37. Rubidium: What’s Cooler Than Being Cool?

Rubidium is more explosive than the alkali metals above it on the table, but it’s much more interesting when it’s standing almost perfectly still.

Featured above: Satyendra Nath Bose, who wasn’t just a genius pentalingual theoretical physicist, but a skilled musician, too.

Show Notes

Uh, Everything’s Perfectly All Right Now. We’re Fine. We’re All Fine Here, Now, Thank You. How Are You? Thoisoi is one of my go-to resources when producing each episode, and his rubidium video is a great showcase of element 37’s explosive properties.

Please note, I mistakenly said that this video includes an accidental explosion — that actually takes place in Thoisoi’s cesium video, which genuinely made me gasp aloud.

State Of The Matter: I mentioned that solids, liquids, and gases are the most common states of matter that are found on Earth, and that’s one of those true statements that deserves a little follow-up. I alluded to plasmas, which are the fourth fundamental state of matter. A plasma is made when a gas gets so hot that its atoms’ nuclei disassociate from their electrons, sharing those electrons among all the nuclei in the plasma.

Plasma, it turns out, is the most common state of matter in the universe. This is related to the fact that hydrogen and helium are the two most common elements in the universe, but not on Earth.

But there aren’t five different states of matter. There are dozens, and they get pretty weird: Rydberg matter, quantum fog, degenerate matter… One strange state of matter that was discovered in the 1930s was the “superfluid.” This video shows that which cannot be described with words alone:

Rubidium came into play once again with the recent discovery of photonic molecules in 2018. Even with all the completely bizarre concepts we’ve been discussing here, photonic molecules are especially strange.

Scientists created an ultra-cold cloud of rubidium atoms, then shone a very weak laser beam through it. At some point, while traveling through that cloud, some of those photons bonded in pairs or triplets. Not only that, but those bonded photons became much, much slower than normal light — about 100,000 times slower. The craziest part: While an individual photon is completely massless, these photonic molecules picked up a fraction of an electron’s worth of mass.

It’s truly wild stuff, but, as Cosmos Magazine put it, “no laws of physics are broken.” These photons don’t bond to each other chemically or mechanically, as atoms do. Instead, they become entangled with each other, a quantum phenomenon that can link two particles over any distance.

These are just a couple highlights from a field of research that’s still new and uncovering stranger and stranger things every year.

A Little Graphical Explanation: Laser cooling can be a tricky thing to understand. Better, then, with some visuals to help.

Not The Speaker Company, Though: Satyendra Bose might need to share top billing with Einstein for their nominative condensate, but Bose also gets something of his very own in the world of physics, too.

You might have heard of the Higgs Boson, sometimes given the moniker, “The God Particle.” That’s its own whole story, but yes, that “boson” is named after our new friend Satyendra Nath Bose. There are two fundamental types of subatomic particle: Bosons and fermions. (Some of them are also hadrons.) The differences in their behavior are rather heady and esoteric, but to simplify a bit, fermions make up the “stuff” of matter — protons, electrons, etc. — and bosons make up particles like photons.

Episode Script

As we make our way down the periodic table, things are going to get a little strange. We’re starting to leave behind those elements we encounter every day in lieu of those that are a bit more… exotic. And we couldn’t ask for a better introduction to the mounting weirdness of period five than rubidium.

At first, it doesn’t seem out of the ordinary. Like all those above it, this alkali metal is highly reactive. But when tossed into water, rubidium doesn’t sizzle across its surface, like lithium, or burst into flames, like sodium. First of all, rubidium is the first alkali metal that’s more dense than water — so it sinks. But it probably never reaches the bottom, because element 37 explodes. Violently.

It’s not a coincidence that the alkali metals get more reactive as they get more massive. As I’m sure you remember, the reason the elements in group 1 are so reactive is because their outermost valence shell has only a single electron, which they’re desperate to lose. Since a heavier element like rubidium has so many more electrons, its valence shell is necessarily farther away from the nucleus than that of a smaller element. Being farther away, the atom loses that single electron much more easily than its lighter counterparts.

