Terbium (Tb). Diagram of the nuclear composition and electron configuration of an atom of terbium-159 (atomic number: 65), the most common isotope of this element. The nucleus consists of 65 protons (red) and 94 neutrons (yellow). 65 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Terbium is a lanthanide in period 6, and the f-block of the periodic table. It melts at 1356 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.