Diagram of the nuclear composition and electron configuration of an atom of hafnium-178 (atomic number: 72), the most common isotope of this element. The nucleus consists of 72 protons (blue) and 106 neutrons (red). Hafnium is a transition metal in group 4, period 6, and the d-block of the periodic table. It has a melting point of 2233 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.