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Cobalt (Co). Diagram of the nuclear composition and electron configuration of an atom of cobalt-59 (atomic number: 27), the most common isotope of this element. The nucleus consists of 27 protons (red) and 32 neutrons (blue). Cobalt is a transition metal in group 9, period 4, and the d-block of the periodic table. It has a melting point of 1495 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.
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