Unstable

 

The story of the development of the periodic table of the elements, and of the characteristics of the elements themselves, has long fascinated me.  Perhaps it was Tom Lehrer’s song. Mr Grimm, my 11th grade chemistry teacher, offered extra credit for anyone who could memorize it; luckily, my grade was good enough to not have to pursue that option, but it did prompt me to buy an album with all of Lehrer’s best songs, including such classics as “Poisoning Pigeons in the Park” and “The Masochism Tango.” But I digress:

There’s antimony, arsenic, aluminum, selenium,
And hydrogen and oxygen and nitrogen and rhenium,
And nickel, neodymium, neptunium, germanium,
And iron, americium, ruthenium, uranium.
Europium, zirconium, lutetium, vanadium,
And lanthanum and osmium and astatine and radium,
And gold, protactinium and indium and gallium,
And iodine and thorium and thulium and thallium.
There’s yttrium, ytterbium, actinium, rubidium,
And boron, gadolinium, niobium, iridium,
There’s strontium and silicon and silver and samarium,
And bismuth, bromine, lithium, beryllium, and barium.
There’s holmium and helium and hafnium and erbium,
And phosphorus and francium and fluorine and terbium,
And manganese and mercury, molybdenum, magnesium,
Dysprosium and scandium and cerium and cesium.
And lead, praseodymium and platinum, plutonium,
Palladium, promethium, potassium, polonium,
And tantalum, technetium, titanium, tellurium,
And cadmium and calcium and chromium and curium.
There’s sulfur, californium and fermium, berkelium,
And also mendelevium, einsteinium, nobelium,
And argon, krypton, neon, radon, xenon, zinc and rhodium,
And chlorine, carbon, cobalt, copper, tungsten, tin and sodium.
These are the only ones of which the news has come to Harvard,
And there may be many others but they haven’t been discovered.

Up there in the second line of the second stanza is astatine, my favourite element. (Yes, I have a favourite element).  Astatine occupies #85 on the periodic table. When Dmitri Mendeleev first devised published his periodic table in 1869 (revising it in 1871), his great innovation–and probably the reason he’s credited as the “father of the periodic table”–was not simply to group elements by period (that is, by shared characteristics), which scientists had been doing for some time, but also to leave gaps in the table where it seemed that elements might be missing.  In Mendeleev’s table, the elements were numbered horizontally by atomic number (the number of protons in the nucleus) and vertically into eight columns of elements with similar properties–now recognized as their oxidation states.  Mendeleev’s eight-column table was later revised by Horace Groves Deming into the format we’re familiar with today, with its eighteen columns and two extra rows for the lanthinides and actinides. We now know that the columns correlate to the way the electrons in the items fill the available shell spaces (s, p, d, and f shells–which you can think of as four sets of “orbits” around the nucleus, although that’s a outmoded way of thinking of it–the electrons don’t really “orbit” as much as they tend to concentrate in certain locations vis-a-vis the nucleus).

Back to Mendeleev. In his 1871 table, there was a space left blank under iodine in the seventh column, indicating that there theoretically should be an element there in the halogen column that would be share characteristics of this class of elements. Halogens are some of the most reactive of elements, particularly the lighter ones (such as fluorine and chlorine).  There were other blanks in the table, too, most of them with higher atomic numbers, except for the elusive #43, which ought to have been an element similar to manganese.  Gradually, over the years, scientists began filling in these blanks in the table, even many of the rare earths (the lanthinides).  But #43 remained unidentified–despite several claims that were disproven–until a physicist named Emilio Segrè got his hands on some used molybdenum strips from the world’s first cyclotron in Berkeley that were emitting odd forms of radiation.  Molybdenum, with its atomic number of 42, was the near neighbour of the elusive #43, and Segrè knew that radioactive isotopes on molybdenum might just be the missing element. He was correct, and the element he and his colleagues discovered in 1937 was christened technetium, after the Greek word for “artificial.”  You’ll find it in the third line of the fifth stanza of Leher’s ditty.

