Of the 118 elements in the periodic table, 93 of them are metals. Mercury stands out among them since it is the only metal that exists as liquid at normal temperatures.
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Unusual Metallic Form
The liquid status of mercury has been known for over three thousand years, although it is not something that has been predicted had it ever been discovered as the periodic table was being filled in. Most of the liquid elements have low density, so it is pretty unusual to encounter one down the periodic table.
Mercury's neighbors on the periodic table, such as gold and thallium, are known to melt at over 1,832 and 5,432 degrees Fahrenheit (1,000 and 3,000 degrees Celsius), respectively. Meanwhile, mercury melts at -38.0 degrees Fahrenheit (-38.9 degrees Celsius).
This strange behavior of mercury is actually useful for many reasons. The combination of its high density and liquid state make it well suited for making thermometers, barometers, and blood pressure monitoring devices.
While mercury is famous for being a liquid metal, it is not the only known liquid element. Bromine also shares the record for existing as liquid at standard temperature and pressure. Meanwhile, there may be two transuranic elements which could add to this list.
Copernicium and flerovium are also suspected of being liquid at room temperature. They do not appear in nature because they decay too quickly to have survived from their formation in supernovae or kilonovae. Scientists do not have much time to study them because of their short half-lives, with only seconds left before dying. This means that there is a fair degree of uncertainties about this claim, making mercury stand out among stable elements.
Why Is Mercury Liquid at Room Temperature?
There are different ways to explain the strange nature of mercury. At the simplest level, the outermost electrons of this metal do not bond very strongly, causing weak attraction between mercury atoms. This means that as soon as mercury absorbs even a modest amount of energy, the solid organization breaks down and the atoms are allowed to start moving around more freely.
Another way to explain this is that when atoms bond chemically, some of their kinetic energy gets converted into the energy of the bond. So little energy is left in mercury's bond with itself that it only takes a few movements to break them apart. At the atomic level, the random kinetic energy is equal to heat, so mercury does not need to be warm or hot to be liquid. Meanwhile, other metals with more stored energy in their bonds do so.
The position of mercury in the periodic table tells why its outer electrons led to weak bonding. First, the atom of mercury has a full outer electron shell, so it is not difficult for electrons in a partially filled shell to get out of the atom and become part of valence electrons that hold the atoms together. For metals with electrons that are more easy to share around, they have higher melting points which can be higher than room temperature.
Additionally, the outer electrons of mercury experience a relativistic contraction which allows them to move very fast. This behavior matters only to heavier elements since the electrons are accelerated by greater mass. Just as the planet mercury orbits around the Sun faster than other members of the Solar System, electrons located closer to the nucleus travel faster. In the atom of mercury, it happens fast enough for relativistic effects to matter.
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