Iron has atomic number 26. Write the configuration of the atom, and find the number of electrons in…

Step 1. The atom. $[\text{Ar}]\,3d^6 4s^2$.

Step 2. $\text{Fe}^{2+}$. Remove both $4s$ electrons: $3d^6$, so $6$ electrons in $3d$. The check is $Z - 20 = 6$.

Step 3. Unpaired electrons in $\text{Fe}^{2+}$. Five boxes, six electrons. Five sit alone and the sixth pairs with one of them. That leaves $10 - 6 = 4$ unpaired electrons.

Step 4. $\text{Fe}^{3+}$. One more electron leaves from $3d$, so $3d^5$. Each of the five boxes has one electron, so $5$ are unpaired.