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We tend to think of metals as hard, strong, and resistant to high temperatures. Just consider iron, aluminum, and steel, although this is generally true. But there is one important exception: mercury, which has 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 type is bromine, which is not a metal.)
But why is mercury so different from its fellow metals?
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The melting point is directly related to the strength of the bond – “the stronger the bond, the The more energy in the form of heat the more energy is required to break the bonds.” Zoe AshbridgeA senior chemistry lecturer at the UK Ministry of Defence, told WordsSideKick.com
Mercury atoms, e.g. atom All other metals are held together by metallic bonds. That’s a lattice of positively charged metal particles called ions. It is surrounded by a sea of separated (free) electrons, and the electrostatic attraction between these oppositely charged particles acts as the glue that holds the metal together. This structure explains other unique properties of metals, such as electrical conductivity, because electrons can move freely through the material. and molding ability This is because layers of positive particles can slide on top of each other to form new shapes. which is lubricated by free electrons But it is specifically the strength of electrostatic attraction that controls the melting point.
The availability of outer electrons to form this fragmented sea is therefore a key factor. “The more positive the metal center and the more separated the outer valence electrons, The gravitational force will be greater. and generally follows from left to right in the periodic table,” explains Ashbridge.
Because group 12 metals, in theory Mercury therefore has 12 outer electrons, which may contribute to metallic bonds. “however All these electrons are in “The sub-floor is filled in,” she added. “When they’re full That will make them more stable and less likely to split. And this makes mercury extremely reluctant to share its electrons. even with other mercury atoms.”
But this added subshell effect is not large enough to explain mercury’s unusually low melting point. Metal bond strength and melting points also decrease from top to bottom of the periodic table. When atoms become larger But when extrapolating from these emerging trends The mercury should still be there. Melting point is approximately 266 F (130 C).This will harden at room temperature.
So what causes this big difference?
Mercury’s liquid state is due almost entirely to relativistic effects. Peter SchwerdtfegerQuantum physicist from Massey University in New Zealand. at the bottom of the periodic table Electrons in the heaviest elements are so strongly attracted to the nuclei of atoms that they move closer. speed of light. At this point They no longer obey the laws of classical physics. and the resulting quantum phenomenon known as the relativistic effect. leading to surprising physical properties How these are displayed depends on the composition.
“Relativistic effects are particularly important for Group 11 and Group 12 elements, where gold And there’s mercury in it,” he told Live Science. So, the strange physical properties arising from these quantum effects are most observable in these elements. Gold has a very unusual yellow color. and mercury is liquid at room temperature.
“They show us what is called the maximum relativistic effect. And as a result, the outer shells of these atoms shrink. It’s huge. For mercury, it’s about 20%,” says Schwerdtfeger. In chemistry, this relativity-induced shrinkage is most easily explained by reconsidering the electron structure of mercury.
The full 4f sublayer, which contains electrons associated with rare earthor lanthanide element It has a very poor ability to shield other electrons from nuclear charge. This means that the outermost electrons are held much closer to the nucleus than normal. This is a phenomenon known as lanthanide shrinkage. These contracted electrons move close to the speed of light and therefore have a relativistic effect.
“This increases their mass. And when they increase in mass due to high speed. It pulls those electrons closer to the nucleus,” Ashbridge said. As a result, the relativistic effect reduces the availability of electrons to contribute to metallic bonds. As a result, the melting point of the metal is lower than room temperature.
However, at the level of quantum mechanics This qualitative description is extremely challenging to back up computationally.
“action Schrödinger equation” — which usually describes the possible positions of particles such as electrons — “did not respond. principle of relativity of Albert Einstein,” Schwerdtfeger explains. As a result, the equation does not apply to high-velocity particles, such as electrons in mercury. Scientists therefore had to turn to particles with significantly higher speeds instead. Dirac equationMake any simulation However, it requires a lot of calculation.
However, in the end Advances in computation allowed Schwerdtfeger to devise a model that could accurately simulate the melting of mercury. and provides a quantum theoretical explanation for the unusual melting point.
“Using what we call density functional theory This allows us to conclude that the melting point has greatly decreased. 200 degrees Celsius (360 F) from relativistic effects,” he said. These quantum contributions dominate Therefore, while periodic trends predict a low melting point of mercury, Relativistic effects cause such elements to become liquids at room temperature.
