Mercury (Hg) is the only metal that is a liquid at room temperature and pressure. What makes mercury so special? It's the only-child of the liquid metal family and it hates sharing electrons.
Image from here.
Most metal atoms are solids because they readily share their valence electrons with surrounding metal atoms. As we discussed before, a metallic crystal is one in which metal atoms sit at the lattice points of the cell and the valence electrons swarm around them, acting as an electronic "glue." This is the reason metals conduct electricity—the shared electrons are free to move around all of the metal atoms.
Mercury is different. Mercury hangs on to its valence 6s electrons very tightly, just like this baby monkey riding on a pig. Because each individual mercury atom has a tighter grip on its own individual valence electrons and does not share these electrons very much with its neighbors, the mercury-mercury bond is very weak. Heat easily overcomes the weak binding and mercury boils and melts at lower temperatures than any other metal. This means it is liquid at room temperature.
Why is mercury so greedy? It turns out that these electrons are able to bury themselves very close to the positive core of the mercury atom. The electrons wiz around the very massive mercury nucleus at speeds close to the speed of light, which causes relativistic effects. We'll leave Einstein's theory of relativity for another day, but basically, these fancy relativistic effects cause the electrons to behave as if they are more massive than electrons moving at slower speeds. Because of the added "fake" mass, the electrons hang out closer to the nucleus.