Crystal Structures of Elements

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Face centered cubic or cubic close packed

This structure is exhibited by metals Ag, Al, Au, Cu, Ir, Ni, Pb, Pd, Pt, and Rh, as well as the noble gases Ar, Kr, Ne, and Xe. This arrangement (but without actual contact by atoms of the same element) is also assumed by the anions (or, less often, the cations) in numerous ionic compounds.

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Unit cell

For Ag a=409 pm

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8 unit cells
Space group Fm3m (225)

Hexagonal close packed

This structure is adopted by Be, b-Ca, Cd,a-Co, Hf, Mg, Os, Re, Ru, Sc, b-Sr, Tc, Ti, Tl, Y, Zn, and a-Zr.

Unit cell

For Mg estimated cell dimensions are a=290 and c=473 pm

Extended structure
Space group P6₃/mmc (194)

Body centered cubic

The metals Ba, Cr, Cs, a-Fe, K, Li, Mo, Na, Rb, Ta, V, and W adopt this structure. If the atom in the center of the cube is different than those at the corners, the cesium chloride structure results.

Unit cell

For Fe a=287 pm

8 unit cells
Space group Im3m (229)

Diamond

A diamond structure has a face-centered cubic arrangement of atoms plus more atoms (of the same element) in half of the tetrahedral holes. If the atoms in the tetrahedral holes are different, the zinc blende structure results.

Unit cell

For diamond a=357 pm

8 unit cells
Space group Fd3m (227)

Graphite

Graphite has a structure containing layers of atoms arranged at the corners of contiguous hexagons (not to be confused with hexagonal close packed). The ease with which layers slide against each other is consistent with the much larger distance between carbon atoms in different layers (335 pm) than between carbon atoms in the same layer (142 pm).

Unit cell.
Space group P6₃mmc (194)

For graphite a=246; c=669 pm

Extended structure showing hexagonal symmetry

Buckminsterfullerene

In the 1980s scientists characterized a new set of allotropes of carbon in which a large number of carbon atoms join to make a structure resembling the stitching on a soccer ball. The molecule C₆₀ is the most easily prepared of these allotropes and has been extensively studied. This structure and that of C₇₀ appear below.

C₆₀

C₇₀

Sulfur

Sulfur exist in three polymorphs know as orthorhombic a, monoclinic b, and monoclinic g. In all three, molecules of S₈ form rings in the shape of crowns. The structure here is that of orthorhombic a-sulfur.

S₈ unit

Unit cell
Space group Fddd (170)

Boron

Elemental boron exists in icosahedral clusters. An icosahedron is a regular polyhedron having 12 vertices and 20 faces of equilateral triangles. Althouigh the vertices may be viewed as existing in two pentagons plus additional vertices above the top pentagon and below the bottom pentagon, all vertices are equivalent. The icosahedra arrange themselves in different manners in various allotropes, one of which is shown below.

Icosahedral unit

Extended structure of a-rhombohedral boron.
Space group R3m (166)

Iodine

The halogens Cl₂, Br₂, and I₂ adopt similar structure in which diatomic molecules lie in layers. The distances between the atoms in a molecule are longer in the solid state than in the gas phase, reflecting the strength of intermolecular interactions.

Unit cell.
Space group Cmca (64)

For iodine a=714; b=469; c=978 pm

Extended structure showing layers of molecules

Black Phosphorus

Phosphorus exists in a variety of allotropes. Black phosphorus forms a layed structure in which the atoms in a layer are each bonded to three other atoms.

Unit cell.
Space group Cmca (64)

a=331a; b=1048; c=438 pm

Extended structure showing layers of phosphorus atoms.

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