Editing Promethium (section)

===Physical properties===
A promethium atom has 61 electrons, arranged in the [[electron configuration|configuration]] &#91;[[xenon|Xe]]&#93;&nbsp;4f<sup>5</sup>&nbsp;6s<sup>2</sup>. The seven 4f and 6s electrons are [[valence electron|valence]] electrons.<ref name="Cotton">{{Greenwood&Earnshaw|page=1233}}</ref> In forming compounds, the atom loses its two outermost electrons and one 4f-electron, which belongs to an open subshell. The element's atomic radius is the second largest among all the lanthanides but is only slightly greater than those of the neighboring elements.<ref name="Cotton" /> It is the most notable exception to the general trend of the contraction of lanthanide atoms with the increase of their atomic numbers ([[lanthanide contraction]]<ref>{{Cotton&Wilkinson5th|pages=776, 955}}</ref>). Many properties of promethium rely on its position among lanthanides and are intermediate between those of neodymium and samarium. For example, the melting point, the first three ionization energies, and the hydration energy are greater than those of neodymium and lower than those of samarium;<ref name="Cotton" /> similarly, the estimate for the boiling point, ionic (Pm<sup>3+</sup>) radius, and standard heat of formation of monatomic gas are greater than those of samarium and less than those of neodymium.<ref name="Cotton" />

Promethium has a [[Close-packing of equal spheres|double hexagonal close packed]] (dhcp) structure and a hardness of 63&nbsp;kg/mm<sup>2</sup>.<ref name="str" /> This low-temperature alpha form converts into a beta, [[Cubic crystal system|body-centered cubic]] (bcc) phase upon heating to 890&nbsp;°C.<ref name="CRCrare" />