Periodic table (crystal structure)

This articles gives the crystalline structures of the elements of the periodic table which have been produced in bulk at STP and at their melting point and predictions of the crystalline structures of the rest of the elements.

Standard temperature and pressure

The following table gives the crystalline structure of the most thermodynamically stable form for elements that are solid at standard temperature and pressure. Each element is shaded by a color representing its respective Bravais lattice, except that all orthorhombic lattices are grouped together.

Melting point and standard pressure

The following table gives the most stable crystalline structure of each element at its melting point at atmospheric pressure Note that helium does not have a melting point at atmospheric pressure, but it adopts a magnesium-type hexagonal close-packed structure under high pressure.

Predicted structures

The following table give predictions for the crystalline structure of elements 85–87, 100–113 and 118; all but radon have not been produced in bulk. Most probably Cn and Fl would be liquids at STP. Calculations have difficulty replicating the experimentally known structures of the stable alkali metals, and the same problem affects Fr; nonetheless, it is probably isostructural to its lighter congeners. The latest predictions for Fl could not distinguish between FCC and HCP structures, which were predicted to be close in energy. No predictions are available for elements 115–117.

Structure types

The following is a list of structure types which appear in the tables above. Regarding the number of atoms in the unit cell, structures in the rhombohedral lattice system have a rhombohedral primitive cell and have trigonal point symmetry but are also often also described in terms of an equivalent but nonprimitive hexagonal unit cell with three times the volume and three times the number of atoms.

