“Sometimes when the two theories touch, they complement each other brilliantly: relativistic corrections of the speed of electrons help explain why mercury…..is a liquid and not the expected solid at room temperature”
Mercury is the only metal that is liquid at room temperature, but why? The low melting point of Mercury is unusual because it doesn’t follow the rest of the periodic trends. Dmitri Mendeleev created the periodic table and he designed it so that elements near each other would have fairly similar properties. However, Mercury is an unusual element that needs more investigation to comprehend its properties. A Chemist and a Physicist might argue over whose field is more important, but Mercury is one heck of an element that beat both of them-- by combining them. Albert Einstein's theory of relativity can be used to better understand why Mercury is a liquid.
The theory of General relativity states that when “an object approaches the speed of light, its mass becomes infinite” (Howell). As for Mercury, it has an average atomic mass of 200.59 atomic mass unit. This average mass of Mercury is large compared to other elements in the periodic table. The electrons and their orbits are the key to understanding why Mercury is a liquid at room temperature. Atomic orbitals have different levels of s, p, d and f orbitals. These orbitals are distinguished by “different quantum numbers and different shapes” (Jogalekar). Most of the metals are characterized by significantly occupying the d orbitals. When orbitals are full, it implies the electrons are stable. The single quality that makes Mercury unique from its neighboring elements is that “it has a filled outermost 6s atomic orbital” (Jogalekar).
This means that the electrons in the orbital are paired up with each other and are unwilling to be shared among other Mercury atoms. This is where the theory of relativity comes in. It accounts for small changes in the masses of the electrons in Mercury and the atomic radii which nonetheless have profound effects on the physical properties of Mercury. According to special relativity, the mass of an object increases as its velocity (speed) approaches the speed of light. Niels Bohr's theory of atomic structure tells us that “the velocity of an electron is proportional to the atomic number of an element” (Jogalekar). For lighter elements like hydrogen, the velocity has little impact compared to the speed of light. Also, with lighter objects relativity can be essentially ignored, but it becomes significant for Mercury -- being a heavy element. For the 1s electron orbitals of Mercury “the electron approaches about 58% of the speed of light, and its mass increases to 1.23 times its rest mass” of 200.59 atomic mass unit (Jogalekar). This increase in mass is due to the relativistic effect of the electrons. And it is the key to understanding Mercury’s low melting point at room temperature.
The Bohr theory explains the structure of an atom that consists of a proton in the nucleus, with a single electron moving in distinct circular orbits around it. Each orbit corresponding to a specific quantized energy state. The radius of an electron orbital goes inversely as the mass, meaning that two electrons occupying the same orbital have opposite spins.
Based on the Bohr theory, “this mass increase results in a 23% decrease in the orbital radius” (Jogalekar). In simpler terms, as the effect of relativity increases with increasing mass, the electrons move closer to the nucleus reducing its radii. This shrinkage of Mercury's radius results in stronger attraction between the nucleus and the electrons. The effect is intensified by the more hybridization of the d and f orbitals which is not shielded by the s orbitals as much. The concept of hybridization is to mix atomic orbitals (s, p, d, f) into new hybrid orbitals that is suitable for the pairing of electrons to form chemical bonds. The shrinkage of the radii also translates to the outermost 6s orbital as well as to other orbitals, which makes Mercury very reluctant to share its outermost electrons and form strong bonds with other Mercury atoms.
The interaction amongst Mercury atoms are mainly in small clusters as a result of weak Van der Waals forces. The Van der Waals forces are short-range electrostatic attractive forces between uncharged molecules, arising from the interaction of permanent electric dipole moments. Therefore, Mercury has a very weak Van der Waals force because of its dipole moment that makes them less interactive and have a lower melting point. However, this low interaction of the Mercury atoms was not enough to explain the low melting point. Chemists from New Zealand, Germany and France solved for the Schrodinger equation, which is a mathematical equation that describes the changes over time of a physical system, in which quantum effects such as relativity become significant. First, they did the calculations without the effect of relativity and then by including it. The results found after the calculation including the effects of relativity made clear as of why Mercury had such a low melting point. When the effect of relativity were taken into the calculation, “the melting point of Mercury dropped from 355 kelvin to 250 kelvin” (Jogalekar). Since the temperature of Mercury dropped by 105 kelvin, Mercury is able to liquify below standard temperature of 273 kelvin. This dramatic change of the melting point was clearly caused by the effect of Einstein's general theory of relativity. This explains why Mercury is a liquid at room temperature. Now Physicist and Chemist needs to stop arguing and just deal with Mercury!
