By Ron B. Davis Jr., Georgetown University
At the bottom of group 7, on the edge of the early transition metals, we find rhenium, an element whose high melting point makes it attractive in alloys. The early transition metals also have a strange bedfellow in the form of technetium, the lightest element on the table that has no stable isotope. In spite of its inherent radioactivity, technetium has chemistry that makes it potentially helpful for special cases in the preservation of steel.
Technetium
Because of its radioactive properties, a briefly ‘meta-stable’ form of technetium is one of the most commonly employed radiological pharmaceuticals in the world.
Technetium is the element whose relatively short half-life helped Paul Merrill prove that there must be ongoing synthesis of heavy elements in stars. Technetium is different from all the other d-block metals in that it has no stable isotopes at all.
Technetium’s longest half-lives, of a few million years, made it useful because they are so short compared to the much longer life of the stars where it was found.
Other uses of technetium exploit the very short half-life what is called a metastable isotope, technetium-99m, which lasts only a few hours before rearranging its nucleus to a lower configuration, releasing gamma radiation. This short half-life is just long enough to conduct radiological imaging experiments on human patients.
Corrosion Resistance to Steel
Now, consider technetium-99, the isotope most commonly made in labs and found in spent nuclear reactor fuel. With a half-life of 200,000 years, a human lifetime might as well be the blink of an eye. At that decay rate, after three thousand years only 1% of that technetium sample will have decayed radioactively.
This slow radioactive decay, paired with its remarkably powerful ability to provide corrosion resistance to steel, has actually made technetium—despite being radioactive—somewhat useful as a corrosion inhibitor, not unlike its close neighbors on the table chromium, vanadium, manganese and molybdenum.
While the typical chrome moly, stainless or magnesium steel alloy contains as much as 10% of these non-iron elements, it has been discovered that a significant corrosion resistance in steel can also be obtained by treating it with extremely dilute solutions of technetium.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Limited Applications
The upside to this passivation technique is that very little technetium is needed to produce strong corrosion resistance. The downside is that the very small amount of technetium has to be a radioactive isotope, since there are no stable nuclei of this element. So, the protection of steel by technetium, while remarkable, has only limited applications in confined systems not accessible to humans who might be sickened by the radiation.
Studies have also shown that when cooled to about 11° above absolute zero, technetium becomes a superconductor—yet another property that may one day lead to technologies that are so valuable scientists are willing to tolerate the risks that its radioactivity poses.
In 1925, an attempt to discover elements below manganese succeeded in finding an element just below technetium. That element is rhenium, element 75.
Rhenium
Rhenium’s detection in 1925, made it the second-to-last stable element ever discovered. German chemists, Walter Noddack, and his future wife, Ida, named element-75, Rhenium, in honor of the Rhine River, in Germany, near Ida’s birthplace.
The couple had hoped to name element-43, masurium, after the region in Prussia that Walter had come from, which would have made them the only husband and wife- pair with neighboring elements on the table. Unfortunately, they never actually discovered technetium.
They managed to find rhenium because stable atoms of element 75 are present, though rare, in our Earth’s crust. This heavy, siderophilic, ‘iron-loving’ element with an odd atomic number has rare written all over it. And indeed, it is only about one part per billion in our Earth’s crust.
High Melting Point
Although it is rare, rhenium does have a stable isotope and is a valued element for its high melting point, third highest on the table after tungsten and carbon. It can be found alloyed with tungsten and molybdenum in oven and lamp filaments. Some of its alloys even have superconducting properties.
The remarkable traits of the early transition metals result in part from having somewhat sparsely populated d-subshells of electrons that are half-full or less.
Their increasingly complex sets of oxidation states as more electrons are added, and the dance of their d-subshell electrons from d-orbital to d-orbital, give many of them the ability to absorb colored light. One sees the evidence of this in the many colorful gems that they make possible, the chemical indicators that they are used in, and even in their very names, like chromium and vanadium.
The unpaired electrons in these sparsely populated d-subshells give many of them extreme hardness and high melting points that have made them popular manufacturing materials and alloying agents that improve the strength of common materials like steel.
Thus, whether one needs a material with high density and heat resistance, light weight and durability, or even a splash of color, the early transition metals with their allocation of available d-subshell electrons are just the ticket.
Common Questions about Technetium and Rhenium
Technetium’s longest half-lives, of a few million years, made it useful because they are so short compared to the much longer life of the stars where it was found.
The upside to the passivation technique is that very little technetium is needed to produce strong corrosion resistance. The downside is that the very small amount of technetium has to be a radioactive isotope, since there are no stable nuclei of this element.
Rhenium has a stable isotope and is a valued element for its high melting point, which is the third highest on the table after tungsten and carbon.
