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The thermodynamically stable structures of metallic elements adopted at [[Standard conditions for temperature and pressure|standard temperature and pressure (STP)]] are color-coded and shown below.<ref name = "Greenwood">{{Greenwood&Earnshaw}}</ref> The only exception is [[mercury (element)|mercury]], Hg, which is  a liquid and the structure refers to the low temperature form. The melting points of the metals (in [[Kelvin|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 of carbon|allotropes]] at STP which of course can also occur for typical metals like e.g. [[tin]].
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==Table==
 
{{periodic table (crystal structure)}}
 
==Unusual structures ==
 
{| class="wikitable"
! Element!! [[crystal system]]!! coordination number!! notes
|-
| [[manganese|Mn]] || cubic ||distorted  bcc – unit cell contains Mn atoms in 4 different environments <ref name = "Greenwood"/>|| 
|-
|[[zinc|Zn]] || [[Hexagonal crystal system|hexagonal]] ||distorted from ideal hcp.  6 nearest neighbors in same plane- 6 in adjacent planes 14% farther away<ref name = "Greenwood"/>|| 
|-
| [[gallium|Ga]] || [[Orthorhombic crystal system|orthorhombic]] || each Ga atom has one nearest neighbour at 244 pm, 2 at 270 pm, 2 at 273 pm, 2 at 279 pm.<ref name = "Greenwood"/> || The structure is related to Iodine.
|-
| [[cadmium|Cd]] || hexagonal ||distorted from ideal hcp. 6 nearest neighbours in the same plane- 6 in adjacent planes 15% farther away<ref name = "Greenwood"/>||
|-
| [[indium|In]]|| [[Tetragonal crystal system|tetragonal]] || slightly distorted fcc structure<ref name = "Greenwood"/>||
|-
| [[tin|Sn]] || tetragonal ||4 neighbours at 302 pm; 2 at 318 pm; 4 at 377 pm; 8 at 441 pm <ref name = "Greenwood"/>|| white tin form (thermodynamical stable above 286.4 K)
|-
|[[antimony|Sb]]|| [[Rhombohedral crystal system|rhombohedral]]  || puckered sheet; each Sb atom has 3 neighbours in the same sheet at 290.8pm; 3 in adjacent sheet at 335.5 pm.<ref name = "Greenwood"/>  ||grey metallic form.
|-
| [[mercury (element)|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 <ref name = "Greenwood"/>
|-i
|[[bismuth|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.<ref name = "Greenwood"/>          || Bi, Sb and grey As have the same space group in their crystal
|-
| [[polonium|Po]] || cubic || 6 nearest neighbours            || simple cubic lattice. The atoms in the unit cell are at the corner of a cube.
|-
| [[samarium|Sm]]|| trigonal || 12 nearest neighbours  || complex hcp with 9 layer repeat, ABCBCACAB....<ref>A.F Wells (1962) ''Structural Inorganic Chemistry'' 3d Edition  Oxford University Press</ref>
|-
| [[protactinium|Pa]] || tetragonal || body centred tetragonal unit cell, which can be considered to be a distorted bcc|| 
|-
| [[uranium|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.<ref>Harry L. Yakel, ''A REVIEW OF X-RAY DIFFRACTION STUDIES IN URANIUM ALLOYS''. The Physical Metallurgy of Uranium Alloys Conference, Vail, Colorado, Feb. 1974</ref> || 
|-
| [[neptunium|Np]] ||orthorhombic|| highly distorted bcc structure. Lattice parameters: a=666.3 pm, b=472.3 pm, c=488.7 pm <ref>Lemire,R.J. et al.,''Chemical Thermodynamics of Neptunium and Plutonium'', Elsevier, Amsterdam, 2001</ref><ref>URL http://cst-www.nrl.navy.mil/lattice/struk/a_c.html</ref>         ||
|-
| [[plutonium|Pu]] || [[Monoclinic crystal system|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 ° <ref>Lemire,R.J. et al.,2001</ref><ref>URL http://cst-www.nrl.navy.mil/lattice/struk/aPu.html</ref> ||
|}
 
==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 [[space group|P6<sub>3</sub>/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, [[space group|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 [[space group|P6<sub>3</sub>/mmc]], Pearson Symbol hP4, strukturbericht  A3' ).<ref>URL http://cst-www.nrl.navy.mil/lattice/struk/a3p.html</ref>
*α-Sm packing has a period of 9 layers A,B,A,B,C,B,C,A,C,.... ([[space group|R3m]], Pearson Symbol hR3, strukturbericht C19).<ref>URL http://cst-www.nrl.navy.mil/lattice/struk/c19.html</ref>
 
====Hexagonal close packed====
In the ideal hcp structure the unit cell [[axial ratio]] is <math>\scriptstyle 2\sqrt{\frac{2}{3}}</math> ~ 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 <math>\scriptstyle\frac{c}{a}=</math>
<math>\scriptstyle4\sqrt{\frac{2}{3}}</math> ~ 3.267. In real dhcp structures of the 5 lanthanides (including β-Ce) <math>\scriptstyle\frac{c}{2a}</math> 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).<ref>Nevill Gonalez Swacki &amp; Teresa Swacka, ''Basic elements of Crystallography'', Pan Standford Publishing Pte. Ltd., 2010</ref>
 
===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 metal]]s (group 1) and [[alkaline earth metal]]s (group 2) the melting point decreases as atomic number increases, but in transition metal groups with incomplete [[Electron shell|d-orbital subshells]], the heavier elements have higher melting points. For a given [[Period_(periodic_table)|period]], the melting points reach a maximum at around [[group 6 element|group 6]] and then fall with increasing atomic number.
 
==See also==
*[[Crystal structure]]
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==
{{reflist|30em}}
 
;General
*{{cite book|title=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, |isbn=0-7923-4968-7|page= pp.59–61}}
*{{cite book|title=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}}
 
==External links==
*[http://cst-www.nrl.navy.mil/lattice/struk/atype.html  Strukturbericht Type A  – structure reports for the pure elements]
*[http://wwwhomes.uni-bielefeld.de/achim/ele_structures.html  Crystal Structures for the solid chemical elements at 1 bar]
 
{{PeriodicTablesFooter}}
 
[[Category:Periodic table]]

Revision as of 09:47, 1 March 2014

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