Only two elements are liquid at standard room temperature: bromine and mercury. Bromine is a nonmetal made of loosely attracted molecules, so that makes some sense. Mercury is a metal, and it sits in the periodic table right next to gold, which melts at over a thousand degrees.
A strange place in the table
Mercury is in group 12, below zinc and cadmium. Going down a group, you might expect a steady trend, but mercury breaks it sharply:
| Metal | Melting point |
|---|---|
| Zinc | 420 °C |
| Cadmium | 321 °C |
| Mercury | −39 °C |
| Gold, its neighbour | 1064 °C |
| Thallium, its other neighbour | 304 °C |
Metals hold together by sharing
Atoms in a metal are bonded by sharing their outer electrons across the whole piece of metal. The more readily an atom shares, the stronger the bonding tends to be, and the higher the melting point.
Mercury has very little to offer. Its outer electrons are a filled 6s pair, sitting on top of a filled 5d subshell. Both are complete, stable arrangements, so mercury atoms hold on to their electrons and behave a little like the atoms of a noble gas.
Relativity tips the balance
Filled subshells are only part of the explanation, since zinc and cadmium have the same arrangement and are solid. What sets mercury apart is its heavy nucleus. With 80 protons pulling on them, its inner electrons move at a large fraction of the speed of light, and relativistic effects shrink the 6s orbital and bind that pair even more tightly.
In 2013, a team led by Peter Schwerdtfeger tested this with computer simulations of melting mercury, run with and without relativity. Switching relativity on lowered the melting point by about 105 °C. Without it, mercury would be a solid on a normal day.
What that means in practice
Mercury’s weak bonding explains more than its melting point. It also has a relatively low boiling point for a metal, at 357 °C, and it gives off vapour even at room temperature. That vapour is toxic, which is why mercury thermometers have largely been replaced.
It isn’t entirely alone, either. Gallium melts at about 30 °C, so it will turn liquid in your hand, and caesium melts at about 28 °C. Mercury is still the only metal that stays liquid on an ordinary day.