Question

How many effective sodium ions are located at the edge center of a unit cell is a sodium crystel?

Answer 1

There are 12 Na atoms at the edges of unit cell each making a contribution of 1/4. so number of sodium atoms present here is 12*1/4=3. One sodium atom at bogy centre: 1*1=1

The contribution of chlorine atom is shown below

Eight Cl atoms in corner: 8*1/8=1

Six chlorine units in phase: 6*1/2=3

So there are 4 Na atoms and 4 Cl atoms ina unit cell