In order to write the Iron electron configuration we first need to know the number of electrons for the Fe atom (there are 26 electrons). Once we have the configuration for Fe, the ions are simple. When we write the configuration we'll put all 26 electrons in orbitals around the nucleus of the Iron atom.
Video: Fe, Fe 2+ , and Fe 3+ Electron Configuration Notation
In writing the electron configuration for Iron the first two electrons will go in the 1s orbital. Since 1s can only hold two electrons the next 2 electrons for Iron 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 six electrons in the 3d orbital and end with 3d6.
Therefore the Iron electron configuration will be 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 .
Note that when writing the electron configuration for an atom like Fe, 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 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2
For the Fe2+ ion we remove two electrons from 4s2 leaving us with: 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6
For the Fe3+ ion we remove a total of three electrons (two from the 4s2 and one form the 3d6) leaving us with 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 1s2 2s2 2p6 3s2 3p6 3d5
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.
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