Why is mercury a liquid at room temperature?

Mercury’s atoms are so heavy (atomic number 80) that their inner electrons move at a significant fraction of the speed of light. Relativity causes these electrons to become more massive and contract inward, which in turn shields the outer electrons more effectively. The outermost electrons form a tightly held shell that is reluctant to share or bond with neighbouring mercury atoms. This weakens the metallic bonds between atoms enough that mercury melts at −39 °C, far below room temperature. Gold, its neighbour in the periodic table, shows a milder version of the same effect, which gives it its yellow colour.