Electron configurations

Energy level diagrams are useful when you need to figure out chemical reactions and bonding, but they’re very bulky to work with. Wouldn’t it be nice if there were another representation that gives just about the same information but in a much more concise form? Well, there is. It’s called the electron configuration.

The electron configuration for oxygen is 1s²2s²2p⁴. Compare that notation with the energy level diagram for oxygen in Figure 1.1.

Figure 1.1: Energy level diagram for oxygen.

Doesn’t the electron configuration take up a lot less space? You can derive the electron configuration from the energy level diagram. The first two electrons in oxygen fill the 1s orbital, so you show it as 1s² in the electron configuration.

The 1 is the energy level, the s represents the type of orbital, and the superscript 2 represents the number of electrons in that orbital. The next two electrons are in the 2s orbital, so you write 2s². And finally, you show the four electrons in the 2p orbital as 2p⁴. Put it all together, and you get 1s²2s²2p⁴.  The sum of the superscript numbers equals the atomic

number, or the number of electrons in the atom.

Here are a couple of electron configurations you can use to check your conversions from energy level diagrams:

Chlorine (Cl): 1s²2s²2p⁶3s²3p⁵

Iron (Fe): 1s²2s²2p⁶3s²3p⁶4s²3d⁶