That alone is pretty impressive. But as we’ll see today, rubidium is most interesting when it’s very nearly perfectly still.

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’re slowing down to investigate rubidium.

No overview of chemistry’s history would be complete without a discussion of Dr. Robert Bunsen. Even among his brilliant contemporaries, Bunsen stands out as one of the most notable scientists of the 19th century.

He first rose to prominence by discovering an antidote for that frightening chemical killer, arsenic. And it’s an awfully good thing he did, too: Nine years after that discovery, Bunsen suffered a terrible disaster while working with an arsenic compound called “cacodyl cyanide.” To get an idea of just how nasty this stuff is, “cacodyl” comes from the Greek for “evil smell.” Not just a bad smell — evil.

That’s not even the worst part, though. Cacodyl cyanide can also spontaneously explode, and that’s precisely what happened to the sample Bunsen was working on. It shattered his face mask, permanently blinding him in his right eye. As if that weren’t enough, he was also severely poisoned with arsenic. If it hadn’t been for the antidote that he had invented, he almost certainly would have died.1

Thank goodness he didn’t, because he had an entire life’s worth of inventions and discoveries yet to uncover.

He invented a much cheaper battery by replacing its expensive platinum cathode with one made of carbon. He helped make the furnaces in steel plants far more efficient. He discovered how geysers work. But his most impressive work was yet to come.2

We have, on several occasions, made mention of “spectral signatures.” That is, when an element is burned, it emits a pattern of light that can be seen with an instrument called a spectroscope. Every element’s light pattern is unique, allowing the emission spectrum to act as an identifier for that element, like a fingerprint.

Bunsen did not invent the spectroscope — that was Joseph von Fraunhofer, if you remember way back to episode 2 — but along with his colleague Gustav Kirchhoff, Bunsen discovered this use for the spectroscope.3

There was a minor problem, though: The gas burners chemists used to heat those elements could not produce flames that were especially hot, and they put off a lot of highly colored light. That’s rather an issue when you’re trying to heat materials until they glow so you can see what color they are.

To tackle this dilemma, the good doctor created his most famous invention, the one that bears his name. You’ve probably heard of it: The Bunsen burner. A mechanic named Peter Desaga actually conducted the design and production of the thing at Bunsen’s request, but I suppose Desaga Burner just doesn’t sound as catchy.4

By adding ventilation holes near the nozzle, Bunsen and Desaga’s invention could produce a blisteringly hot flame that was nearly invisible — and unlike the lab equipment that came before, it was a clean, smokeless flame, too.

With a spectroscope in hand, his eponymous burner ready to fire, and his old friend Kirchhoff by his side, Bunsen found two new elements hiding in a sample of the mineral lepidolite. Both were alkali metals, and both emitted impressively colorful spectra when burned. The first one displayed a bright blue line when viewed through the spectroscope, so Bunsen named it “caesium,” after the Latin word for the color of the sky. The second emitted an equally brilliant line of the deepest red, like a ruby — so this one he named rubidium.5

We have barely scratched the surface of what Bunsen accomplished over the course of his illustrious, seven-decade career. But even for a scientist as proficient and prolific as he, Bunsen was equally notable for his good humor and eccentric behavior. For example: When he was a young man, he proposed marriage to a woman he fancied. She happily accepted.

Immediately after, though, he became totally and utterly consumed by his work with organoarsenic compounds. For weeks, he scarcely left his laboratory, and no one saw hide nor hair of the man. Apparently that work occupied his mind as completely as his time, because when he finally reappeared, he could not remember if he had already proposed marriage or not.

Just to be on the safe side, he figured he would ask her again — better to ask her twice than not at all. The young lady, however, did not agree, and was furious about his prolonged and unexplained absence, too. Long story short, Bunsen remained a bachelor for his entire life.6

Absent-minded though he may have been, Bunsen was, by all accounts, a good man whose company everyone enjoyed. He refused to engage in the bitter feuds that typified so much of scientific discourse at the time, leaving him with many friends and no rivals to speak of. And even though he was as notable an inventor as he was a chemist, as a matter of principle, in his entire life he never took out a single patent.7

As he grew older, Bunsen’s quirks grew more exaggerated. He appeared amused by his own disheveled appearance, and he reserved an entire room in his house for unwanted mail. Eventually, his curious mind even found an unconventional personal use for the spectroscopic identification method he had discovered.

