The unit cell of a NaCl lattice

In NaCl crystal Cl- occupies the corners and face centre of the unit cell so the total no. of Cl- ions in unit cell is 1 1 8 6 1 3 4Cl ions 8 2 x x ( contribution of atoms at corners towards unit cell is 1/8 and at face centre is ½) Na+ Ocupies the octahedral voids in Fcc unit cell we know that octahedral voids are equal to the no. of effective atoms in unit cell. So in fcc no. of atoms = 4 = 4 Oh voids = 4Na+ ion 4Na 4Cl 4NaCl -> Hence the no. of NaCl unit = 4