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Almost exactly 200 years ago, sodium was isolated as an elemental metal during electrolysis of molten sodium hydroxide (caustic soda). This was announced on November 19, 1807, by Sir Humphry Davy, in the Bakerian Lecture of the Royal Society, along with another new alkali metal, potassium (from molten caustic potash). Sodium made its initial appearance as liquid metal " ... globules having a high metallic lustre", and Davy established an approximate "fusion" (or freezing) point from liquid to solid between 120F and 180F, now accepted as 208F (371 K).
The article by McMahon et al. in a recent issue of PNAS concerns the structure of a solid phase of sodium, not at the one atmosphere conditions of Davy, but at pressures corresponding to almost 4-fold relative compression. For homogeneous densification, this corresponds to reductions in average near-neighbour separations by upward of 40% that are sufficient to bring into play the hitherto mostly passive core states of the sodium ion. The arrangements assumed by the ions are surprisingly complex, and the implications for our understanding of the physics of sodium and other metals variously described as "simple" in the electronic context but now at higher densities may be considerable.

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