Prototype	Strukturbericht	Diagram	Lattice system	Space group	Atoms per unit cell	Coordination	notes
α-Pu			Monoclinic	P2₁/m	16		slightly distorted hexagonal structure. Lattice parameters: a = 618.3 pm, b = 482.2 pm, c = 1096.3 pm, β = 101.79°
β-S			Monoclinic	P2₁/c	32		-
α-Np	A_c		Orthorhombic	Pnma	8		highly distorted bcc structure. Lattice parameters: a = 666.3 pm, b = 472.3 pm, c = 488.7 pm
α-U	A20		Orthorhombic	Cmcm	4	Each atom has four near neighbours, 2 at 275.4 pm, 2 at 285.4 pm. The next four at distances 326.3 pm and four more at 334.2 pm.	Strongly distorted hcp structure.
α-Ga	A11		Orthorhombic	Cmce	8	each Ga atom has one nearest neighbour at 244 pm, 2 at 270 pm, 2 at 273 pm, 2 at 279 pm.	The structure is related to that of iodine.
b-P	A17		Orthorhombic	Cmce	8		Specifically the black phosphorus form of phosphorus.
Cl	A14		Orthorhombic	Cmce	8		-
α-S	A16		Orthorhombic	Fddd	16
In	A6		Tetragonal	I4/mmm	2		Identical symmetry to the α-Pa type structure. Can be considered slightly distorted from an ideal Cu type face-centered cubic structure which has.
α-Pa	A_a		Tetragonal	I4/mmm	2		Identical symmetry to the In type structure. Can be considered slightly distorted from an ideal W type body centered cubic structure which has.
β-Sn	A5		Tetragonal	I4₁/amd	4	4 neighbours at 302 pm; 2 at 318 pm; 4 at 377 pm; 8 at 441 pm	white tin form
β-B			Rhombohedral	Rm	105 315		Partly due to its complexity, whether this structure is the ground state of Boron has not been fully settled.
α-As	A7		Rhombohedral	Rm	2 6	in grey metallic form, each As atom has 3 neighbours in the same sheet at 251.7pm; 3 in adjacent sheet at 312.0 pm. each Bi atom has 3 neighbours in the same sheet at 307.2 pm; 3 in adjacent sheet at 352.9 pm. each Sb atom has 3 neighbours in the same sheet at 290.8pm; 3 in adjacent sheet at 335.5 pm.	puckered sheet
α-Sm			Rhombohedral	Rm	3 9	12 nearest neighbours	complex hcp with 9-layer repeat: ABCBCACAB....
α-Hg	A10		Rhombohedral	Rm	1 3	6 nearest neighbours at 234 K and 1 atm	Identical symmetry to the β-Po structure, distinguished based on details about the basis vectors of its unit cell. This structure can also be considered to be a distorted hcp lattice with the nearest neighbours in the same plane being approx 16% farther away
β-Po	A_i		Rhombohedral	Rm	1 3		Identical symmetry to the α-Hg structure, distinguished based on details about the basis vectors of its unit cell.
γ-Se	A8		Hexagonal	P₃21	3
Mg	A3		Hexagonal	P6₃/mmc	2	Zn has 6 nearest neighbors in same plane: 6 in adjacent planes 14% farther away Cd has 6 nearest neighbours in the same plane- 6 in adjacent planes 15% farther away	If the unit cell axial ratio is exactly the structure would be a mathematical hexagonal close packed structure. However, in real materials there are deviations from this in some metals where the unit cell is distorted in one direction but the structure still retains the hcp space group—remarkable all the elements have a ratio of lattice parameters c/a < 1.633. In others like Zn and Cd the deviations from the ideal change the symmetry of the structure and these have a lattice parameter ratio c/''a'' > 1.85.
g-C	A9		Hexagonal	P6₃/mmc	4		Specifically the graphite form of carbon.
α-La	A3'		Hexagonal	P6₃/mmc	4		The Double hexagonal close packed structure. Similar to the ideal hcp structure, the perfect dhcp structure should have a lattice parameter ratio of In the real dhcp structures of 5 lanthanides variates between 1.596 and 1.6128. For the four known actinides dhcp lattices the corresponding number vary between 1.620 and 1.625.
β-N			Hexagonal	P6₃/mmc	4
α-Po	A_h		Cubic	Pmm	1	6 nearest neighbours	simple cubic lattice. The atoms in the unit cell are at the corner of a cube.
γ-O			Cubic	Pmn	16		Closely related to the β-W structure, except with a diatomic oxygen molecule in place of each tungsten atom. The molecules can rotate in place, but the direction of rotation for some of the molecules is restricted.
α-Mn	A12		Cubic	I3m	58	Unit cell contains Mn atoms in 4 different environments.	Distorted bcc
W	A2		Cubic	Imm	2		The Body centered cubic structure. It is not a close packed structure. In this each metal atom is at the centre of a cube with 8 nearest neighbors, however the 6 atoms at the centres of the adjacent cubes are only approximately 15% further away so the coordination number can therefore be considered to be 14 when these are on one 4 fold axe structure becomes face-centred cubic.
Cu	A1		Cubic	Fmm	4		The face-centered cubic structure. More content relating to number of planes within structure and implications for glide/slide e.g. ductility.
d-C	A4		Cubic	Fdm	8		The diamond cubic structure. Specifically the diamond form of Carbon.

Close packed metal structures

The observed crystal structures of many metals can be described as a nearly mathematical close-packing of equal spheres. A simple model for both of these is to assume that the metal atoms are spherical and are packed together as closely as possible. In closest packing, every atom has 12 equidistant nearest neighbours, and therefore a coordination number of 12. If the close packed structures are considered as being built of layers of spheres, then the difference between hexagonal close packing and face-centred cubic is how each layer is positioned relative to others. The following types can be viewed as a regular buildup of close-packed layers:

Mg type has alternate layers positioned directly above/below each other: A,B,A,B,...
Cu type has every third layer directly above/below each other: A,B,C,A,B,C,...
α-La type has layers directly above/below each other, A,B,A,C,A,B,A,C,.... of period length 4 like an alternative mixture of fcc and hcp packing.
α-Sm type has a period of 9 layers A,B,A,B,C,B,C,A,C,...

Precisely speaking, the structures of many of the elements in the groups above are slightly distorted from the ideal closest packing. While they retain the lattice symmetry as the ideal structure, they often have nonideal c/a ratios for their unit cell. Less precisely speaking, there are also other elements are nearly close-packed but have distortions which have at least one broken symmetry with respect to the close-packed structure:

In type is slightly distorted from a cubic close packed structure
α-Pa type is distorted from a hexagonal close packed structure