Why mercury is the only metal liquid at room temperature

Mercury is the only metal that is liquid at ordinary room temperature: it does not freeze until −38.8 °C (234.3 K) and does not boil until about 357 °C, so in a house, a lab or an old thermometer it stays a dense, silvery liquid.

“The only liquid metal” is true of a cool room, not a hot one. Los Alamos National Laboratory counts five metals that are liquid at or near room temperature: mercury, caesium, gallium, rubidium and francium. Two of the near-misses need only a warm afternoon. Caesium melts at 28.5 °C, though it is so reactive that the moisture on your palm could set it off before it melted. Gallium melts at about 29.8 °C, low enough to liquefy in your hand; magicians have used gallium spoons that seem to bend by willpower.

Rubidium and francium sit just above and below caesium in the periodic table. So at 30 °C, mercury has company; at 20–25 °C, it stands alone.

Why is mercury so different? Its atoms bond weakly. As MIT materials scientist Craig Carter explains, the energy of a bond in mercury is very low, so modest warmth is enough to jumble the atoms out of a solid lattice.

The deeper cause is Einstein’s special relativity. Mercury’s heavy atom contains electrons moving at a large fraction of the speed of light, which changes how they behave and leaves the atoms reluctant to bond. In 2013 a team from New Zealand, France and Heidelberg University tested this with computer simulations published in Angewandte Chemie. Switch off the relativistic effects, they found, and solid mercury’s melting point rises by about 105 °C, to somewhere near 66 °C: a metal that would sit solid on any desk.

“Mercury frequently behaves more like an inert gas than a metal.” — Michael Wormit, Heidelberg University, 2013