to
become unwieldy. For example, the electron configuration for bromine (Br) is:
1s22s22p63s23p64s23d104p5. For elements beyond this it becomes confusing, so we use an abbreviated
form; which also carries a side benefit in that it enables us to focus more closely on the valence
electrons—which are the electrons that get involved in reactions.
Noble Gas Notation
The way we abbreviate the electron configuration is by referencing the previous noble gas in
square brackets. For sodium this would be: [Ne] 3s1. For bromine it would be: [Ar] 4s23d104p5. By
writing “[Ne]” we are in essence listing the “1s22s22p6” portion (which is the electron configuration of the
noble gas neon) without having to write it out. Similarly, “[Ar]” represents the “1s22s22p63s23p6” portion,
which is the electron configuration of argon, the previous noble gas to bromine. A note of caution: If
your instructor asks you for the electron configuration do not use noble gas notation, you will lose points.
Similarly, if the instructor asks for noble gas notation, do not write out the whole electron configuration;
these are different methods for depicting the electron distribution within an atom and are not to be
interchanged on a test or quiz.