We tend to think of metals as hard, strong, and resistant to high temperatures. Look at iron, aluminum, steel. While this is generally true, there is one important exception: mercury. Mercury has a melting point of -37.9 degrees Fahrenheit (-38.8 degrees Celsius) and 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 other metals?
you may like
Sign up for newsletter
Sign up for our weekly Life’s Little Mysteries newsletter to get the latest mysteries before they’re posted online.
Melting point is directly correlated to the strength of the bonds, and “the stronger the bonds, the more energy in the form of heat is required to break them,” Zoe Ashbridge, a senior lecturer in chemistry at the UK Ministry of Defence, told Live Science.
Atoms of mercury, like atoms of all other metals, are held together by metallic bonds. A lattice of positively charged metal particles known as ions is surrounded by a sea of delocalized (free) electrons, and the electrostatic attraction between these oppositely charged particles acts as the glue that holds the metals together. This structure explains many of the other characteristic properties of metals, such as electrical conductivity, because electrons can move freely through the material, and formability, because layers of positive particles can slide over each other to adopt new shapes, lubricated by free electrons. However, it is especially the strength of electrostatic attraction that governs the melting point.
Therefore, the availability of external electrons to create this delocalized ocean is a key factor. “The more positive the metal center and the more delocalized the outer valence electrons, the greater the attraction, and generally this progresses from left to right on the periodic table,” Ashbridge explained.
Mercury, a Group 12 metal, has 12 external electrons that could theoretically contribute to metallic bonding. “But all of these electrons are in a ‘filled subshell.’ When an electron is filled, it becomes more stable and less likely to delocalize. This makes mercury particularly reluctant to share its electrons, even with other mercury atoms,” she added.
However, this filled subshell effect is not large enough to explain the unusually low melting point of mercury. As atoms grow larger, the strength of the metallic bond, and therefore the melting point, also decreases from the top to the bottom of the periodic table. However, extrapolating from these established trends, mercury’s melting point should still be around 266 F (130 C), making it a solid at room temperature.
So what causes this huge disparity?
Peter Schwertfeger, a quantum physicist at New Zealand’s Massey University, said Mercury’s liquid state is almost entirely due to relativistic effects. As you move toward the bottom of the periodic table, the electrons of the heaviest elements experience a very strong attraction to the atomic nucleus, moving at speeds 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) give rise to surprising physical properties. How these appear depends on the element.
What to read next
“Relativistic effects become very important for Group 11 and Group 12 elements, which include gold and mercury,” he told Live Science. Therefore, strange physical properties arising from these quantum effects are most observed in these elements. Gold has a very unusual yellowish tint, and mercury is liquid at room temperature.
“They show a maximum of the so-called relativistic effect, which causes the outer shells of these atoms to contract. It’s huge. In the case of mercury, it’s about 20%,” Schwertfeger said. In chemical terms, this relativity-induced contraction is most easily explained by considering again the electronic configuration of mercury.
A complete 4f subshell contains electrons associated with rare earth or lanthanide elements, but is very poor at shielding other electrons from the nuclear charge. This means that the outermost electrons are held closer to the nucleus than normal. This is a phenomenon called lanthanide contraction. These contracted electrons experience relativistic effects because they move at speeds close to the speed of light.
“This increases the mass of the electron, and the increased mass due to this higher velocity pulls the electron further toward the nucleus,” Ashbridge said. As a result, relativistic effects reduce the availability of electrons contributing to metallic bonding, lowering the melting point of the metal below room temperature.
However, at the quantum mechanical level, this qualitative explanation is extremely difficult to support computationally.
The Schrödinger equation, which normally describes the possible positions of particles such as electrons, “does not satisfy Albert Einstein’s principle of relativity,” Schwertfeger explained. As a result, this equation does not apply to fast particles such as the electrons in mercury. Scientists must instead rely on the highly complex Dirac equation, making simulations extremely computationally intensive.
Eventually, however, advances in computing enabled Schwertfeger to devise a model that accurately simulated the melting of mercury and provided a quantum theoretical explanation for its unusual melting point.
“Using something called density functional theory, we were able to prove that the melting point can be lowered by more than 200 degrees Celsius. [360 F] Because these quantum contributions are dominant, periodic trends predict a low melting point for mercury, while relativistic effects render the element liquid at room temperature.
Source link
