# How many valence electrons does iron have?

Dec 15, 2014

Iron has 8 valence electrons.

#### Explanation:

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 $1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{2}$.

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?

Iron has an electron configuration $1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} 4 {s}^{2} 3 {d}^{6}$ or $\left[\text{Ar}\right] 4 {s}^{2} 3 {d}^{6}$.

The electrons outside the noble gas core are?

Right! The $4 {s}^{2} 3 {d}^{6}$ 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