Number of atoms in the unit cell of Na (bcc type crystal) and Mg (fcc type crystal) are, respectively

In a cubic cell, the total volume of an atom belongs to eight corners, hence one unit cell has only 1/8 part of the corner atom. Since, BCC unit cell has one atom at the center of the cube along with 8 atoms at corners of the cube, Therefore total atoms in BCC = 1× 8 + 1 8 = 2 atoms An FCC unit cell has total six faces and each face has one atom. But the facial atom is always divided between two unit cells, henceforth only half part of facial atom belongs to one unit cell. From above discussion, it is clear that the number of atoms in FCC cell = 1× 8 + 3 8 = 4 atoms Hence the correct answer is