By Ron B. Davis, Jr., Georgetown University
Why is Mercury a liquid at room temperature? Based on our observations of the rest of the d-block elements, we would at least expect mercury to have a melting point higher than that of its smaller group members, cadmium and zinc. So then, what’s the deal with mercury? Read on to find out more.
Mercury’s Unique Properties
One of the d-block element, mercury, melts at a surprising −39° Celsius. Mercury’s high density and low melting point have made it a material of choice in electrical contact switches, barometers, and thermometers for decades.
Equally important is its toxicity, which has been understood for many decades, too. Nonetheless, scientists and engineers are sometimes willing to accept the risks that mercury poses because of its unique properties—a dense, electrically conductive liquid—which made it so very useful in so many applications.
Mercury’s Unusual Phase Behavior
Mercury’s metallic bonds to other atoms have to be remarkably weak. Each atom of mercury must be hanging on to its outermost electrons more tightly. But why would mercury cling to its valence electrons more than the smaller atoms of cadmium or zinc do, in the same group?
Mercury’s unusual phase behavior remained a mystery to chemists even after the discovery of atomic subshells and the resulting long-form periodic table that explained many of the other metallic elements physical properties.
The Theory of Relativity
An explanation for mecury’s melting point had to wait until the middle of the 20th century, when physicists began to apply the theory of relativity to the behavior of atoms.
One of the principles of general relativity is that objects become more massive as they move faster.
As nuclear charge in atoms increase, electrons are forced to move faster to remain in orbit, and the 6s electrons of mercury are being pulled by a nucleus with eighty protons. This makes mercury’s 6s electron subshell contract, holding the 6s electrons closer to the nucleus and therefore more tightly held than one would expect in the absence of this so-called ‘relativistic effect.’
This mystery was long suspected to be caused by relativity, but was only finally demonstrated in 2013 when scientists at Massey University, in Aukland, New Zealand, demonstrated that computer modeling of mercury’s behavior without relativistic effects predicts a melting point of 82° Celsius. Inclusion of these relativistic effects, however, brought the model into perfect agreement with mercury’s actual melting point of −39° Celsius.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The D-block Elements
The many metals that appear in the d-block have the d-subshell as the highest-energy subshell for their atoms. And yet, although all of the d-block elements are metals, they are diverse in their own ways.
Among the d-block elements are some of the most common metals in our environment, like iron, titanium, manganese, and zirconium. But it also includes rare and precious metals, like palladium, platinum, and gold. Some, across the top row of the block, are essential nutrients—like iron for hemoglobin, cobalt in vitamin B12—zinc is for enzymes, DNA synthesis, and the immune system.
Others, just below zinc, are weak metals that make for infamous poisons, like cadmium and mercury. Those in the upper left of the d-block, like titanium, are valued for very low density. But down on row 6 of the block, we find the densest naturally-occurring element of all, osmium.
The D-subshell
Down on row 7, are synthetic elements that, predictions say, are probably among the densest elements of all—though in some cases we can’t be sure, since some of them are so unstable that only a few atoms of them have ever been made.
As diverse as some of their properties might be, the real driving force behind some of their properties however, is the additional fact that the d-subshell is in an interior energy level for each atom.
One consequence of filling the d-subshell is that d-electrons do not screen valence electrons from the nucleus very well, resulting in a steady decrease in atomic radius across the d-block. This same phenomenon is even more pronounced for row 6, where f-electrons experience even less screening, making many row-5 and row 6 transition metals roughly the same atomic size, and difficult to separate.
Closed or Half-closed Subshells
There are also important effects due to closed or half-closed subshells. Elements that meet this special condition are more reluctant to release their electrons into the valence band of the metallic bond. Reluctance to release electrons lowers melting points, one of the most fundamental physical properties of matter.
Clearly, mercury is an element with a nucleus so massive that relativistic effects start to take hold. This helps to explain mercury’s apparently unique phase properties.
Common Questions about Mercury’s Unique Phase Properties
Mercury’s high density and low melting point have made it a material of choice in electrical contact switches, barometers, and thermometers for decades.
In 2013, scientists at Massey University, in Aukland, New Zealand, demonstrated that computer modeling of mercury’s behavior without relativistic effects predicts a melting point of 82° Celsius. Inclusion of these relativistic effects, however, brought the model into perfect agreement with mercury’s actual melting point of −39° Celsius.
Among the d-block elements are some of the most common metals in our environment, like iron, titanium, manganese, and zirconium. But it also includes rare and precious metals, like palladium, platinum, and gold.
