NaCl Stoichiometry

• The NaCl structure is derived from the FCC structure with the Cl^- ions centered at the corners and the center of each face of a cubic unit cell.

• The Na⁺ ions occupy positions centered on the cube edges between the Cl^- ions and at the center of the cube.

• Each atom at an edge of the unit cell is part of 4 unit cells; thus, it is 1/4 in the unit cell of interest.

• There are 12 edge positions occupied by Na⁺ ions. Thus 12 positions at 1/4 atom per position equals 3 atoms. One additional Na⁺ ion is in the center of the unit cell, making a total of four Na⁺ ions.

• Stoichiometry (empirical formula): Na₄Cl₄ or NaCl.