# Periodic table (crystal structure)

The thermodynamically stable structures of metallic elements adopted at standard temperature and pressure (STP) are color-coded and shown below.[1] The only exception is mercury, Hg, which is a liquid and the structure refers to the low temperature form. The melting points of the metals (in K) are shown above the element symbol. Most of the metallic elements crystallize in variations of the cubic crystal system, with the exceptions noted. "Non-metallic" elements, like the noble gases, are not crystalline solids at STP, while others, like carbon, may have several meta stable allotropes at STP which of course can also occur for typical metals like e.g. tin.

## Table

Crystal structure of elements in the periodic table

H

He

453.69
Li
bcc
1560
Be
hcp
B C N O F Ne
370.87
Na
bcc
923
Mg
hcp
933.47
Al
fcc
Si P S Cl Ar
336.53
K
bcc
1115
Ca
fcc
1814
Sc
hcp
1941
Ti
hcp
2183
V
bcc
2180
Cr
bcc
1519
Mn

1811
Fe
bcc
1768
Co
hcp
1728
Ni
fcc
1357.8
Cu
fcc
692.68
Zn
hcp
302.91
Ga

Ge As Se Br Kr
312.46
Rb
bcc
1050
Sr
fcc
1799
Y
hcp
2128
Zr
hcp
2750
Nb
bcc
2896
Mo
bcc
2430
Tc
hcp
2607
Ru
hcp
2237
Rh
fcc
1828
Pd
fcc
1235
Ag
fcc
594
Cd

430
In

505
Sn

904
Sb

Te I Xe
301.59
Cs
bcc
1000
Ba
bcc
2506
Hf
hcp
3290
Ta
bcc
3695
W
bcc
3459
Re
hcp
3306
Os
hcp
2719
Ir
fcc
2041.4
Pt
fcc
1337.33
Au
fcc
234.32
Hg

577
Tl
hcp
600.61
Pb
fcc
544.7
Bi

527
Po

At Rn
Fr 973
Ra
bcc
Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo

1193
La
dhcp
1068
Ce
fcc
1208
Pr
dhcp
1297
Nd
dhcp
1315
Pm
dhcp
1345
Sm

1099
Eu
bcc
1585
Gd
hcp
1629
Tb
hcp
1680
Dy
hcp
1734
Ho
hcp
1802
Er
hcp
1818
Tm
hcp
1097
Yb
fcc
1925
Lu
hcp
1323
Ac
fcc
2115
Th
fcc
1841
Pa

1405.3
U

917
Np

912.5
Pu

1449
Am
dhcp
1613
Cm
dhcp
1323
Bk
dhcp
1173
Cf
dhcp
1133
Es
fcc
Fm

Md

No

Lr

Legend:
The top number in the cell is the melting point (in K)
fcc: face centered cubic (cubic close packed)
unusual structure
nonmetal
unknown or uncertain

## Unusual structures

Element crystal system coordination number notes
Mn cubic distorted bcc – unit cell contains Mn atoms in 4 different environments [1]
Zn hexagonal distorted from ideal hcp. 6 nearest neighbors in same plane- 6 in adjacent planes 14% farther away[1]
Ga orthorhombic each Ga atom has one nearest neighbour at 244 pm, 2 at 270 pm, 2 at 273 pm, 2 at 279 pm.[1] The structure is related to Iodine.
Cd hexagonal distorted from ideal hcp. 6 nearest neighbours in the same plane- 6 in adjacent planes 15% farther away[1]
In tetragonal slightly distorted fcc structure[1]
Sn tetragonal 4 neighbours at 302 pm; 2 at 318 pm; 4 at 377 pm; 8 at 441 pm [1] white tin form (thermodynamical stable above 286.4 K)
Sb rhombohedral puckered sheet; each Sb atom has 3 neighbours in the same sheet at 290.8pm; 3 in adjacent sheet at 335.5 pm.[1] grey metallic form.
Hg rhombohedral 6 nearest neighbours at 234 K and 1 atm (it is liquid at room temperature and thus has no crystal structure at ambient conditions!) this structure can be considered to be a distorted hcp lattice with the nearest neighbours in the same plane being approx 16% farther away [1]
Bi rhombohedral puckered sheet; each Bi atom has 3 neighbours in the same sheet at 307.2 pm; 3 in adjacent sheet at 352.9 pm.[1] Bi, Sb and grey As have the same space group in their crystal
Po cubic 6 nearest neighbours simple cubic lattice. The atoms in the unit cell are at the corner of a cube.
Sm trigonal 12 nearest neighbours complex hcp with 9 layer repeat, ABCBCACAB....[2]
Pa tetragonal body centred tetragonal unit cell, which can be considered to be a distorted bcc
U orthorhombic strongly distorted hcp structure. 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.[3]
Np orthorhombic highly distorted bcc structure. Lattice parameters: a=666.3 pm, b=472.3 pm, c=488.7 pm [4][5]
Pu monoclinic slightly distorted hexagonal structure. 16 atoms per unit cell. Lattice parameters: a= 618.3 pm, b=482.2 pm, c=1096.3 pm, β= 101.79 ° [6][7]

