Question: how many atoms are there in (100) of fcc crystal?
Answer: For the (100) plane, there are 4 atoms at the 4 corners and one atom in the middle. One fourth of each corner atom is enclosed within the unit cell, and middle atom is entirely within the unit cell, so the number of atoms on the (100) plane within the unit cell is N100 = 4 × (1/4) + 1 × 1 = 2.