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Lutetium (Lu). Diagram of the nuclear composition and electron configuration of an atom of lutetium-175 (atomic number: 71), the most common isotope of this element. The nucleus consists of 71 protons (red) and 104 neutrons (blue). Lutetium is a lanthanide in group 3, period 6, and the d-block of the periodic table. It melts at 1652 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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