What is the electron configuration of iron?

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Oct 31, 2017

Iron is on the fourth row of the periodic table, sixth column of the transition metals, atomic number #26#.

What we have is:

  • Its core orbitals are the #1s#, #2s#, #2p#'s, #3s#, and #3p#'s.
  • Its valence orbitals are the #4s# and #3d#'s.

Writing the electron configuration, you really only need the valence orbitals, and you can omit the core orbitals by notating it via the noble gas shortcut.

So, #[Ar]# can be written instead of #1s^2 2s^2 2p^6 3s^2 3p^6#.

Note that the total number of electrons in the neutral atom adds up to the atomic number, so #2+2+6+2+6 = 18#, which is the atomic number of #"Ar"#.

Finally, the #4s# orbitals are higher in energy than the #3d# orbitals by about #"3.75 eV"# when they are partially filled like this, so we write the #3d# before the #4s#.

Therefore, we get:

#color(blue)([Ar]3d^6 4s^2)#

just like it should be.

Or:

https://en.wikibooks.org/ (note, the original image was wrong. The #4s# is higher in energy, not lower, when the orbitals are filled. They are only lower in energy when they are empty.)

The long version is:

#1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2#

Indeed, #2+2+6+2+6+6+2 = 26#, the atomic number of #"Fe"#.

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