For elements that are solid at
standard temperature and pressure
Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...
the table gives the crystalline structure of the most thermodynamically stable form(s) in those conditions. In all other cases the structure given is for the element at its melting point (H, He, N, O, F, Ne, Cl, Ar, Kr, Xe, and Rn are gases at STP; Br, Hg, and probably Cn and Fl are liquids at STP). Data is presented only for the elements that have been produced in bulk (the first 99, except for
astatine
Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-li ...
and
francium
Francium is a chemical element with the symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium K after the natural decay chain it appears in), has a half-life of only 22 ...
). Predictions are given for astatine, francium, elements 100–113 and 118. The latest predictions for flerovium (element 114) could not distinguish between face-centred cubic and hexagonal close-packed structures, which were predicted to be close in energy.
No predictions are available for elements 115–117.
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 is how each layer is positioned relative to others. Whilst there are many ways that can be envisaged for a regular buildup 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 packi