1. In simple cubic unit cell particles are present only at the comers of the cube and this is shared by 8 other unit cells. Hence, the contribution of the point at the comers to the given unit cell is
∴ Z = 8 × 1/8 = 1
i.e, A simple cubic unit cell has a single constituent (atom/molecule/ion) unit per unit cell.
2. In the body-centered cubic unit cell, comers, as well as the center of the unit cell, are occupied.
∴ Z = 8 × 1/8 + 1 = 2
Hence body-centered cubic (bcc) has 2 constituent units per unit cell.
3. In face-centered cubic unit cell (fee) comers as well as face centers are occupied.
∴ Z = 8 × 1/8 + 6 + 2 = 4
Hence, the face-centered cubic unit cell has four constituent units per unit cell.
[Note: Remember that the number of atoms per unit cell is in the same ratio as the stoichiometry of the compound. Hence, it helps to predict the formula of the compound.]
