The density is given by the formula zMa3NA
Each unit cell of NaCl has 4 Na− and 4 Cl− ions →z=4
Avagadro's number NA=6.022×1023/mol
The total mass of NaCl =22.99+34.34=58.5g/mol
⇒ Density ρ=zMa3NA=4×58.5(564×10−12)36.022×1023gm−3≈2.16×106gm−3
NaCl has a cubic unit cell. It is best thought of as a face-centered cubic array of anions with an interpenetrating fcc cation lattice (or vice-versa). The cell looks the same whether you start with anions or cations on the corners. Each ion is 6-coordinate and has a local octahedral geometry.