Most metal elements melt at temperatures of several hundred degrees, but for mercury that temperature is -38.9°C (-38.0°F). So why is this metal different from all the others? It all depends on the external electrons and a combination of factors that cause them to bind unusually poorly.
The first thing to note is that the title question may not be entirely accurate. There may be two transuranic elements, which do not occur in nature because they decay far too quickly to have survived their creation in supernovae or liquid kilonovae at room temperature. The same short half-lives that necessitate their artificial production mean we don’t have much time to study them. Copernicium and Flerovium are believed to be liquid at room temperature, but since one lasts a few seconds before disintegrating, and the other even less, there is some degree of uncertainty about this. We certainly didn’t do much studying either.
Beyond these curiosities, mercury stands out among the stable elements. At the simplest level, the reason is that mercury’s outermost electrons don’t bind very strongly, weakening the attraction between one mercury atom and another. This weakness means that as soon as mercury captures even a modest amount of energy, the organization of a solid disintegrates and atoms begin to move more freely.
Another way to look at it is that when atoms bond, some of their kinetic energy is converted into bonding energy. There is so little energy in mercury’s bonds to itself that it doesn’t take much movement to break them. Since at the atomic level, random kinetic energy equates to heat, mercury does not need to be hot, much less hot, to become liquid, but other metals, with more energy stored in their connections, do it.
Mercury’s liquid state was known more than three thousand years ago, but it’s not something we would have predicted if the element had only been discovered when the periodic table was being populated. Most familiar liquids have a fairly low density, so encountering a liquid so far down the periodic table goes completely against our expectations. Its neighbors on the periodic table, gold and thallium, melt at over 1,000 and 300 degrees centigrade, respectively. It is, however, useful: Mercury’s combination of density and liquid is why it is so well suited to thermometers, barometers, and blood pressure measurement.
-So what is it about mercury’s outer electrons that lead to a much weaker bond than its metallic counterparts? It turns out that mercury is in a sweet spot on the table, where three effects combine. The first is that its outer electronic layer is full. It is much easier for electrons in a partially filled shell to escape and become part of a mist of valence electrons that hold atoms together. Metals with more easily shared electrons generally have higher melting points, certainly well above room temperature.
However, mercury is not the only metal to have a full shell, so this cannot be the only reason. The other two factors cause the outer electrons of affected atoms to stay closer to their nucleus, interfering with their ability to bond with other atoms.
Members of the lanthanide series of elements, which share the sixth period of mercury in the periodic table, undergo what is known as “lanthanide contraction.” The electrons in the 4f subshell do not shield the electrons from the positive charge of the nucleus as much as the others, causing the outer electrons to be attracted inward. Therefore, most elements in period 6 have atomic radii of similar size to those in the higher period, leading to a much greater density.
Additionally, Mercury’s outer electrons undergo a relativistic contraction, moving so quickly that the effects of approaching the speed of light come into play. This is something that only really matters with heavier elements, because greater mass accelerates electrons more. Just as the planet Mercury moves around the Sun faster than more distant objects, electrons attracted near the core move faster, in cases like Mercury, fast enough for relativistic effects to be important.
The combination of these two effects interferes with the bonding between mercury atoms. In addition to keeping it liquid at room temperature, they ensure that when it is heated to the point of forming a gas, the mercury atoms do not combine, like most elemental gases (think H).2Oh2 or N2). Instead, mercury atoms remain isolated like noble gases.
