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One that the good moments in twentieth century scientific research came once Paul Dirac married quantum mechanics through Einstein"s Special theory of Relativity to create relativistic quantum mechanics. Dirac"s theory did plenty of things - suspect electron spin and the positron, analyze atomic collisions, jump-start the transformation in quantum electrodynamics - however it additionally had very significant repercussions because that chemistry. Yet these effects did not end up being known because that another couple of decades because it turned the end that for solving most troubles in chemistry you can neglect relativistic effects. Figuring the end chemical bonding, predicting the thermodynamic nature of molecules and rates of chemical reactions, expertise the molecular glue that holds proteins together; every these troubles succumbed to calculation without chemists worrying about relativity.

You are watching: Why mercury is liquid at room temperature

All except one problem, the is. And it deals with a question that every child since antiquity has asked: Why is mercury liquid at room temperature? Mercury - the only metal with this residential or commercial property - has beguiled and also fascinated men for centuries; a glittering substance that flows v studied gravity, supports the load of coins, magically seems to dissolve other metals and also resists every attempts come scoop that up. A substance the can aid health once calibrated inside a thermometer and also can kill when it accumulates in life tissues. But the one top quality of mercury the is apparent to everyone who has also the little acquaintance through it is its fluid nature.

Why is this so? It transforms out that sometimes straightforward observations in science can have complex although very interesting explanations, and also this is just one of those cases. Fortunately the crux that the matter is simple, and it has actually received its many complete and also satisfying therapy in a recent document published in the journal Angewandte Chemie. But an initial let"s go earlier to the basics. Mercury is a metal, which way that it rectal the center of the routine table in addition to other steels like gold, zinc and cadmium. In truth it is in the same group as zinc and also cadmium, and also yet it couldn"t be an ext different native them. Zinc and cadmium are not liquids at room temperature and they crystallize in a different type from mercury. In addition mercury is right alongside gold, and yet their properties room utterly dissimilar.

Recall from college chemistry the atomic orbitals come in different flavors; s, p, d and f orbitals are identified by different quantum numbers and also different "shapes". Metals are characterized by considerably occupied d orbitals. In addition, fill orbitals suggest special stability. The singular reality that distinguishes mercury indigenous its next-door neighbors is that it has a filled outermost 6s atom orbital. This way that the electron in the orbital room happily combine up with each other and are wake up to it is in shared amongst neighboring mercury atoms. Wherein the concept of relativity comes in is in bookkeeping for subtle transforms in the masses of the electron in mercury and also the atom radii i m sorry nonetheless have profound effects on the physical properties of the metal.

According to unique relativity, the evident mass of things increases together its velocity viewpoints the rate of light. From Niels Bohr"s theory of atomic framework we recognize that the velocity of an electron is proportional to the atomic variety of an element. For light aspects like hydrogen (atomic number 1) the velocity is insignificant compared to the speed of irradiate so relativity have the right to be essentially ignored. But for the 1s electron the mercury (atomic number 80) this result becomes significant; the electron approaches about 58% the the speed of light, and its mass boosts to 1.23 time its rest mass. Relativity has kicked in. Since the radius of one electron orbit in the Bohr theory (orbital to it is in precise) go inversely together the mass, this mass increase results in a 23% decrease in the orbital radius. This shrinkage provides a people of difference since it outcomes in more powerful attraction in between the nucleus and also the electrons, and also this effect translates come the outermost 6s orbital and to various other orbitals. The effect is compounded by the much more diffuse d and f orbitals insufficiently shielding the s electrons. An unified with the filled nature the the 6s orbital, the relativistic shrinkage makes mercury very reluctant undoubtedly to share its outermost electrons and type strong bond with various other mercury atoms.

The bonding between mercury atom in little clusters hence mainly results from weak valve der Waals pressures which happen from local charge fluctuations in surrounding atoms quite than the sharing of electrons. Yet all this to be conjecture; someone had actually to perform the rigorous calculations, dealing with every electron in the facet relativistically and calculating the relevant properties. In this situation the relevant property is the heat capacity that a problem which dramatically transforms during a phase transition, say from solid to liquid. The inquiry was simple; using the many state-of-the-art calculations, can you guess the temperature at which mercury melts as indicated by a sudden readjust in warmth capacity? In a paper published in Angewandte Chemie this month, chemistry from new Zealand, Germany and France have detailed a result which is the most complete one come date. They actually simulated the melt of mercury making use of quantum molecular dynamics, addressing the Schrodinger equation, calculating forces and velocities from quantum mechanics and permitting the atom clusters come sample different geometric orientations randomly. They lugged out the calculations first by not included relativity and also then by consisting of it, and also the outcomes were unambiguous; when relativistic results were taken into account, the melting suggest of mercury dropped indigenous 355 kelvin to 250 kelvin, in excellent commitment with experiment and also accompanied by a sudden readjust in the heat capacity.

The liquid nature of mercury is not the only thing that the special concept explains. It additionally explains why gold is yellow while silver is white. In this case, the splitting of orbitals and the lower power of the 6s orbital outcomes in gold absorbing blue light and also emitting yellow and also red. Due to the fact that the 6s level is greater in silver, the power required to wake up an electron coincides to the UV region instead the the clearly shows region; subsequently silver shows up devoid the colors native the visible an ar of the spectrum.

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I always feel a twang of pleasure as soon as I come throughout studies like this. Over there are couple of things an ext satisfying than the successful application the our most cherished and accurate theories to explaining life"s many humdrum and yet fascinating phenomena. That"s what science is about.


1. Proof for Low-Temperature melting of Mercury fan to Relativity; F. Calvo et al. Angew. Chem. Intl. Ed. Engl. 2013, 10.1002/anie.201302742