71. Lutetium: Swampland Centaurs

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Let us ensure we remember the lanthanide series before we leave it behind once and for all.

Featured above: Not really Nasty, but a Little Kid Rubbing Cats Fur.

Show Notes

Please forgive my lack of niceties in these show notes. I’m moving house this week (finally!) and that tends to take up a lot of time. I’m pleased that I was able to stick to the podcast schedule regardless!

In this episode, I very briefly touched on electron orbitals. I didn’t get too in-depth because they are weird.

Atoms place electrons wherever it’s easiest for them to do so — i.e., wherever it takes the least energy to do so. The first electron shell consists of one subshell: 1s. It can only hold two electrons. The second shell has two subshells: 2s and 2p. The third contains three subshells; 3s, 3p, and 3d. Next is the fourth shell, with subshells 4s, 4p, 4d, and 4f. These shells do not get filled in a way that makes any kind of intuitive sense. For instance, the 4s subshell gets filled before the 3d subshell.

Khan Academy does a good job explaining the inexplicable:

Electron shells occur in the following order: s, p, d, f, g, h, and k (theoretically). To remember this order? Well, there’s one more mnemonic: “Sober Physicists Don’t Find Giraffes Hiding in Kitchens”

More works cited and possible show notes coming later. Stay safe, my friends.

Episode Script

It might not be obvious at first glance, but today’s subject is one of the dozen-ish elements that take their names from cities around the world. But you won’t find “Lutetia” on any maps — at least, not any modern ones. The Romans founded that city over two thousand years ago among the fertile floodplains of Gallia Celtica, The name they gave it, Lutetia, meant “mid-water dwelling,” or somewhat less charitably, “Swamp City.”^{1 2}

With a name like that, it’s not so shocking that the residents might want to call the place something else. They were a people called the Parisii, and so over time, their city became popularly known as Paris.

Georges Urbain discovered lutetium at the same time as ytterbium — right around the same time that George Auer von Welsbach also independently discovered those elements.

Von Welsbach wanted to name element 71 cassiopeium after constellation Cassiopeia, and in turn, the mythological queen of Aethiopia — but once again, Urbain won the naming rights.

It’s a little unfortunate. Cassiopeium would have been a beautiful name. But it’s not a total loss for the chemists of the world, because we’ll always have Paris.

You’re listening to The Episodic Table Of Elements, and I’m T. R. Appleton. Each episode, we take a look at the fascinating true stories behind one element on the periodic table.

Today, we’ll finish off the lanthanide series with lutetium.

“Languid Centaurs Praise Ned’s Promise of Small European Garden Tubs; Dinosaurs Hobble Erratically Thrumming Yellow Lutes.”

I’m sure you already figured out what’s going on there, but I’ll explain nonetheless. That is a mnemonic to help memorize all fifteen elements in this series: Lanthanum, Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium. Each word in that sentence sounds kind of like the corresponding element in the row.

There are several such mnemonic phrases for various segments of the periodic table. For instance, to memorize the first nine elements in sequence: “Happy Henry Liked Beer, But Could Not Obtain Food.” That’s Hydrogen, Helium, Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine. Or if you’d like to remember the group of alkali metals, top to bottom: “Little Nasty Kids RUB CatS FuR.” Lithium, Sodium, Potassium, Rubidium, Caesium, Francium. It’s worth noting that this mnemonic, particularly the “Nasty Kids,” correspond more to the elements’ chemical symbols than their names.³

Those are among the easier ones to remember. If the whole bit about “Languid Centaurs” and “Dinosaurs Hobbling” sounds a bit more forced, well, you have my sympathies — but there are far less helpful phrases that people have cooked up. To wit: “Yes sir, Niobi, Most Technicians, Ruth and Rhonda Paddle Silver Cadillacs.” That’s supposed to help one remember the fifth period’s transition metals: Yttrium, Zirconium, Niobium, Molybdenum, Technetium, Ruthenium, Rhodium, Palladium, Silver, Cadmium. But it’s not even a coherent sentence. At that point, you’d need a mnemonic to remember your mnemonic. At least “Languid Centaurs Praising Ned’s Promise of Small European Garden Tubs” while “Dinosaurs Hobble Erratically, Thrumming Yellow Lutes” conjures a vivid and memorable mental image, even if it is a nonsensical one.

Surely it couldn’t be too difficult to devise some more helpful and meaningful phrases as mnemonics, but this is left as an exercise for the listener.

At any rate, lutetium is the last of the lanthanides — a position that’s quite fitting. Of the fourteen natural lanthanides, this was the last one to be discovered, the final, tiniest matryoshka doll, nesting inside a sample of ytterbium, which was found inside erbium, which in turn had been found inside yttrium.

Coincidentally, an atom of lutetium really is smaller than any of its fellow rare earth elements. Remember, the lanthanides tuck their electrons away in a peculiar manner — so each one, with one more positive proton in its nucleus and one more negative electron orbiting it, holds it all together a little more tightly than the last. It’s a phenomenon called “lanthanide contraction.”⁴ Since each one is packing a little more matter into a slightly smaller space, it’s no surprise that lutetium is the densest of the rare earths. For similar reasons, it is also the hardest one, and the one with the highest melting point.⁵

Among those superlatives, lutetium also used to be the most expensive of all the lanthanides, but alas, no longer. Modern manufacturing processes have brought its price down in recent years. Thulium, terbium, and promethium all fetch prices far higher than lutetium does today.

