Mercury (Hg). Diagram of the nuclear composition and electron configuration of an atom of mercury-202 (atomic number: 80), the most common isotope of this element. The nucleus consists of 80 protons (red) and 122 neutrons (yellow). 80 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Mercury is a transition metal in group 12, period 6, and the d-block of the periodic table. It is the only metal to be liquid at room temperature, freezing at minus 39 degrees Celsius. This is because the d-subshell shielding effect ends. It is a toxic metal, used in thermometers, barometers and manometers.