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Thulium (Tm). Diagram of the nuclear composition and electron configuration of an atom of thulium-169 (atomic number: 69), the most common isotope of this element. The nucleus consists of 69 protons (red) and 100 neutrons (blue). Thulium is a lanthanide in period 6, and the f-block of the periodic table. It melts at 1545 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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