r = 127.8 pm
r = 127.8 × 10⁻¹⁰ cm
r = 1.278 × 10⁻⁸ cm
NA = 6.02 × 10²³ mol⁻¹
M = 63.55 g/mol
From the formula, d = Z × M / (a³ × NA)
In fcc lattice, Z = 4 atoms
For fcc lattice for copper,
a = 2√2r
a³ = (2√2r)³ ⇒ a³ = 8 × 2√2 (1.278 × 10⁻⁸ cm)³ ⇒ a³ = 4.723 × 10⁻²³ cm³
d = 4 × 63.55 g/mol / (4.723 × 10⁻²³ cm³ × 6.02 × 10²³ mol⁻¹)
⇒ d = 254.2 g/mol / 28.43 = 8.94 g/cm³