Why is mercury liquid at room temperature?

Mercury is unusual among metals because its atoms do not form the strong metallic bonds seen in iron or copper. Relativistic effects in its heavy atoms make those bonds weaker, which lowers the melting point dramatically. As a result, mercury remains liquid at ordinary room temperatures, unlike most metals. It freezes only below about -38.8 °C and boils at 356.7 °C. This combination of properties made mercury useful historically in thermometers, barometers, and some electrical switches, though its toxicity has led to reduced use.