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The structure of many of the metallic and non-metallic elements can be described in terms of a close packing of equal spheres. There is a clear relationship between the structure and the position of an element in the Periodic Table. Elements in the same group tend to have the same structure at room temperature; for example, the alkali metals and Be, Mg, Zn and Cd (group IIA and IIB) are hcp; Cu, Ag and Au (group IB) are ccp. The elements of the rare-earth series crystallize in the ccp structure, the hcp structure or the 4H (ABCB) structure (e.g. Sm). The stable modification of Co at room temperature is probably hcp but it undergoes transformation to a ccp structure at high temperature.
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