Unit Cell
The smallest repeating three-dimensional unit that, when stacked, reproduces the entire crystal lattice.
Formula / equation
ρ = ZM / (a³N_A)
Unit
pm
In depth
A unit cell is defined by three edge lengths and three interaxial angles, giving seven crystal systems and fourteen Bravais lattices in total. Knowing the cell dimensions and the number of formula units it contains allows crystal density to be calculated from first principles.
Examples in the real world
NaCl's cubic cell has a = 564 pm and contains four formula units, giving the observed density of 2.16 g/cm³.