Question

NaCl crystallizes in a cubic unit cell with Cl- ions on each corner and each face. How many Na+ and Cl- ions are in each unit cell of NaCl?

Answer 1

it's 4 Na+ ions and 4 Cl- ions per unit cell:

Na+ Cl- and behind it Cl- Na+
Cl- Na+ ..................... Na+ Cl-

to make a 2x2x2 cubic unit cell.The NET number of sodium cations per unit cell is FOUR, and the NET number of chloride anions per unit cell is also FOUR, so Z (the multiple of the simplest/empirical formula present in the cell) is 4. The 4 : 4 or 1 : 1 ratio of Na : Cl ions present in the unit cell is responsible for the chemical formula of NaCl.