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Promethium (Pm). Diagram of the nuclear composition and electron configuration of an atom of promethium-147 (atomic number: 61), the most common isotope of this element. The nucleus consists of 61 protons (red) and 86 neutrons (blue). Promethium is a lanthanide in period 6, and the f-block of the periodic table. It melts at 1042 degrees Celsius. The trend across the lanthanide series arises from the failure of electrons filling an inner f-subshell (here, within the 4th ring) to shield outer electrons from increasing nuclear charge.
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