As an older gentleman, Bunsen became quite fond of cigars, especially Cuban cigars. Unfortunately, such a luxury came at a steep price, and he suspected that his tobacconist might have been trying to unload the cheap stuff on him at a premium. So Bunsen did his homework and discovered that Cuban soil contained abnormally high levels of lithium. The solution was obvious: Whenever he bought a new box of Cuban cigars, Bunsen would incinerate a small sample of the tobacco inside, and with his spectroscope, look for the telltale fingerprint of lithium.

That’s the kind of at-home, practical application that they never really tell you about in chemistry 101. Enjoy it, because everything else we’re going to learn about rubidium is about as far from home and practicality as you can get.

Temperature measures how hot something is, of course, but to be a little more technical about it, it’s a measure how much the molecules of something are vibrating. If a material is very hot, its molecules are vibrating very rapidly.

Sometimes those molecules become so energetic that they bounce around and fly far apart from each other. This is what we call a gas. When that gas cools down, losing energy, those molecules come closer together to form a liquid. Cooler still, and that liquid becomes rigid, forming a solid.

Gases, liquids, and solids are the most common states of matter on Earth. But they’re not the only states of matter. If a substance cools down further — way, way down — it can become something truly bizarre.8

Since temperature measures how much molecules are moving, there’s a limit to how cold anything can get. If you could somehow get the molecules in a material to stop moving entirely, that would be the coldest it could possibly get. It’s a temperature that scientists call “absolute zero.”

It’s a purely hypothetical thing, at least for now. No one knows of any way to get molecules to stop vibrating completely. But we can get very, very, very close. Like, 170 billionths of a degree above absolute zero. But traditional cooling methods can only go so far. To get something that cold, scientists need to use some unconventional methods — like shooting it with lasers.9 10

We tend to think of lasers as hot — and they can be, if they’re powerful enough. But if you use an extremely weak and finely tuned laser, you can achieve the opposite effect. When shot at a cloud of matter, that weak laser can push those atoms around very gently, sort of like a softly tapped billiards ball. For ultra-cold experiments, scientists line up lasers in every direction, forming an intersection where those atoms become trapped and stand very still. They call that intersection “optical molasses.”11

It was in 1995 that scientists at the University of Colorado Boulder first trapped about two thousand rubidium atoms in optical molasses, bringing it down to that fraction of a hair above absolute zero. And it turned into something that wasn’t a gas, nor a liquid, nor a solid. For any of you wiseacres out there, no, it wasn’t a plasma, either.12

At that low, low temperature, those two thousand atoms collapsed in on themselves, and began acting as one single, enormous rubidium atom. The whole cloud started to exhibit the kind of odd behavior that’s usually only apparent at the sub-atomic level. For instance, the rubidium stopped behaving like particles of matter, and more like a wave of light.13

It’s a truly difficult thing to comprehend, even if you have some familiarity with quantum physics. But amazingly, this new state of matter was comprehended, about 70 years before its actual creation, by Satyendra Nath Bose.

Bose was a true genius, a theoretical physicist who was born in Calcutta, India in 1894, making him a contemporary of Albert Einstein. In 1919, he actually created the first English translations of Einstein’s theory of relativity from French and German versions. Oh, right — on top of being a brilliant physicist, Bose was also a polyglot who spoke at least five languages.14

But even then, Bose didn’t have any personal connection to Einstein by the time he wrote his paper theorizing this new state of matter. So he must have been pretty confident in his work when he blindly sent Einstein a letter that read,

Respected Sir, I have ventured to send you the accompanying article for your perusal and opinion. I am anxious to know what you think of it. … I do not know sufficient German to translate the paper. If you think the paper worth publication I shall be grateful if you arrange for its publication in Zeitschrift für Physik. Though a complete stranger to you, I do not feel any hesitation in making such a request. Because we are all your pupils though profiting only by your teachings through your writings. I do not know whether you still remember that somebody from Calcutta asked your permission to translate your papers on Relativity in English. … I was the one who translated your paper on Generalised Relativity.15

Between Satyendra Bose and Leo Szilard’s letter about developing the atomic bomb, it seems like any good physicist had a decent shot of getting through to Einstein with a cold-call.

