A close-packed ABAB layer sequence with coordination number 12 and efficiency 74 percent.
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Hexagonal close packing is a close-packed arrangement in which layers of spheres stack in an ABAB sequence, with each layer sitting in alternate positions. Each atom has a coordination number of twelve and a packing efficiency of about 74 percent, the same as face-centred cubic. The ideal ratio of the unit cell height c to the base edge a is about 1.633, although real metals deviate slightly from this value. Metals such as magnesium, zinc and titanium crystallize in this structure. The main difference from face-centred cubic is the stacking sequence rather than the packing density.
Key ideas: Per hexagonal cell Z = 6, or 2 per primitive cell. The c/a ratio is sqrt(8/3) = 1.633. Layer sequence ABAB differs from fcc ABCABC; both have 12 neighbours and 74 percent packing. Void counts match fcc, with 1 octahedral and 2 tetrahedral voids per atom. Mg, Zn, Cd, Ti and Co adopt it.
Common mistakes and connections: Students often think hcp is less dense than fcc, but both fill the same fraction. Another error is mixing up the stacking sequences. The hcp structure has fewer slip systems, so hcp metals like zinc are less ductile than fcc metals. Links to fcc, voids, close packing and the layers in ionic structures.
Hexagonal close packing stacks ABAB layers with coordination number 12 and 74% efficiency; the hexagonal layer shown is the building plane, as in Mg, Zn and Ti. The interactive lesson shows this as a live animation or chart.
Magnesium crystallizes in hexagonal close packing, with a c/a ratio close to the ideal value of 1.633.
A close-packed ABAB layer sequence with coordination number 12 and efficiency 74 percent.
Hexagonal Close Packing is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in States of Matter & Solutions first: Simple Cubic Lattice, Body-Centered Cubic, Face-Centered Cubic.
The next topics in States of Matter & Solutions are Packing Efficiency and Tetrahedral and Octahedral Voids.
Yes. The interactive lesson includes a self-check question on Hexagonal Close Packing, and the States of Matter & Solutions quiz draws on every topic in this area.
Hexagonal Close Packing is topic 61 of 109 in States of Matter & Solutions.