Determine the number of formula units of NaCl in one unit cell. NaCl is face centred cubic.

Explanation

In the fcc arrangement, there are Eight corners `8 xx 1/8 =Cl^(-)` ion
At the six faces `6 xx 1/2 = 3 Cl^(-)` ions
Along the 12 edges `12 xx 1/4 = 3Na^(+)` ion
At the cube centre `(1 xx 1 = 1Na^(+)" ion")/(" Total 4 " Cl^(-)" ions")`
Hence, the unit cell contains four NaCl units.