Niobium (Nb). Diagram of the nuclear composition and electron configuration of an atom of niobium-93 (atomic number: 41), the most common isotope of this element. The nucleus consists of 41 protons (red) and 52 neutrons (yellow). 41 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Niobium is a transition metal in group 5, period 5, and the d-block of the periodic table. It has a melting point of 2477 degrees Celsius. The trends across the transition metals are due to electrons filling an inner d-subshell (here, within the 4th ring), shielding the outer electrons from the increasing nuclear charge.