Exactly why do people value gold? Is all of the allure of gold due to its color? What if gold metal did not have the golden color? Instead, what if it had a silver luster like its neighbors on the periodic table of elements? Would we find it quite so appealing?

There are many reasons why people might desire gold.  The motivation to possess gold would surely vary based upon where in the value chain the metal was encountered.  Gold prospectors might value gold because it was an item of trade. Artisans would value gold for more pragmatic reasons relating workability.  Rulers would value gold because it was an asset that could be put in the treasury and later used to buy influence or fund military adventures. Thieves and plunderers valued gold owing its high value per unit volume  and the ability to offer it in trade virtually anywhere.  

Here is what we can say for sure about gold.  It’s high degree of inertness means that it can retain its golden luster indefinitely and bestow an everlasting aspect. Its malleability and ductility means that metalsmithing with fairly primitive tools was feasible. Gold could be hammered into thin sheets that could be cut, punctured, and otherwise worked by artisans to produce impressive art objects. Gold could be worked to produce all manner of ornamentation for the sake of religiosity, as an ostentatious display of wealth and power, or for coinage. Whatever the context, gold leaves an impression on people, aesthetic or otherwise.

Here is where it all gets interesting. You see, one of the consequences of Einstein’s theory of relativity is that as an object approaches the speed of light, c, its mass increases by an amount defined by a fairly simple mathematical relationship. An object’s rest mass is less than its mass appreciably near lightspeed.  The term “relativistic” refers to effects relating to objects traveling near lightspeed.

It turns out that some of the outer electrons around heavy atoms like gold and mercury are moving at an appreciable fraction of the speed of light- they are relativistic electrons.  If these relativistic electrons are at the outer, valence level, then aspects or behaviors affected by relativity may become apparent by how the atom interacts with light or other atoms. 

Chemistry is about the behavior of electrons confined to the space in the immediate vicinity of nuclei, or bound electrons. In particular, the electrons outer, valence, electrons. This is the realm of chemistry.  Chemists go about their business manipulating these electrons for fun and profit. Virtually our entire material experience of life is dictated by the manner in which these electrons interact.

In the case of gold, the 6s electrons are moving at a significant fraction of the speed of light. The magnitude is 58 % of c, according to one internet reference. At this velocity, the electron mass has increased by a factor of 1.22 times its rest mass. This being the case, the Bohr radius of the orbital is contracted by 22 %. 

The implication of this perturbation in orbital size is that an electronic transition between the 5d and 6s orbitals shifts out of the UV range and into the visible band. The molar extinction from the UV cutoff to about 500 nm is high enough that metallic gold takes on its characteristic golden hue from the reflected light.

Gold is not the only element to be affected at the valence level by relativistic effects. Mercury is also affected. The contraction of the 6s orbital results in relative inertness of the 6s^2 lone pair and poor interatomic (metallic) bonding, resulting in the unusually low melting point of mercury.  Indeed it is likely that most of the interatomic attraction is due to van der Waals forces, which is notably weak.

The inertness of the 6s lone pair reveals itself in the oxidation states of bismuth, which has stable oxidation states at +3 and +5. Like other pnictogens, bismuth (III) compounds have a lone pair. But unlike nitrogen and phosphorus lone pairs which are reactive and an important part of their ordinary chemistry, bismuth’s 6s lone pair is rather inert and not significantly hybridized. Triarylbismuth (III) compounds are trigonal planar with the lone pair taking spherical s-orbital symmetry.  UV-Vis experiments will show that for some simple BiAr3 compounds, the n->pi* transition has a very low extinction coefficient, unlike the analogous Ph3P.  Exposure to Pd(II), for instance, will show scant indication of coordination in the UV spectrum, again unlike Ph3P.

This is quantum chemistry stuff that the reader can run down later. What is of interest to me in this post is the fact that, without knowing it, gold prospectors, miners, and mill operators of the 19th century took full advantage of certain relativistic effects in their search for gold.

The first relativistic effect the early miners took advantage of was the simple fact that gold is a colored and relatively inert metal. It could be spotted by simple inspection in streams and quartz veins. The color of gold made it impossible to confuse with other metals. Ofcourse, iron pyrite was always a problem, but there were simple ways to test for pyrite.

The other relativistic tool used by miners was amalgamation of gold (and silver). Mercury, being a metallic liquid by virtue of relativistic valence electrons, could be intimately contacted with gold dust or larger particles to form a solution that would remain liquid up to some modest fraction of gold. Mercury, being quite dense, would naturally seek the low points where the gold would also be found. This dissolution could be affected by simple sloshing or by grinding the mercury with the ore in an arrastra or an amalgamation pan.  After agitation, the mercury would pool and could be easily collected.

Later amalgamation techniques would combine aqueous cyanidation of the ore in the presence of mercury in hopes of better gold and silver  recovery. Reduction of gold or silver chloride occured in-situ to provide amalgam.  Amalgamation of ore that had been chlorinated by roasting in the presence of NaCl was a common solution to the serious problem of sulphuretted auriferous or argentiferous ore.

The miners of the 19th century American gold rush certainly didn’t know that their task of extracting gold would be aided by the effects of high velocity electrons. Most people walking around today don’t know or even care about this more than 55 years after the passing of Albert Einstein.  But it goes to show how subtle effects of nature can affect our lives in unexpected ways. And this is just one of many such nuances of physics.