By Ron B. Davis Jr., Georgetown University
When it comes to the rare earth elements, with the lone exception of promethium, the name ‘rare earth’ is really a misnomer. We now know these hidden elements are as abundant in Earth’s crust as many other useful metals from outside of the f-block. What made these elements appear ‘rare’ is how similar and how intimately comingled their geochemistry is.
Lanthanide Contraction
The fate of rare earth elements in the natural world hinges in part on some of their remarkable similarities, but also made more nuanced, by a noteworthy trend called the ‘lanthanide contraction.
The atomic and ionic radii of the lanthanides are remarkably similar from one end of the series to the other, but they aren’t quite identical. The poor screening effect of f-electrons means that as we move from left to right across the periodic table, the pull of an ever-more-positive nucleus on the outermost electrons of the atom increases, making each element’s atoms and most common ions progressively smaller in diameter as we go across the group.
Because of the lanthanide contraction, those rare earth elements further to the left on the table tend to be found together in nature more often, as do rare earth elements farther to the right. This observation has led some to sub-divide the rare earth elements into a first half called the light rare earth elements—and a second half called the heavy rare earth elements.
Heavy Rare Earth Elements
The heavy lanthanides contract enough that they resemble element 39 Yttrium. This is why element 39 yttrium is often included in the ‘heavy’ list—not because yttrium is heavy, or because ‘heavy’ is the relevant similarity. It’s because the smaller atomic radius causes chemical similarities between mid-sized yttrium and the more massive lanthanides.
None other than Victor Goldschmidt himself forwarded this idea. And there is some practical support for such a classification, based on where each of the minerals are found.
Light Rare Earth Elements
The lightest lanthanides, lanthanum, cerium, and neodymium, for example, are often found in the earth’s crust in combination in a class of minerals called mozanites. Another mineral, cerite, contains high levels of the most common lanthanides, cerium and lanthanum, but also many traces of the lighter rare earths, including praseodymium, neodymium, samarium.
Similar minerals can also contain heavier rare earths, from gadolinium to lutetium. Nonetheless, due to their greater atomic mass and the lanthanide contraction, the presence of these elements can increase the density of the minerals that contain them, causing them to partition deep within the Earth’s crust near (or possibly even in) the mantle—a place that no human drill has ever penetrated. Naturally, this makes heavy lanthanides harder to come by in our immediate environment.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Promethium
So the heavy lanthanides are a bit harder to come buy near the surface of the Earth than the light lanthanides. But there’s one exception. A quick glance at the rare earth metals reveals that only one member of the light lanthanides sub-group sports an integer atomic mass—element 61, promethium.
This is reminiscent of element 43, technetium, the lightest element for which there is no stable isotope at all. There’s no natural abundance of isotopes to provide an average atomic mass.
Instead, the mass of promethium’s most stable known isotope—145—is displayed in the corner of its cell on the table. Because no stable isotope of promethium exists, it gave elemental explorers all the same fits that technetium famously did.
Bohuslav Brauner
An element for this location was proposed as early as 1902 by Czech chemist, Bohuslav Brauner, who noted the significant gap in atomic mass between neodymium and samarium might leave room for a new undiscovered element. Shortly after that, Moseley’s atomic number measurements made it clear that Brauner was absolutely right. Element 61 should exist, but had not yet been observed.
Yet for decades after that, one highly capable chemist after another, specializing in rare-earth elements, tried and failed to find element 61.
Element 61
It was not until 1945, when element 61 was first positively detected at what we now call Oak Ridge National Laboratory, in Tennessee, as physicists there analyzed the byproducts of uranium fission. As larger uranium atoms split into smaller pieces during test reactions, small amounts of element 61 were detected for the first time.
In recognition that this element was forged in the most powerful ‘fire’ ever created, element 61 was named promethium, in honor of Prometheus, the Titan in Greek mythology, who stole fire from the gods and delivered it into the hands of humanity.
To conclude, with respect to rare- earth elements, once we understood the lanthanide contraction trend, and learned where to look and how to extract practical amounts of these elements in the mid-20th century, it opened the door to a wide variety of new elemental materials for use in new applications. This was true, especially in cases involving light and electromagnetism, which not only opened up many possibilities but in many instances, enhanced many high-tech products of the 21st century.
Common Questions about Promethium
It is because of the lanthanide contraction that rare earth elements further to the left on the table tend to be found together in nature more often.
It was not until 1945, when element 61 was first positively detected at what we now call Oak Ridge National Laboratory, in Tennessee, as physicists there analyzed the byproducts of uranium fission.
In recognition that this element was forged in the most powerful ‘fire’ ever created, element 61 was named promethium, in honor of Prometheus, the Titan in Greek mythology, who stole fire from the gods and delivered it into the hands of humanity.
