Question

What is the electron configuration of iron?

Answer 1

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:

(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"`.