Periodic Table (Crystal Structure) Information

The thermodynamical stable structures of metallic elements adopted at standard temperatures and pressures (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) is shown above the element symbol. Most of metallic elements are 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

Unusual structures

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 ~ 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 ~ 3.266. In real dhcp structures of the 5 lanthanides (including β-Ce) 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 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 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 metallic bond between their various atoms. the atoms of the d-block elements have covalent bond along with the metallic bond present. so the strength of interactions is more in the elements of d-block.

References

^{[11] [12]}

1. ^ ^{a b c d e f g h i j} Greenwood, N. N.; Earnshaw, A. (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. ^ URL http://cst-www.nrl.navy.mil/lattice/struk/a_c.html
6. ^ Lemire,R.J. et. al.,2001
7. ^ URL http://cst-www.nrl.navy.mil/lattice/struk/aPu.html
8. ^ URL http://cst-www.nrl.navy.mil/lattice/struk/a3p.html
9. ^ URL http://cst-www.nrl.navy.mil/lattice/struk/c19.html
10. ^ Nevill Gonalez Swacki & Teresa Swacka, Basic elements of Crystallography, Pan Standford Publishing Pte. Ltd., 2010
11. ^ 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, pp.59-61
12. ^ The Chemistry of the Actinide and Transactinide Elements, Edited by L.R. Morss, Norman M. Edelstein, Jean Fuger, 3rd. Edition, Springer 2007, ISBN-10: 1402035551, ISBN-13: 978-1402035555

External links

• Strukturbericht Type A -- structure reports for the pure elements
• Crystal Structures for the solid chemical elements at 1 bar

Categories:

• Periodic table

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