Technetium (Tc). Diagram of the nuclear composition and electron configuration of an atom of technetium-98 (atomic number: 43), the most common isotope of this element. The nucleus consists of 43 protons (red) and 55 neutrons (yellow). 43 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Technetium is a transition metal in group 7, period 5, and the d-block of the periodic table. It has a melting point of 2157 degrees Celsius. The transition metal trends are due to electrons filling an inner d-subshell (here, within the 4th ring), shielding the outer electrons from the increasing nuclear charge.