Most metal elements melt at temperatures of hundreds of degrees, but for mercury that is -38.9° C (-38.0° F). So why is this metal different from all others? It’s all about the outer electrons and a combination of factors that make them bond unusually poorly.
The first thing to note is that the title question may not be entirely accurate. There may be two transuranic elements, which don’t appear in nature because they decay far too quickly to have survived from their creation in supernovae or kilonovae that are liquid at room temperature. The same short half-lives that mean they have to be produced artificially means we don’t get much time to study them. Copernicium and Flerovium are suspected of being liquid at room temperature, but since one lasts seconds before decaying, and the other even less, there’s a fair degree of uncertainty about this. We certainly haven’t been making a lot of either to study.
Leaving these curiosities aside, mercury stands out among stable elements. At the simplest level, the reason is that mercury’s outermost electrons don’t bond to very strongly, weakening the pull between one mercury atom and another. That weakness means that as soon as mercury picks up even quite a modest amount of energy the organization of a solid breaks down and the atoms start moving around more freely.
Another way to look at this is that when atoms bond together some of their kinetic energy is converted to the energy of the bond. There’s so little energy in mercury’s bonds with itself that it doesn’t take a lot of movement to break them apart. Since at the atomic level the random kinetic energy amounts to heat, mercury doesn’t need to be warm, let alone hot, to become liquid, but other metals, with more energy stored in their bonds, do.
Mercury’s liquid status was known more than three thousand years ago, but it’s not something we would have predicted had the element only been discovered as the periodic table was being filled in. Most familiar liquids have quite low density, so encountering a liquid so far down the periodic table goes quite against our expectations. Its neighbors on the periodic table, gold, and thallium, melt at more than 1000 and 300 degrees centigrade respectively. It is useful though: mercury’s combination of density and being liquid is why it is so well suited to thermometers, barometers, and measuring blood pressure.
So what is it about mercury’s outer electrons that lead to bonding so much weaker than its fellow metals? It turns out mercury is in a sweet spot on the table where three effects combine. The first is that its outer electron shell is full. It’s much easier for electrons in a partially filled shell to escape, becoming part of a fog of valence electrons that hold atoms together. Metals with more easily shared electrons to share around usually have higher melting points, certainly far higher than room temperature.
Mercury isn’t the only metal with a full shell, however, so that can’t be the only reason. Both the other two factors cause the outer electrons of affected atoms to stay closer to their nucleus, interfering with their capacity to bond with other atoms.
Members of the lanthanide series of elements, which share mercury’s sixth period on the periodic table, experience what is known as “lanthanide contraction”. The electrons of the 4f subshell shell don’t shield electrons further out from the positive charge of the nucleus as much as others, causing the outer electrons to be pulled inwards. Consequently, most of the elements in period 6 have atomic radii of similar size to those on the period above them, leading to much greater density.
Moreover, mercury’s outer electrons experience a relativistic contraction, moving so fast that the effects of approaching the speed of light come into play. This is something that only really matters with heavier elements, since the greater mass accelerates the electrons more. Just as the planet mercury moves around the Sun faster than objects further out, electrons drawn close to the nucleus travel faster, in cases such as mercury fast enough for relativistic effects to matter.
The combination of these two effects interferes with the bonding between mercury atoms. Besides keeping it liquid at room temperature, they ensure that when heated to the point that it forms a gas mercury atoms don’t pair up, like most elemental gases (think H2, O2 or N2). Instead, mercury atoms keep to themselves like the noble gases.
Dr. Thomas Hughes is a UK-based scientist and science communicator who makes complex topics accessible to readers. His articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.