In fact, some people consider lutetium to not even count as an “actual” lanthanide. All the elements from lanthanum through thulium put their valence electron in the same place, an orbital referred to as the 4f shell. But after thulium, there’s no more room for additional electrons in the 4f shell, so lutetium places its additional electron in the next shell up. For that reason, some scientists argue that lutetium should be considered a transition metal, among the likes of hafnium, palladium, vanadium, and more.

But the periodic table is not exclusively a catalog of electron configurations. If it were, then helium’s proper place would be at the top of Group 2, above beryllium and the other elements with two electrons in their s shells. Indeed, periodic tables that emphasize electron configurations do place helium in that spot, for precisely that reason. But the standard, run-of-the-mill periodic table is more concerned with grouping elements by their behavior. Since helium rarely reacts with other chemicals, it’s right at home sitting atop the column of noble gases. And since lutetium’s chemical behavior is more similar to the lanthanides than the other transition metals, its place as last among lanthanides is generally not terribly controversial. It would certainly function anywhere a chemical formula might contain the symbol Ln.

Ln sounds like it could be the symbol for lanthanum, but you probably remember that La is the correct symbol for that element. Actually, no chemical element has the symbol Ln — but nonetheless, Ln is occasionally found among various chemical formulae and diagrams.

That’s because Ln is a pseudoelement symbol. Because the lanthanides are so similar to each other, there are plenty of compounds that are functionally identical, no matter which lanthanide is participating. For instance, samarium boride, gadolinium boride, terbium boride, dysprosium boride, holmium boride, erbium boride, thulium boride, ytterbium boride, and lutetium boride are all structurally identical. Rather than write all of that out every time, chemists will sometimes write “LnB₂” as shorthand.

There are scores of these pseudoelement symbols, although a chemist could go their entire career without encountering most of them. For example, an inorganic chemist will probably never scribble down “Me” or “Et” to represent methyl and ethyl groups, respectively, because methyl and ethyl groups are the domain of organic chemistry. Inorganic chemists are more likely to work with metals. Sometimes it doesn’t matter which metal is part of a compound, only that there is some metal included — in which case the inorganic chemist might write “M” for short.

It’s important that these symbols don’t clash with existing symbols of actual elements. “Be” is already the symbol for beryllium, so it would be inappropriate to use as a symbol for the 1-benzotriazolyl group. Bn would work, except that Bn is already used as a placeholder for the benzyl group; similarly, Bz symbolizes the benzoyl group. Bt was free, though, so that became the symbol used to indicate the 1-benzotriazolyl group’s presence in a chemical compound.

It works in the reverse direction, too. No chemical element has the symbol “D,” and it’s a virtual certainty that no chemical element ever will. D has fallen into common usage to indicate the presence of deuterium, which is a hydrogen atom containing a neutron in addition to its one proton. Giving some new, obscure element the symbol “D” would require literally rewriting all the books to change deuterium’s symbol to something else, and that’s probably not worth it.

Unlike a couple of boron atoms, you’re not searching for any old lanthanide. You’re in search of lutetium, preferably as pure a sample as possible, to add to your element collection.

We’ve had to hunt down some rare samples before, but unfortunately, lutetium might be the rarest. Since it is so uncommon, and so hard to get, it has scarcely any uses at all. It’s used in petroleum cracking, and radioactive lutetium can be used to determine the age of meteorites that have fallen to earth. Lutetium texaphyrin is a medicine used in photodynamic therapy — that is, highly targeted beams of light that kill cells in very specific areas. The lutetium-based medicine in particular is sometimes used as a cancer treatment.

So even though lutetium might not have many applications, those that it does have are fairly impressive. As a member of the general public, your best bet is to ask a rare earths processing plant if they might let you sift through their cast-offs.

That’s about it for the lanthanide series. At first blush, this group of fifteen elements looks like the periodic table’s flyover territory — just a bunch of dull metals, more similar to each other than different, with little practical value. In fact, a seminal textbook published in 1971 said, “Lanthanum has only one important oxidation state in aqueous solution, the +3 state. With few exceptions, this tells the whole boring story about the other 14 elements.”⁶

I hope, after this eight-month-long tour through the series, that you might disagree.

Thanks for listening to The Episodic Table of Elements. Music is by Kai Engel. To learn more about electron shells and what they have to do with epicurean giraffes, visit episodic table dot com slash L u.

Next time, we’ll get the whole story behind hafnium.

Until then, this is T. R. Appleton, reminding you, Caesar Procrastinated at the Nile, Permitting So Many Europeans who were Getting Turbid and Dying of Home Urges to Yell “Lutetium!”

1. Ancient History Encyclopedia, Lutetia. Livius, February 15, 2011.
2. Encyclopedia Britannica, Paris. Last updated August 20, 2020.
3. Chegg Prep, Periodic Table Mnemonics By Groups And Periods. Wellesley S.
4. Chemistry LibreTexts, Periodic Trends – Atomic Radius. Last updated August 12, 2020.
5. Encyclopedia Britannica, Lutetium. Last updated April 16, 2019.
6. Chemistry Of The F-Block Elements, p.17. Helen C. Aspinall, 2017.