Why is mercury a liquid?

Mercury stands out as the only metal that remains liquid at room temperature, with a melting point of minus 37.9 degrees Fahrenheit. While mercury's filled electron subshells make it reluctant to share electrons for bonding, the primary cause is relativistic effects that occur when electrons move near light speed in heavy elements. These quantum effects cause mercury's outer electrons to contract about 20% closer to the nucleus, dramatically reducing their availability for metallic bonding and lowering the melting point by over 360 degrees Fahrenheit below what periodic trends would predict.