Corner atomIt is shared by six cells. One-sixth of it counts here.
Face-center atomIt is shared by two cells, so half counts here.
Interior atomThree such atoms lie wholly inside the cell. They form its middle layer.
Hexagonal baseIt is the top or bottom face of the prism. Six corner atoms ring one atom at its center.
Edge aIt is the length of one side of the hexagon.
Height cIt is the distance from the bottom face to the top face. It is not equal to a.
The stacking 3
First layer (A)It is a close-packed layer. Six spheres touch each one.
Second layer (B)Its spheres rest in hollows of the first layer.
Third layer (A)It sits directly above the first layer. That makes the order ABAB.
How this list was compiled: Every name and count checked on 2026-10-01, kept only where two independent sources agree. (1) Iowa State University, Center for Nondestructive Evaluation, NDE-Ed.org, Primary Metallic Crystalline Structures, https://www.nde-ed.org/Physics/Materials/Structure/metallic_structures.xhtml : three layers of atoms; in the top and bottom layers six atoms form a hexagon round a seventh; three atoms in the middle layer sit in the grooves of the layers beside it; six atoms in the cell, the twelve corner atoms counting one-sixth each, the two in the centers of the hexagons one-half each and the three in the middle layer whole; coordination number 12; packing factor 0.74, as for face-centered cubic; beryllium, magnesium and zinc among the examples. (2) Purdue University, Bodner Research Web, The Structure of Metals, https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch13/structure.php : planes of closest-packed spheres stacked ABAB, the third plane directly above the first; each sphere touches six in its own plane, three above and three below; 74 percent of the space is filled and no more efficient packing is known; Be, Mg and Zn; ABCABC stacking gives cubic closest packing, whose cell is face-centered cubic. (3) Wikipedia, text only: Atomic packing factor, https://en.wikipedia.org/wiki/Atomic_packing_factor (the cell is a hexagonal prism holding six atoms: three inside, the central atom of each base shared with the next cell, each atom at a vertex shared by six cells; a is the side of the base and c the height); Close-packing of equal spheres, https://en.wikipedia.org/wiki/Close-packing_of_equal_spheres (HCP is AB AB, FCC is ABC ABC; coordination number 12; packing 0.74); Hexagonal crystal family, https://en.wikipedia.org/wiki/Hexagonal_crystal_family (two equal axes a and a height c that can differ from a). (4) W. D. Callister, Fundamentals of Materials Science and Engineering (2001), section 3.4, read by full-text search at https://archive.org/details/fundamentalsofma00call : the top and bottom faces of the cell are six atoms in a regular hexagon round a single atom in the center, and a plane between them gives three more atoms; the cell holds six atoms, one-sixth of each of the twelve corner atoms, one-half of each of the two center face atoms and all three midplane interior atoms; a and c are the short and long dimensions of the cell. (5) University of Cambridge, DoITPoMS teaching package Crystallography, https://www.doitpoms.ac.uk/tlplib/crystallography3/printall.php : the hexagonal system has a equal to b and not equal to c. Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
The hexagonal close-packed (hcp) unit cell is a hexagonal prism with atoms at its corners, in the centers of two faces and inside. A second view pulls it apart into its layers, stacked in the order ABAB.
Read the full guide
Counting the atoms in one cell
Twelve corner atoms are each shared by six cells, so together they count as two. The atoms in the centers of the top and bottom faces are each shared by two cells, which makes one more. Three atoms lie wholly inside. That gives six atoms in the cell.
The stacking
Each layer is close-packed: every sphere touches six others. The second layer rests in hollows of the first, and the third sits directly above the first. The order repeats as ABAB.
Stack the same layers in the order ABCABC and the result is cubic close packing, whose unit cell is face-centered cubic.
Questions and answers
How many atoms are in an hcp unit cell?
Six. The twelve corner atoms count one-sixth each, the two face-center atoms count one-half each, and the three interior atoms count whole.
What is the coordination number of hcp?
Twelve. Each atom touches six atoms in its own layer and three each in the layers above and below.
What is the packing efficiency of hcp?
About 74 percent of the space is filled, the same as in a face-centered cubic cell. No tighter way to pack equal spheres is known.
Which metals have an hcp structure?
Magnesium, zinc and beryllium are three examples. Each crystallizes with its atoms in hexagonal close-packed layers.
What is the difference between hcp and fcc?
The order of the layers. In hcp the third layer sits directly above the first, giving ABAB. In fcc it sits over a different set of hollows, giving ABCABC.