Scandium (Sc). Diagram of the nuclear composition and electron configuration of an atom of scandium-45 (atomic number: 21), the most common isotope of this element. The nucleus consists of 21 protons (red) and 24 neutrons (yellow). 21 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Scandium is a transition metal in group 3, period 4, and the d-block of the periodic table. It has a melting point of 1541 degrees Celsius. The trends across the transition metals are due to electrons filling an inner d-subshell (here, within the 3rd ring), shielding the outer electrons from the increasing nuclear charge.