Promethium
Physical properties

A promethium atom has 61 electrons, arranged in the configuration 4f 6s. The seven 4f and 6s electrons are valence electrons. 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. 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). 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; similarly, the estimate for the boiling point, ionic (Pm) radius, and standard heat of formation of monatomic gas are greater than those of samarium and less than those of neodymium.

Promethium has a double hexagonal close packed (dhcp) structure and a hardness of 63 kg/mm. This low-temperature alpha form converts into a beta, body-centered cubic (bcc) phase upon heating to 890 °C.
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