Hexagonal Close Packing
A close-packed arrangement in which layers of spheres repeat in an ABAB sequence.
Formula / equation
6 atoms per cell, 74% packing
Unit
Dimensionless
In depth
Hexagonal and cubic close packing achieve identical 74% efficiency and coordination number twelve, differing only in whether the third layer sits above the first or in the remaining set of hollows. The lower symmetry of hcp gives fewer slip planes, making hcp metals less ductile.
Examples in the real world
Magnesium, zinc, titanium and cobalt; zinc's brittleness at room temperature reflects its hcp structure.