Relativistic Effects In Your Wedding Ring

Gold wedding ringsBoundaries 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 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.

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The ratio of the relativistic radius for 6s orbital to the non-relativistic radius. Au is the most affected.

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 5d electrons 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.

The expected gaps between levels from a non-relativistic treatment, and the more realistic ones by considering the high velocity of electron compliments of gold’s nucleus.

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.electron-affinity-page_4-feb-2013

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.

In 2011, 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
http://voh.chem.ucla.edu/vohtar/fall02/classes/172/pdf/172rpint.pdf

Geoffrey Bond. Relativistic effects in coordination, chemisorption and catalysis

M. Concepción Gimeno. The Chemistry of Gold. 

Image from http://exagger-art.artistwebsites.com/featured/albert-einstein-art.html

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Monsters From The Armpits

They would have made sculptors drop their chisels. The two women were preparing lunch for us when my friend suddenly became self-conscious of his armpits’ stench. There was no deodorant in sight, but I assured him we could easily concoct something before they returned from the kitchen. A bit of ashes and soap should do the trick, I told him.

When I woke up from the dream, I dismissed the ashes recipe, but I was reminded of an idea I had when I was in college. The nauseating smell of armpits comes from organic acids produced by bacteria. I reasoned that if an enzyme could play the role of a strong acid usually required to catalyze the conversion of organic acids and alcohols to esters, we would end up with pleasant smells.  The alcohol, if in excess, would also inhibit the growth of bacteria. In those days I was under the false impression that butyric acid was the main culprit of armpit odor. If that was the case, ethanol and a catalyst, butyric acid would be converted into one of the sweet smells of pineapple, ethyl butyrate.

3-hydroxy-3-methylhexanoic acid

Of course, throughout the day I could not stop sneaking a peak at the current knowledge of deodorant chemistry. One of the compounds secreted by armpit sweat glands is 3-hydroxy-3-methylhexanoyl-glutamine  (C12H22N2O5). This compound has no odor; however, it’s not left intact by the most abundant armpit bacteria Corynebacterium jeikeium,  a harmless member of a genus that also includes a species responsible for diphtheria. With the help of zinc-dependent enzyme, C. jeikeium cleaves off the glutamine component and leaves behind the cheesy and rancid compound 3-hydroxy-3-methylhexanoic acid:

They don’t have exclusive control over this semi-closed and moist environment— prime real estate for bacteria. Staphylococcus haemolyticus also hangs out here and converts a different precursor into 3- 3-methyl-3 sulfanylhexan-1-ol This molecule is not as repulsive as the C. jeikeium’s byproduct. It has a fruity, onion-like smell. Not surprisingly, female armpits produce more of the latter. They have, on average, lower ratios of C. jeikeium to S. haemolyticus bacteria. If you look closely at the data of the ratio of the will-turn-to-cheesy-smell to will-turn-to-onion-smell secretions, you can see a wide variety of compositions in men, but none of the women tested showed the high peaks that appear in more than half the male samples.

Most deodorants use the right strategy: their ingredients curb the growth of bacteria. Speedstick, for example, uses propylene glycol, soap(sodium stearate), salt and stearyl alcohol. Some more innovative deodorants include pleasant-smelling molecules similar in shape to the organic acids so that they compete for spots on nasal receptors. Unfortunately for women, their discriminating noses aren’t as easily fooled as those of men. Other additives in some preparations attempt to block active sites on enzymes that bacteria use to generate the offensive smells.

ratiosRegardless of the precise formulation, rub-on deodorants and anti-perspirants are often dissolved in clear, odorless silicones known as cyclomethicones. Their advantage is that these non-irritants apply smoothly, evaporate quickly, leaving the active residue as their only trace.

Cyclomethicones are also used in antiperspirants, whose active ingredient is commonly aluminum zirconium tetrachlorohydrex with glycine hydrated in its structure. Some investigators were under the impression that the compound combines with intraductal keratin fibers to temporarily block sweat pores, which starves bacteria. What’s more likely is a precipitation reaction involving the complex salt, and the insoluble solid blocks the pore.

Eventually, with a deodorant additional sweat dilutes and washes away soap and other soluble ingredients. With an antiperspirant, a pressure buildup eventually forces insoluble products out of the duct. Either way, monsters of the armpits eventually regain control.

Sources: 

M Troccaz and al. Chem Senses. 2009 Mar;34(3):203-10. Epub 2009 Jan 15. full text : 

Gautschi, Markus; Natsch, Andreas; Schröder, Fridtjof  Biochemistry of Human Axilla Malodor and Chemistry of Deodorant Ingredients  Volume 61, Numbers 1-2, February 2007 

http://cen.acs.org/articles/90/i27/Deodorants-Antiperspirants.html

http://www.colgate.com/app/Speedstick/US/EN/Products/SpeedStick.cvsp
http://www.rosehulman.edu/~devasher/CHEM470/classnotes/23The%20Dry%20Facts%20About%20Wet%20Perspiration.pdf

http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0846.2011.00532.x/full