Electron Configuration Notation:

-shows the arrangment of electrons around the nucleus of an atom.

- helps chemist understanding how elements form chemical bonds.

- can be written using the period table or an electron configuration chart.

How to Write the Electron Configuration for Chromium (Cr, Cr2+, and Cr3+)

In order to write the Chromium electron configuration we first need to know the number of electrons for the Cr atom (there are 24 electrons). Once we have the configuration for Cr, the ions are simple. When we write the configuration we'll put all 24 electrons in orbitals around the nucleus of the Chromium atom.

NOTE: Chromium is an exception to the rules for writing electron configurations!

In writing the electron configuration for Chromium the first two electrons will go in the 1s orbital. Since 1s can only hold two electrons the next 2 electrons for Chromium go in the 2s orbital. The next six electrons will go in the 2p orbital. The p orbital can hold up to six electrons. We'll put six in the 2p orbital and then put the next two electrons in the 3s. Since the 3s if now full we'll move to the 3p where we'll place the next six electrons. We now shift to the 4s orbital where we place the remaining two electrons. After the 4s is full we put the remaining four electrons in the 3d orbital and end with 3d4.

Therefore the expected electron configuration for Chromium will be 1s²2s²2p⁶3s²3p⁴4s²3d⁹.

Note that when writing the electron configuration for an atom like Cr, 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 (here is an explanation why).

Therefore we have (still incorrect) 1s²2s²2p⁶3s²3p⁶3d⁴4s²

Correct Electron Configuration for Chromium (Cr)

Half-filled and fully filled subshell have got extra stability. Therefore, one of the 4s2 electrons jumps to the 3d5 so that it is half-filled (see video below). This give us the (correct) configuration of:

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

For the Cr²⁺ ion we remove one electron from 4s1 and one from the 3d5 leaving us with: 1s²2s²2p⁶3s²3p⁶3d⁴

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

1s²2s²2p⁶3s²3p⁶3d³

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.