Iridium (Ir). Diagram of the nuclear composition and electron configuration of an atom of iridium-193 (atomic number: 77), the most common isotope of this element. The nucleus consists of 77 protons (red) and 116 neutrons (yellow). 77 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Iridium is a transition metal in group 9, period 6, and the d-block of the periodic table. It has a melting point of 2466 degrees Celsius. The trends across the transition metals are due to electrons filling an inner d-subshell (here, within the 5th ring), shielding the outer electrons from the increasing nuclear charge.