By Ron B. Davis Jr., Georgetown University
When it comes to the debate about transition metals, the question often asked is that, if the d-10 ground state is full, then do we include the coinage metals as true transition metals? The unusual ground-state electron configurations of these three elements—copper, silver and gold—do not meet the IUPAC definition. So then, what are the two ways to qualify as a transition metal?
The Transition Metals Club
The first way to qualify as a transition metal is to be like the Groups 3 through 10 metals. They qualify because they are elements with incomplete d-subshells in their neutral ground state.
However, there is a second way into the transition metals club. We have to find out if any of the group 11 or 12 metals can give rise to ions with an incomplete d-subshell. To answer this question, we need to think about some compounds of these metals.
To begin with, copper, silver, or gold—in combination with mildly oxidizing sulfur—are each known to lose their only s-valence electron to become a plus one ion that still contains a full d-subshell. Hence, copper (I), silver (I) and gold (I) ions found in sulfides still don’t get us to the modern definition of a transition metal.
Meeting the IUPAC Standard
Yet, copper can adopt an oxidation state of +2 when combined with highly-electronegative elements like oxygen.
Silver is a bit more stubborn, but in the presence of the voraciously electronegative element fluorine, silver can also form a +2 ion. The electron configurations of these +2 ions is d-9. So, the existence of these ions with an incomplete d-subshell, however rare they may be, qualifies copper and silver as transition metals based on the IUPAC definition.
When it comes to relinquishing its electrons, gold is even more stubborn. But in the presence of an even more powerful oxidizing solvent, like aqua regia (a mixture of nitric acid and hydrochloric acid), gold takes on an oxidation state of plus 3, making it a d8 ion—two electrons less than full!
So, here the answer is yes, copper, silver and gold do indeed meet the standard to be called transition metals by the IUPAC definition, but just barely!
The Pure Coinage Metals
As influential in human history and useful as pure coinage metals are, they are also more than elements with a pretty face. In fact, it’s their interaction with other elements that have made them mainstays of manufacturing throughout history.
Their d-10 ground state means that they do not release their d-electrons into the valence band for strong metallic bonds. Instead, these elements are famously soft, a property that makes them easy to mold into useful or beautiful objects.
Gold is often classified as one of Victor Moritz Goldschmidt, a Norwegian mineralogist’s, ‘iron-loving’ siderophiles. It is famous for being found most often as a dense native metal that can be separated from surrounding rock and soil through the simple act of panning. This is only possible because gold is so unreactive that it is often classified as a noble metal, together with members of the platinum group.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The Copper Age
Copper was the metal of choice for tools as early as 5000 BCE in what came to be known as the ‘Copper Age’, as evidenced by copper tools and smelting equipment discovered in eastern European archaeological sites from this era.
But sometime around 3000 BCE, the Mesopotamian civilization discovered that tin could be included with copper to produce bronze, a much harder and therefore sometimes more useful alloy.
And copper’s influence on the world around us extends beyond both its native metal and its alloys. Copper has some very interesting and useful chemistry when oxidized to copper ions.
Copper Oxidation
Probably the most iconic example of copper oxidation in action, is the world-famous Statue of Liberty, a structure that was originally plated with a thin layer of elemental copper. For decades, that thin layer of copper has itself been covered in an even thinner layer of green copper (II) compounds. Oxides, sulfides, and other compounds that formed as the copper reacted with components of the air and rainwater.
Comparison of early color photography of the statue to modern images reveals a marked change over the years as this reaction slowly took place. The thin but equally colorful outer layer we see today, sometimes called a patina, is relatively tough and passivating. This new green patina protects the underlying metal from the elements.
Silver
Like copper, silver is beautiful when in its pure form, but isn’t quite as noble a metal as gold or the platinum-group elements. Silver will tarnish over time, slowly reacting with its environment to produce silver ions.
However, unlike copper, the element reacting with silver is not oxygen, it’s sulfur. The resulting silver sulfide gives silver an unattractive dark tarnish, instead of the more attractive (or tolerable) patina of copper. Physical polishing of the object removes the silver sulfide. However, cleaning with an abrasive removes a little of the pure silver metal.
Silver’s reluctance to become oxidized can also be an asset in photography.
Silver Halide
Before the advent of the digital camera, photographic films relied heavily on the element silver. Black-and-white photographic films all contained silver halide, with silver in its +1 oxidation state. Yet, when exposed to light, silver reclaims an electron from halide ions, releasing silver metal in the reaction.
The act of developing photographic film involves chemically stripping away the remaining silver halide salts on regions of the film that were not exposed to light, leaving the metallic silver behind on the film. This creates a negative of the image on the film, with regions of the film exposed to light now darkened by the presence of metallic silver, while regions not exposed to light are transparent.
Common Questions about What Counts as a Transition Metal
Probably the most iconic example is the world-famous Statue of Liberty, a structure that was originally plated with a thin layer of elemental copper.
The element reacting with silver sulfur. The resulting silver sulfide gives silver an unattractive dark tarnish.
Before the advent of the digital camera, photographic films relied heavily on silver. Black-and-white photographic films all contained silver halide, with silver in its +1 oxidation state.
