Update 12_band_structures_in_higher_dimensions.md - added a question

src/12_band_structures_in_higher_dimensions.md

@@ -44,6 +44,10 @@ $$ N_{states} = 2 \frac{L^3}{(2\pi)^3} \int_{BZ} dk_x dk_y dk_z  = 2 L^3 / a^3 $
 
 Here, $L^3/a^3$ is the number of unit cells in the system, so we see that a single band can host 2 electrons per unit cell (because of spin). If there are no overlapping bands, a system with 2 electrons per unit cell will therefore be an insulator/semiconductor.
 
+??? question "Can we now understand why diamond is an insulator?"
+    Hint: how many atoms per unit cell does diamond have (see exercises Lecture 7)? And how many valence electrons does a carbon atom have?
+
+
 We come to the important rule:
 
 > Any material with an odd number of electrons per unit cell is a metal.