Wednesday, November 20, 2019

Element of the Month: Cerium!

November's Element of the Month:


Atomic Mass: 141.116 amu
Melting Point: 795 °C
Boiling Point: 3443 °C

Back in October we got to learn about Jöns Jakob Berzelius's role in the discovery of Selenium. Let's "change things up" a little this month, and learn about Jöns Jakob Berzelius's role in the discovery of Cerium. For discover it he did, along with his buddy Wilhelm Hisinger, in 1803. (A brainy German guy named Kaproth discovered it more or less simultaneously, but he tends to get third billing behind the brainy Swedes.) As we've remarked so many times in our coverage of the Elements, brainy Swedes are all over the early history of Chemistry! You'd think that early investment in materials science would have put Sweden on the path to great things, but here they are struggling along with only the world's 22nd largest economy, despite having the 89th largest population.

Anyway, Cerium is one of the Lanthanides or, as chemists don't like us to call them, the "rare earth" elements. Since it's been a while let's review: the Lanthanides are the 15 metallic elements #57 through #71 which you can find at the bottom of most squarish elemental tables with the Actinides, although properly the elemental table should be short and wide, with the Lanthanides and Actinides taking up much of the left half. Lanthanides are promiscuously reactive, so they are never found in a pure form and are very difficult to isolate even in the lab. They have names like Neodymium and Cadolinium and Holmium and Lutetium, the unfamiliarity of which suggest materials that were late to be discovered and that sit a bit out of the mainstream of human commercial and industrial use relative to, say, "Tin" or "Iron."

The Centerfold!

All of this means that Cerium's status as probably the most abundant, easiest to isolate, and most heavily used of the Lanthanides is not really much to write home about. It's the 26th most common element in the Earth's crust, up there with Copper, Zinc, and Nickel, but since it is so diluted by its reactivity, it's never found in a concentration that would have made it useful to pre-industrial metallurgy. A cubic centimeter of Cerium left exposed to the atmosphere would spall (meaning rust, essentially) to dust in about a year; that makes it pretty resiliant by Lanthanide standards, but ephemeral as a mayfly in geological terms. Mind you, it also reacts with water and can burst into flame if abraided. You just aren't going to find pure Cerium lying around.

Cerium's practical uses are esoteric and dull, unless you are keenly interested in the photostability of pigments or how Aluminum can be alloyed for improved high temperature performance. Such things are important, of course, but when the big news about Cerium is that you can sprinkle a little into your Aluminum to make it even better, that gives you a pretty clear picture of which is really the A-list element of the two. Cerium is at best the Robin to Aluminum's Batman, know what I'm sayin?

Remember how we've been talking about "allotropes" recently? Cerium has four of them, which is to say that it has four different solid forms that it can take depending on pressure and temperature. But you know what? It doesn't matter one little bit! Since pure Cerium only happens in the lab, its allotropic forms are pretty moot. However, it's worth mentioning that, in whichever form, it is a silvery grey metal. Most descriptions add that it is so "ductile" that you can cut it with a knife. Not with a Cerium knife, though.

Cerium.  The image search says that the artist is "Paul Ch...", and is attached to a dead link, so I'm
honestly not sure who our painter is, nor can I send you to his website.  I like the piece, though.

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