Electron Configuration for Cr, Cr2+, and Cr3+ (Exception to Rules)

To write the configuration for the Chromium ions, first we need to write the electron configuration for just Chromium (Cr). We first need to find the number of electrons for the Cr atom (there are 24 electrons) using the Periodic Table.

When we write the configuration, we'll put all 24 electrons in orbitals around the nucleus of the Chromium atom.

In this video we'll start with using the Periodic Table to help us write the notation for Chromium. Alternatively you can use a chart showing how the orbitals fill (   • Using the Electron Configuration Chart  ).

This would give us the (incorrect) Chromium electron configuration of

1s2 2s2 2p6 3s2 3p6 3d4 4s2

--Chromium is an exception! --

Half-filled and fully filled subshell have got extra stability.

Therefore, one of the 4s2 electrons jumps to the 3d4. This give us the (correct) configuration of:

1s2 2s2 2p6 3s2 3p6 3d5 4s1

Note that when writing the electron configuration for an atom like Cu, the 3d is usually written before the 4s. Both of the configurations have the correct numbers of electrons in each orbital, it is just a matter of how the electronic configuration notation is written (see below for link to an explanation why).

For the Cr2+ ion we remove an electron from 4s1 and 3d5 leaving us with:

1s2 2s2 2p6 3s2 3p6 3d4 4s0


For the Cr3+ ion we remove a total of three electrons (one from the 4s1 and two from the 3d5) leaving us with

1s2 2s2 2p6 3s2 3p6 3d3 4s0

• Introduction to Writing Electron Configurations:    • Introduction to Electron Configurations  
• Electron Configurations Chart:    • Using the Electron Configuration Chart  
• Writing Electron Configs Using only the Periodic Table:    • Writing Electron Configurations Using...  
• Order of d and s Orbital Filling: https://eic.rsc.org/Coature/the-troub...

The configuration notation provides an easy way for scientists to write and communicate how electrons are arranged around the nucleus of an atom. This makes it easier to understand and predict how atoms will interact to form chemical bonds.