By Ron B. Davis Jr., Georgetown University
When we turn to the silver-colored metals of zinc, cadmium, and mercury in group 12, their physical appearance as hard, lustrous, silvery metals, might give the misleading impression that we’re back in the realm of good, old-fashioned transition metals. Yet, the Aufbau order predicts that group-12 elements have both full s and d subshells. The consequence of filling both these subshells has a dramatic impact on their properties.
Lower Melting Points
With no available s-orbital for electronic transitions, the group 12 metals lack the characteristic d to s electron transitions that give copper and gold their distinctive colors.
Their melting points are even lower than the coinage metals. The melting point of zinc, for example, is much closer to that of its diagonal p-block neighbors, indium and lead than it is to any of the nearby transition metals.
In addition, like the nearby copper and silver, they’re all sulfur-loving chalcophiles—but again, this is similar to the p-block elements than most elements of the d-block.
In many ways, with a full s and d subshell, the group 12 metals act more like the alkaline earth metals, a dramatic impact on their properties. They lose only their s-subshell electrons on oxidation to form ions like zinc (II) and cadmium (II), while the d-subshell is still full. This means that the group 12 metals fail to meet the second transition metal standard that their neighbors, copper, silver and gold do. So, even though they are in the d-block, zinc, cadmium and mercury are not transition metals, they are post-transition metals.
Zinc’s relatively high cosmic abundance, along with its ‘sulfur-loving’ tendencies, means that a fair amount remains near Earth’s surface, where it can be collected and used.
And having all of its subshells filled gives zinc some useful properties.
Let’s compare zinc with calcium and magnesium, which also have filled s subshells driving their chemistry. Zinc, from group IIb, has some similarities in electronic structure with magnesium, of group IIa. And yet, zinc’s larger nucleus holds its electrons more tightly, making metallic bonding weaker and reducing its melting point.
Zinc’s melting point of about 420° Celsius means that it can be liquified at temperatures well below the melting point of many other common metals, like iron. This gives metallurgists a simple way to create protective coatings on other metals that are more prone to corrosion.
A classic example of this is what is known as hot-dip galvanization. In this process, an object made of metal with a higher melting point, such as iron, is dipped into a pool of molten zinc where the zinc is incorporated into the outer layer of the object. There, just like the green patina that forms on copper, or the oxide layer that passivates aluminum, zinc reacts with oxygen and carbon dioxide in the air to form a tough outer layer that protects the surface of the object beneath.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The Story of King Midas
Zinc can also be alloyed with other metals, most notably with copper to produce brass. Brass is a valued metal that combines the durability of bronze with the color and luster of gold. In his book, The Disappearing Spoon, Sam Kean recounts the story, ‘King Midas’, using chemistry.
The legend of the king’s famous golden touch is likely inspired by attempts to make bronze that led to golden-colored brass, an unintended consequence that might have seemed magical, but brought about by the high zinc content of ores, native to the land ruled by King Midas.
Just below zinc, we find cadmium, named for an ore of zinc from which it was first extracted. Cadmium sulfide is a yellow pigment used by painters like Monet. In pottery glazes, cadmium can produce a distinct orange color.
Although cadmium is now widely known to be toxic, it makes appearances in solar panels and semiconductor technologies. The immediate cost of technologies like these can be affordable, but only time will tell if the environmental issues of working with cadmium can be mitigated to the point that they are truly viable.
Famous since ancient times, mercury has the lowest melting point of all the metallic elements. It is often found in its elemental form. Being such a heavy and unusual liquid gave mercury special status among ancient cultures, many of which believed that elemental mercury actually had healing powers.
But in reality, elemental mercury’s liquid state makes it particularly dangerous to our health.
Those health problems are thought to be linked to mercury’s chalcophile or ‘sulfur-loving’ nature. Mercury, much like its famously-poisonous neighbor, thallium, associates strongly with sulfur atoms in critical enzymes in the body, disrupting our biochemistry and making us sick. Thus, much like thallium poisoning, acute mercury poisoning is sometimes treated with selenium, a much less toxic metalloid that can impersonate sulfur, interacting with the mercury and keeping it tied up, where it can’t interfere with our biochemistry.
Mad as a Hatter
Mercury is also responsible for the famous trope ‘mad as a hatter’. This expression originated in 1830s England, where it was commonly known that those in the hat-making profession seemed to have a high incidence of mental illness in their later years. At the time, felt production for hats involved a process known as carroting, in which fur was matted down and a pleasant orange color was imparted to the material.
Mercurous nitrate was often used for this process, which proved deadly when inhaled on a daily basis over the course of a career making hats.
Inspite of its toxicity, mercury continues to be widely-used, due to its usefulness and uniqueness, even in more recent decades. Mercury used in thermometers can measure temperatures as low as minus 39° Celsius before the mercury freezes. Electrical switches used the movement of liquid mercury to create or interrupt a circuit. Barometers, in which dense liquid mercury shifts by a just few millimeters under changing pressure, allow for a more compact instrument.
Common Questions about Zinc, Cadmium and Mercury
Zinc’s melting point of about 420° Celsius means that it can be liquified at temperatures well below the melting point of many other common metals.
The legend of King Midas’s famous golden touch is likely inspired by attempts to make bronze that led to golden-colored brass, an unintended consequence.
Felt production for hats involves a process, known as carroting, in which fur is matted down and a pleasant orange color is imparted to the material.