Q.Write the complete ground-state electronic configuration of iron ($Z = 26$) and find the number of unpaired electrons in it.

Iron has $Z = 26$ electrons. Filling in Aufbau order gives

which, written in the conventional shell order, is

(Iron is not one of the exceptions like chromium or copper; it follows the standard Aufbau prediction.)

To find the number of unpaired electrons, apply Hund's rule to the $3d^6$ subshell, which has 5 orbitals. The first 5 electrons occupy all 5 orbitals singly with parallel spin; the 6th electron must then pair up in one of them (conventionally the first). So the occupancy is:

$3d_{xy}\!\uparrow\downarrow \quad 3d_{yz}\!\uparrow \quad 3d_{xz}\!\uparrow \quad 3d_{x^2-y^2}\!\uparrow \quad 3d_{z^2}\!\uparrow$ …

Fill the configuration in Aufbau order using the total electron count, then apply Hund's rule within the $3d^6$ subshell to coun …

Treating iron as an exception like chromium/copper and writing $3d^7\,4s^1$ (iron is NOT such an exception); counting all 6 $d$ electrons as …