Boundaries between academic disciplines are like borders between European countries. They’re crossed without blinking. You can’t understand what’s going on in your gut without knowing its chemistry, and gold’s properties make little sense without considering special relativity.
Gold is in the same periodic table family as copper and silver, but while its siblings form patina and a dark tarnish, respectively, gold retains its characteristic color in the presence of either smog or sea spray. Less known is the fact that gold can actually mimic chlorine’s relatives and forms salts with rubidium and cesium metals.
Some scientific ideas are greater than others. Like loved ones, they can be old but resurface in a different context to enlighten you. Who hasn’t been wowed at least a few times by special relativity? For instance, it has to be taken into account by engineers designing GPS systems. But there are even more tangible relativistic effects happening right now in something as prosaic as a gold ring. In heavy atomic nuclei, the strong coulombic force has a significant effect on the velocity of inner electrons. Close enough to that of light, electrons’ speed increases their mass, enough to contract the Bohr radius. Specifically, gold’s innermost electrons move at 58% of the speed of light, and instead of the typical < 0.01c and ensuing negligible rest mass-increase for a hydrogen atom’s electron, there is a 23% increase in mass for a gold 1s electron. Although the relativistic effect doesn’t affect all types of atomic orbitals, it draws all s orbitals closer to the nucleus, including gold’s 6s orbital, where its valence electron resides. If the relativistic radii for various atomic numbers are plotted, we notice that gold is the most affected in the entire periodic table.
Gold’s 79 electrons are configured as such: [Xe]4f145d106s1. If it wasn’t for relativistic effects, there would be a bigger energy gap between the 5d level and the Fermi band at the 6s orbital. An excited electron would absorb in the ultraviolet. But instead as the 6s orbital is pulled closer to the nucleus and the 5ds are shielded and brought closer to the 6s, there’s a strong absorption in the blue and violet, leading to the beautiful blend of red and yellow we perceive as gold.
It takes energy to remove an electron from an atom, so if an electron is instead returned to an atom, energy will be released. The latter is known as electron affinity. With a half-filled orbital that’s more attracted by its nucleus than usual, gold has a high affinity for electrons, sharing something in common with with the halogens(see graph). It explains not only the existence of compounds like RbAu but of a more recently created one like Rb5Au3O. Both of these feature the gold (-1) ion, an unusual charge for metals.
If you have a conventional mercury(Hg) thermometer, you can also watch special relativity impact gold’s period-6 neighbor. With one more electron than Au, Hg’s 6s orbital is filled and because it’s also tightly held due to relativity, the electrons don’t flow as easily from one mercury atom to the next. This weakens metallic bonds, rendering mercury a liquid at any temperature above -38.4 oC.
Thallium is next to mercury on the periodic table. Although the relativistic effects are a bit less pronounced, as shown in the first graph, the 6s2 electrons are still jealously guarded, so to speak. In most cases only the 6p1 electron is lost, which is why thallium salts, formerly used as rat poisons, are typically in a +1 oxidation state. This makes thallium the black sheep of its family. Other members including aluminum and gallium normally form compounds containing +3 ions. Interestingly, before thallium ions do their mysterious damage, they get through cell membranes by serving as K+ -impostors, thanks to their single positive charge and similar ionic radius.
Last year, a pharmaceutical chemist who allegedly was more interested in the relativistic effects of thallium than in those of her wedding ring, sneaked a Tl compound out of her lab and poisoned her husband. She was arrested after her flight to China was delayed, not by a relativistic effect but by a snowstorm.
For relativistic effects in Pb see Focus: Relativity Powers Your Car Battery
RJ Hoffman. Thallium toxicity and the role of Prussian blue in therapy. Toxicological Reviews. http://www.ncbi.nlm.nih.gov/pubmed/14579545
Lars J Norrby. Why is Mercury a Liquid. Journal of Chemical Education
Geoffrey Bond. Relativistic effects in coordination, chemisorption and catalysis
M. Concepción Gimeno. The Chemistry of Gold.
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