Of all metals, why does mercury have the lowest melting point?

A metallic bond is characterized by free electrons and generally forms a specific geometric pattern of compact arrangement of atoms. All metals (except mercury) thus exist in a solid state at room temperature and have a specific crystal structure. Mercury belongs to the zinc group of elements in the periodic table, which comprises zinc, cadmium and mercury. They all have completely filled-up atomic orbitals. It becomes difficult to knock out these electron(s) from their orbitals and form metallic bonds. Therefore, they are soft (low melting point) and do not show multiple valence states unlike other transitional or rare earth metals. In mercury, the binding energy of outer electrons (towards nucleus) is highest and no electron is available to participate in metallic bond formation. Therefore, mercury does not have a specific crystal structure and is the only metal which is liquid at room temperature with a melting point of -38 degree Celsius.