Periodic table (crystal structure)

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The structures of metallic elements adopted at normal temperatures are colour coded and shown below1, 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) is shown above the element symbol.

Contents

Table

bcc
body centred cubic		 hcp
hexagonal close packed		 fcc
face centred cubic (cubic close packed)		 unusual structure not known / uncertain non metal



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
 301.91
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
302
Cs
bcc		 1000
Ba
bcc		 2506
Hf
hcp		 3290
Ta
bcc		 3422
W
bcc		 3186
Re
hcp		 3033
Os
hcp		 2446
Ir
fcc		 1768
Pt
fcc		 1337.33
Au
fcc		 234.32
Hg		 577
Tl
hcp		 600.61
Pb
fcc		 544.7
Bi		 Po At Rn
Fr Ra
bcc		 Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uup Uuh Uus Uuo
↓
La
 Ce
fcc		 Pr
 Nd
 Pm
hcp		 Sm Eu
bcc		 Gd
hcp		 Tb
hcp		 Dy
hcp		 Ho
hcp		 Er
hcp		 Tm hcp Yb
fcc		 Lu hcp
Ac
fcc		 Th
fcc		 Pa U Np Pu Am
hcp		 Cm
hcp		 Bk Cf Es Fm Md No Lr


Unusual structures

Metal structure family 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 neighbours in same plane- 6 in adjacent planes approx. 10% further away1
Ga orthorhombic each Ga atom has one nearest neighbour at 244pm, 2 at 270pm, 2 at 273, 2 at 279pm. 1 The structure is related to Iodine.
Cd hexagonal distorted from ideal hcp. 6 nearest neighbours in the same plane- 6 in adjacent planes approx. 10% further away1
In tetragonal slightly distorted fcc structure1
Sn tetragonal 4 at 302pm; 2 at 318pm; 4 at 377; 8 at 441pm 1
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 this structure can be considered to be a distorted hcp lattice with the nearest neghbours in the same plane being approx 16% further 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
Po cubic
La hexagonal 12 nearest neighbours "double hcp" with a layer structure ABAC...2
Pr hexagonal 12 nearest neighbours "double hcp" with a layer structure ABAC... 2
Nd hexagonal 12 nearest neighbours "double hcp" with a layer structure ABAC...2
Sm hexagonal 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
Np orthorhombic 3
Pu monoclinic

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 centerd 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) .

Hexagonal close packed

In the ideal hcp structure the unit cell axial ratio is 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. In others e.g. zinc the deviations from the ideal change the symmetry of the structure.

Face centered cubic (cubic close packed)

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

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 neighbours, 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 alkali metals (group 1) and alkaline earth metals (group 2) the melting point decreases as atomic number increases, but in the transition metals the melting points if anything increase. Across a period the melting points reach a maximum at around group 6 and then fall away with inreasing atomic number.

See also

References

  1. ^ a b c d e f g h i j Greenwood, Norman N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4. 
  2. ^ a b c d A.F Wells (1962) Structural Inorganic Chemistry 3d Edition Oxford University Press
  3. ^ hume rothery
v • d • e
Periodic tables
Layouts
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Groups
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