So why hadn’t any naturally-occurring technetium been found?  The answer was found in the radioactivity of the isotopes on the molybdenum strips:  Technetium has no stable forms;  all of its known isotopes undergo radioactive decay.  The most stable of these has a half-life of 4.2 million years—a long time, for sure, but much, much shorter than the age of the earth.  It turns out that small amounts of technetium are produced naturally as a result of the decay of heavier radioactive elements, such as uranium, but it took the availability of human-built nuclear reactors in laboratories to discover this.  The reason technetium has no stable forms is itself interesting.  Elements with odd atomic numbers are generally less stable than even ones.  Odd-numbered elements can find stable forms by adding more neutrons.  As a Reddit commenter explained:

Isotopes with an even number of protons and an even number of neutrons are more stable (even-even). Isotopes with odd numbers of both protons and neutrons are unstable (odd-odd)This is because protons and neutrons like to form pairs of “spin up” and “spin down” – an odd number means there is an unpaired spin.

 

Technetium happens to fall such that its most stable isotopes should be odd-odd, and it is surrounded by particularly stable even-even isotopes of other elements. So it is relatively unstable in itself, and there are other more stable isotopes that it can decay to.

So there was technetium, which is now an important element for nuclear medicine.  An isotope of technetium, 99mTc, is what the Chalk River facility produces from spent uranium fuel rods.

Let’s go back to Emilo Segrè, who in 1937 was an established professor in Palermo, Italy. In 1938, he was on the way to visit Berkeley to study some of the shorter-lived isotopes of technetium (ones that had decayed too quickly to be mailed to him) when Mussolini passed laws barring Jews from holding university positions.  Segrè, as a Jew,  was more or less now stuck in the United States.  He managed to get a position as a research assistant for a salary far below what one would expect as a university professor who had recently discovered an element, but he was left with little choice.  He was able to get his wife and children out of Italy, and they eventually became American citizens.  And he and fellow physicist Glenn Seaborg were able to isolate and produce 99mTc, which has a half-life of a little over 6 hours and is perfect for nuclear diagnostic procedures as it decays fairly quickly.

Segrè did, however, now have access to that cyclotron he’d gotten his molybdenum strips from, and so he set his sights on that missing element under iodine.  He proposed to bombard Bismuth-209 with alpha particles to encourage the formation of a new element, and two of his colleagues carried out the experiment. He and another colleague then used chemistry to analyze the results — and found we now call astatine, specifically astatine-211, a radioactive isotope. Astatine was named from the Greek astatos, meaning “unstable.”

Subsequent studies pointed out why astatine had never been found previously.  First, there is no known stable form of astatine.  At this point in the periodic table, everything is radioactive, as the size of the atoms is becoming too unwieldy for stability, making them prone to various forms of radioactive decay into more stable elements.   Second, the most stable form of astatine has a half-life of 8.7 hours.  It turns out astatine does exist naturally, as part of the decay chain for uranium and thorium–but in such tiny amounts that it’s the rarest element on earth.  At any given time, it’s estimated there are about 25 grams of astatine on earth. Sam Kean, in The Disappearing Spoon, has this analogy:

“Imagine that you lost your Buick Astatine in an immense parking garage and you have zero idea where it is. Imagine the tedium of walking down every row on every level past every space, looking for your vehicle. To mimic hunting for astatine atoms inside the earth, that parking garage would have to be about 100 million spaces wide, have 100 million rows, and be 100 million stories high. And there would have to be 160 identical garages just as big–and in all those buildings, there’d be just one Astatine. You’d be better off walking home.”

 

Interestingly enough, it turns out that the powerfully-radioactive, but short-lived and rare astatine does have a use.  Segrè confirmed that he had astatine by injecting it into a guinea pig and checking the animal’s thyroid. As a relative of iodine, it, too concentrated in that organ and was thus confirmed. Because it’s so violently radioactive but short-lived, it can be used in small quantities for cancer treatments.  You do want the isotope 211At, however, with its half life of just of 7.21 hours, rather than 210At, which decays to 210Po.  Yep, Polonium-210 of Russian poisoning fame.  You lose an hour of effectiveness, but you don’t kill the patient.

Emilo Segrè, forced to emigrate to the United States due to the instability in his home country of Italy and the deteriorating situation for Jews that just a year later would erupt into war, became one of the foremost scientists of the unstable, or what we now call nuclear physics. In 1959 shared the Nobel Prize for the discovery of antiprotons. He eventually became a historian of physics, particularly known for his photography.

Perhaps, somewhere out there, he’s located that Buick Astatine.

 

 

One comment

  1. Couple minor points:
    s,p,d,f are sublevels which contain orbitals (1,3,5,7 orbitals respectively) where the “orbitals” represent regions of maximum probability density.
    Once you get beyond element 20, nuclei actually are more stable when additional neutrons are present.
    I can never decide if my favourite element is wolfram or plumbum.

    Like

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