Europium (Eu). Diagram of the nuclear composition and electron configuration of an atom of europium-153 (atomic number: 63), the most common isotope of this element. The nucleus consists of 63 protons (red) and 90 neutrons (yellow). 63 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Europium is a lanthanide in period 6, and the f-block of the periodic table. It melts at 826 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.