Why is mercury a liquid?

​​We tend to think of metals as hard, strong and resistant to high temperatures — just look at iron, aluminum and steel. While this is generally true, there's one key exception: mercury. With a melting point of minus 37.9 degrees Fahrenheit (minus 38.8 degrees Celsius), mercury is one of only two elements that are liquid at room temperature. (The other is bromine, which is not a metal.)

But why is mercury so different from its fellow metals?

The answer comes down to mercury's position in the periodic table and its knock-on effect on how the metal bonds together.

Melting point is directly correlated with bond strength — "the stronger the bonds, the more energy, in the form of heat, is required to break them," Zoe Ashbridge, a senior lecturer in chemistry for the U.K. Ministry of Defence, told Live Science.

Atoms of mercury, like atoms of all other metals, bind together through metallic bonding — a lattice of positively charged metal particles known as ions, is surrounded by a sea of delocalized (freed) electrons, and electrostatic attraction between these oppositely charged particles acts as the glue that holds the metal together. This structure explains many of the other signature properties of metals, such as electrical conductivity, as the electrons can move freely through the material, and mouldability, as the layers of positive particles can slide over one another to adopt a new shape, lubricated by the free electrons. But it is specifically the strength of the electrostatic attraction that governs the melting point.

The availability of outer electrons to create this delocalized sea is therefore a key factor. "The more positive the metal center is and the more delocalized valence electrons on the outside, the greater the attraction is, and generally this tracks from left to right in the periodic table," Ashbridge explained.

As a group 12 metal, mercury theoretically has 12 outer electrons it could contribute to metallic bonding. "However, all of those electrons are in "filled subshells," she added. "When they are full, that makes them more stable and less likely to delocalize, and this makes mercury particularly reluctant to share its electrons, even with other mercury atoms."

Yet this filled-subshell effect isn't big enough to explain mercury's unusually low melting point. The strength of metallic bonding — and, therefore, the melting point — also decreases from the top to the bottom of the periodic table, as the atoms get larger. But extrapolating from these established trends, mercury should still have a melting point of around 266 F (130 C), which would make it solid at room temperature.

So what causes this giant disparity?

Mercury's liquid state results almost entirely from relativistic effects, said Peter Schwerdtfeger, a quantum physicist at Massey University in New Zealand. Toward the bottom of the periodic table, the electrons in the heaviest elements experience such strong attraction to the atomic nucleus that they move close to the speed of light. At this point, they no longer obey the laws of classical physics, and the resulting quantum phenomena — known as relativistic effects — lead to surprising physical properties. How these manifest depends on the element.

"Relativistic effects become extremely important for the group 11 and group 12 elements, where gold and mercury are," he told Live Science. Consequently, the weird physical properties arising from these quantum effects are most observable in these elements. Gold has an extremely unusual yellowish hue and mercury is a liquid at room temperature.

"They show us a so-called maximum of relativistic effects, and the outer shell of these atoms contract as a result. It's enormous. For mercury, it's about 20%," Schwerdtfeger said. In chemistry terms, this relativity-induced contraction is most easily explained by once again considering mercury's electron configuration.

The full 4f subshell, which contains the electrons associated with the rare earth, or lanthanide elements, is extremely poor at shielding the other electrons from the nuclear charge. This means the outermost electrons are held much closer to the nucleus than usual — a phenomenon called lanthanide contraction. These contracted electrons move close to the speed of light and therefore experience relativistic effects.

"This increases their mass, and when they have an increased mass due to this high speed, it pulls those electrons even closer to the nucleus," Ashbridge said. Consequently, the relativistic effects reduce the availability of the electrons to contribute to metallic bonding, thus lowering the melting point of the metal below room temperature.

At a quantum mechanical level, though, this qualitative explanation is extremely challenging to back up with calculations.

"The Schrödinger equation" — which usually describes the possible positions of particles such as electrons — "doesn't fulfill the relativity principle of Albert Einstein," Schwerdtfeger explained. As a result, this equation doesn't work for high-speed particles such as the electrons in mercury. Scientists must instead turn to the significantly more complicated Dirac equation, making any simulations extremely computationally demanding.

Eventually, though, advances in computing enabled Schwerdtfeger to devise a model that could accurately simulate mercury melting and provide a quantum theoretical explanation for the anomalous melting point.

"Using what we call density functional theory, we were able to establish that the melting point is lowered by over 200 degrees Celsius [360 F] by the relativistic effects," he said. These quantum contributions dominate, so while periodic trends predict a low melting point for mercury, the relativistic effects make the element a liquid at room temperature.

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