- Periodic table (crystal structure)
The structures of metallic elements adopted at normal temperatures are colour coded and shown below Greenwood&Earnshaw] , 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.
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 packingor 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.
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