How many valence electrons does iron have?

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Answer 1 · 2014-12-15T23:12:18

Iron has 8 valence electrons.

This is tricky!

You need to have a firm grasp of what you are talking about when you use the term "valence electrons."

For main-group elements, valence electrons are the electrons in the outermost shell of the atom.

But iron is a transition metal.

Transition metals can use the electrons in their $d$ subshells as valence electrons.

Thus, valence electrons for a transition metal are defined as electrons that reside outside a noble-gas core.

For example, silicon (a main-group element) has the electron configuration $1s^2 2s^2 2p^6 3s^2 3p^2$ .

Look at the numbers (not the superscripts) first!

The outermost shell is the third shell ( $n=3$ ) and has $s$ and $p$ orbitals.

O.K. Now we can look at the superscripts (which designate the number of electrons in each orbital).

The total number of electrons in the $n=3$ shell is (2+2=)4, so silicon has 4 valence electrons.

With me so far?

O.K., so let's answer your question!

Iron has an electron configuration $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6$ or $["Ar"] 4s^2 3d^6$ .

The electrons outside the noble gas core are?

Right! The $4s^2 3d^6$ electrons.

Iron thus has 8 valence electrons!

Easy-Peasy, once you know the trick!

Note: Just because iron has 8 valence electrons doesn't mean that it will use them all.

Iron usually uses only two or three of its valence electrons to form compounds