By Ron B. Davis Jr., Georgetown University
There’s something about copper, silver, gold, zinc, cadmium, and mercury that makes them act differently than the other group VIII metals. At the edge of the transition metals, the influence of the d-subshell wanes and the valence electrons take charge once again. This changeover takes place as we pass through groups 11 and 12 of the modern table—the coinage metals and the zinc group.
The Nomenclature
Groups 11 and 12 contain some of the most well-known and influential elements in human history—a fact evidenced by some elemental symbols that evoke ancient names.
Copper has been known for far longer than cobalt. So why didn’t copper get the Co symbol? Because copper’s symbol is an homage to its Latin cognate, cuprum, referring to the island of Cyprus, where most of the Roman empire’s copper was mined.
Silver’s modern name has roots in the Germanic language, but its symbol Ag is also a nod to its Latin name, argentum. This name is also connected to Argentina, but in this unusual case, the country is named for the element, not the other way around. This happened around the same time that Spanish conquistadors came to believe that Argentina was home to a legendary mountain range of silver—the Sierra del Plata.
Gold’s symbol comes from yet another Latin term, aurum, meaning ‘glowing dawn’, an allusion to gold’s signature yellow hue.
Mercury, Zinc, and Cadmium
The letters Hg in the symbol used for mercury refer to its Latin name, hydra-gyrum, which means ‘liquid silver’. Zinc and cadmium, on the other hand, were more recent discoveries, as evidenced by symbols reflecting their modern names. The origin of zinc’s name is not entirely clear, but many believe that it may be tied to the Persian word for stone. Wherever the origin, refining would have been necessary, since zinc does not occur as a native metal in nature—zinc must instead be smelted to collect it from its sulfides, oxides, or silicates.
And cadmium, when it was finally identified in 1817, was isolated from a zinc-containing ore known as cadmia in the ancient Roman world.
Why Are They Called ‘Coinage Metals’?
These elements in group 11 are sometimes referred to as ‘coinage metals’. Their softness and lower melting points, combined with their appearance low on the activity series, make them metals that are enduring, yet easily stamped or molded—whether into coins, or other objects of value.
Copper, silver, and gold border the iron triad and the platinum group metals, some of the most potent catalytic elements on the table. Yet, as pure metals, these three elements are far weaker catalysts than those of the platinum group.
A simple glance at the d-block might suggest that gold, silver and copper should have the requisite incomplete d-subshell for potent catalytic activity. But remember—there’s an interesting exception to the Aufbau order that occurs in group 11, allowing these elements to skip ahead to a full d-subshell.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Distinctive Colors
Jumping ahead to a full d-subshell in the ground-state configuration helps to explain a lot: not only why the coinage metals lack the strong catalytic activity of their platinum-group neighbors, but also many of their own special properties.
A first question that comes up is about their distinctive colors. So many metallic elements appear in pure form as silver or grey—why do copper and gold metals have their distinctive colors in their pure, elemental state?
The source of these signature colors is the full d-subshell of the atomic metals themselves.
This unusual s1-d10 configuration means that the lowest-energy electron transition for the coinage metals is not a d- to d- transition, but rather a jump from a d-subshell to an s-subshell. The energetics of these transitions are somewhat different for each case.
The Electromagnetic Spectrum
For copper, this bigger transition between the 3d and 4s subshells absorbs visible photons in the higher-energy portion of the electromagnetic spectrum.
Silver, on the other hand, lacks a distinctive color, because the additional shielding provided by its larger electron cloud increases the energy gap between the 4d and 5s subshells so much that silver actually absorbs ultraviolet light. If our eyes could detect UV light, silver would, indeed appear to have a unique ‘color’ among the elements.
Gold
So, what about gold? How does gold resume a colored appearance, even though silver does not?
This happens because gold—somewhat like its neighbor, mercury—has an extremely massive nucleus. With 79 protons, gold has such a large, positively-charged nucleus that it holds its outer electrons much more tightly than one might otherwise predict, due to the same relativistic effects that give mercury its low melting point.
In addition, because gold holds its 6s electron tighter, the energy gap for the 5p to 6s transition is reduced compared to silver, shifting it back into the visible region of the spectrum that was possible for copper, but not for silver.
Thus, the Aufbau-order-defying d-10 ground state of group-11 elements helps us to understand their recognizable colors, and the colors of silver and gold themselves are an every-day proof of the theory of relativity.
Common Questions about Copper, Silver, and Gold
It didn’t get the symbol, Co, because copper’s symbol is an homage to its Latin cognate cuprum, referring to the island of Cyprus, where most of the Roman empire’s copper was mined.
Gold’s symbol comes from a Latin term, aurum, meaning ‘glowing dawn’, an allusion to gold’s signature yellow hue.
Silver lacks a distinctive color because the additional shielding provided by its larger electron cloud increases the energy gap between the 4d and 5s subshells so much that silver actually absorbs ultraviolet light.
