METALS are solid and they only melt when heated to their melting point.
Different metals melt at different temperatures and like matter, the atoms of a metal are closely bonded.
However, mercury is the only metal which remains in liquid state at room temperature and pressure.
Unlike all other metals, mercury atoms do not share valence electrons with other
neighbouring atoms. Rather, the electrons in mercury atoms are more tightly bound to
nucleus and they move so quickly that they exhibit relativistic effects.
This means that the electrons behave as if they are larger than slow-moving electrons.
Due to this weak bonding, mercury has low melting as well as boiling points. In other words it requires only a small amount of heat or energy to break the binding between mercury atoms. – BY SCIENCE BRIDGE ACADEMY