Electronic Configurations

Electron configuration is a shorthand notation for us to designate the atomic orbitals for the electrons in a multi-electron atom. In this shorthand notation, we simply denote the subshell of the orbital and the number of electrons in each subshell. For example, nitrogen has 7 electrons. Following the Aufbau Principle and filling from lowest energy subshell to highest we would put 2 electrons in a 1s subshell, followed by 2 electrons in a 2s subshell, and 3 electrons in the 2p subshell. We would denote this by writing the subshell with a superscript that denotes the number of electrons in that subshell. For example, the

\[1s^22s^22p^3\]

This does not denote anything about the spin of the electrons. In fact you need to be able to look at this configuration and know that the 2p electrons in nitrogen will be unpaired (each in a different orbitals) based on Hund's Rule.

This notation can get a bit tedious for configurations with lots of electrons. For example, the ground state configuration for Cl is

\[1s^22s^22p^63s^23p^5\]

To simplify this we often look only at the valence electrons and denote the core electrons with the corresponding noble gas structure. For Cl this would look like.

\[ [Ne]3s^23p^5\]

We typically look at the electron configuration for neutral atoms. However, we can also look at the configuration for atomic ions. These are atoms that are "missing" electrons, or have "extra" electrons. Here the idea is exactly the same. For cations (positive ions) you would simply remove the last electrons you placed into the subshell. For anions (negative ions) you would simply add more electrons (just as you would looking at an element with more electrons). Thus the configuration for a F^- ion is

\[1s^22s^22p^6\]

Where one more electron has been placed into the 2p subshell for F^-compared to F. You can see that F^- has the same configuration that Ne would have. Since the electrons in F^- and Ne have the same configuration we call these two species isoelectronic (same electrons). You will notice that many common atomic ions are isoelectronic with the noble gases as they have a very stable (low energy) electron configuration.

Below are several videos that show you how to systematically write electron configurations for the elements.

E-Config for First 2 Rows of Periodic Table

Dr. LaBrake shows the electron configurations of the elements of the first 2 periods (rows) of the periodic table. Hund's Rule is also illustrated.

Using the Periodic Table to Learn the Aufbau Filling Order

Dr. LaBrake shows you how to use the periodic table to learn the Aufbau filling order of electrons in the atom.

MORE using the Periodic Table to Learn Electron Configurations

Dr. Deborah Walker shows the power of the periodic table when doing electron configurations. She works examples for sulfur (S) and for telluriumm (Te).

E-Config for Iron

Dr. LaBrake uses the Periodic Table to determine the electron configuration of iron. She shows both the shorthand notation and the long form.

E-Config for Bismuth

Dr. LaBrake shows yet AGAIN, the power of using the Periodic Table to determine the electron configuration for bismuth (Bi).