## Usual crystal structures

### Close packed metal structures

Many metals adopt close packed structures i.e. hexagonal close packed and face centred cubic structures (cubic close packed). A simple model for both of these is to assume that the metal atoms are spherical and are packed together in the most efficient way (close packing or closest packing). 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 each layer is positioned relative to others. Whilst there are many ways can be envisaged for a regular build up of layers:

• hexagonal close packing has alternate layers positioned directly above/below each other, A,B,A,B, ......... (also termed P63/mmc, Pearson symbol hP2, strukturbericht A3) .
• face centered cubic has every third layer directly above/below each other,A,B,C,A,B,C,.......(also termed cubic close packing, Fm3m, Pearson symbol cF4, strukturbericht A1) .
• double hexagonal close packing 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 (also termed P63/mmc, Pearson Symbol hP4, strukturbericht A3' ).[8]
• α-Sm packing has a period of 9 layers A,B,A,B,C,B,C,A,C,.... (R3m, Pearson Symbol hR3, strukturbericht C19).[9]

#### Hexagonal close packed

In the ideal hcp structure the unit cell axial ratio is $\scriptstyle 2\sqrt{\frac{2}{3}}$ ~ 1.633, However 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 (best are Mg and Co and worst Be with c/a ~ 1.568). 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.

#### Face centered cubic (cubic close packed)

More content relating to number of planes within structure and implications for glide/slide e.g. ductility.

#### Double hexagonal close packed

Similar as the ideal hcp structure, the perfect dhcp structure should hava a lattice parameter ratio of $\scriptstyle\frac{c}{a}=$ $\scriptstyle4\sqrt{\frac{2}{3}}$ ~ 3.267. In real dhcp structures of the 5 lanthanides (including β-Ce) $\scriptstyle\frac{c}{2a}$ variates between 1.596 (Pm) and 1.6128 (Nd). For the 4 known actinides dhcp lattices the corresponding number variate between 1.620 (Bk) and 1.625 (Cf).[10]

### Body centred cubic

This 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 included. Note that if the body centered cubic unit cell is compressed along one 4 fold axis the structure becomes face centred cubic (cubic close packed).

### Trends in melting point

Melting points are chosen as a simple, albeit crude, measure of the stability or strength of the metallic lattice. Some simple trends can be noted. Firstly the transition metals have generally higher melting points than the others. In the alkali metals (group 1) and alkaline earth metals (group 2) the melting point decreases as atomic number increases, but in transition metal groups with incomplete d-orbital subshells, the heavier elements have higher melting points. For a given period, the melting points reach a maximum at around group 6 and then fall with increasing atomic number.

In general the s-block elements have a lower melting point than d-block elements. The s-block elements have only metallic bonding. The bonding between d-block elements has degrees of both covalent and metallic character, so the strength of interactions is greater for these metals, hence the higher melting points.

## References

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
2. ^ A.F Wells (1962) Structural Inorganic Chemistry 3d Edition Oxford University Press
3. ^ Harry L. Yakel, A REVIEW OF X-RAY DIFFRACTION STUDIES IN URANIUM ALLOYS. The Physical Metallurgy of Uranium Alloys Conference, Vail, Colorado, Feb. 1974
4. ^ Lemire,R.J. et al.,Chemical Thermodynamics of Neptunium and Plutonium, Elsevier, Amsterdam, 2001
5. ^
6. ^ Lemire,R.J. et al.,2001
7. ^
8. ^
9. ^
10. ^ Nevill Gonalez Swacki & Teresa Swacka, Basic elements of Crystallography, Pan Standford Publishing Pte. Ltd., 2010
General
• Actinides and the Environment, Edited by P.A. Sterne, A. Gonis and A.A. Borovoi, NATO ASI Series, Proc. of the NATO Advanced Study Institute on Actinides and the Environment, Maleme, Crete, Greece, July 1996, Kluver Academic Publishers,. p. pp.59–61. ISBN 0-7923-4968-7.
• The Chemistry of the Actinide and Transactinide Elements, Edited by L.R. Morss, Norman M. Edelstein, Jean Fuger, 3rd. Edition, Springer 2007 ISBN 1402035551. ISBN 978-1402035555.