Suffice it to say, Einstein was impressed with Bose’s research, and gladly submitted his translation for publication in the requested journal in 1924. In honor of those two men, the ultra-cold state of matter was called a “Bose-Einstein condensate.” When it was finally made real in 1995, an explosion of new research followed — and it’s only gotten weirder since.16

You don’t need to be a nuclear physicist to acquire a sample of rubidium, but it would probably help. This stuff is expensive — another trend we’re going to start seeing more as we traverse the table’s fifth period. A small ten-gram sample will easily run over a hundred dollars.17

That makes rubidium a good candidate to source secondhand from some kind of tool or instrument, but the kinds of electronics that rubidium is a part of tend to be similarly expensive. Rubidium can be used to create some hyper-accurate clocks, so it tends to wind up in things like GPS satellites.18

Rubidium’s downstairs neighbor, cesium, is capable of creating even more accurate timepieces, so we’ll wait to discuss those properties in episode 55. In the meantime, you might want to see if you can get your hands on one of those cheap atomic clocks to collect element 37..

Thanks for listening to The Episodic Table of Elements. Music is by Kai Engel. To see a video of an accidental mid-experiment rubidium explosion, visit episodic table dot com slash R b.

Next time, we’ll bone up on strontium.

Until then, this is T. R. Appleton, reminding you that while it would be a prestigious honor to be Dr. Robert Bunsen’s lab assistant, you do not want to be the lab assistant of Dr. Bunsen Honeydew.



  1. Famous Scientists, Robert Bunsen. September 23, 2014.
  2. Encyclopedia Britannica, Robert Bunsen. Erik Gregersen, March 26, 2019.
  3. The Discovery Of The Elements, XIII: Some Spectroscopic Discoveries, Mary Elvira Weeks. Journal Of Chemical Education, August 1932.
  4. Chemteam.info, An Aside On The Bunsen Burner.
  5. Elementymology And Elements Multidict, Cesium and Rubidium. Peter van der Krogt.
  6. Robert Bunsen’s Sweet Tooth, William B. Jensen. This PDF is too good to be lost when some server shuts down, so I’ve backed it up here.
  7. The Guardian, Robert Bunsen Did A Whole Lot More Than Invent The Bunsen Burner. James Kingsland, March 31, 2011.
  8. National Institute Of Standards And Technology, Bose-Einstein Condensate: A New Form Of Matter. October 9, 2001.
  9. Institute of Physics Visions16, Bose-Einstein Condensates.
  10. Physics Central, Matters Of State: About The Bose-Einstein Condensate — A New State Of Matter.
  11. Physics Central, How Do Lasers Help Create Both The Coldest And Hottest Spots On Earth?
  12. CU Boulder Today, Bose-Einstein Condensate: A New Form Of Matter. October 8, 2001.
  13. LiveScience, States Of Matter: Bose-Einstein Condensate. Jesse Emspak, August 3, 2018.
  14. EPW Engage, Remembering The Life Of Satyendra Nath Bose. Amitabha Bhattacharya, April 5, 2019.
  15. Satyendra Nath Bose — His Life And Times: Selected Works, p. 427. Edited by Wali Kameshwar C, 2009.
  16. Georgia State University HyperPhysics, Bose-Einstein Condensates With Rubidium Atoms.
  17. Chemicool, Rubidium Element Facts. Dr. Doug Stewart, October 18 2012.
  18. PeriodicTable.com, Rubidium. Theodore Gray.

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