Above (and beyond) the periodic table.
In his recent book The Disappearing Spoon, and Other Tales of Madness, Love, and the History of the World From the Periodic Table of the Elements, American writer Sam Kean recounts the stories behind the building of one of humankind's greatest scientific achievements. Here, we excerpt part of the chapter in which Kean asks how the periodic table, a bulwark of chemistry, might be adapted to reflect what scientists continue to uncover about the nature of the elements.
If aliens ever land and park here, there's no guarantee we'll be able to communicate with them, even going beyond the obvious fact they won't speak "Earth." They might use pheromones or pulses of light instead of sounds; they might also be, especially on the off off chance they're not made of carbon, poisonous to be around. Even if we do break into their minds, our primary concerns--love, gods, respect, family, money, peace--may not register with them. About the only things we can drop in front of them and be sure they'll grasp are numbers like pi and the periodic table.
Of course, that should be the properties of the periodic table, since the standard castles-with-turrets look of our table, though chiselled into the back of every extant chemistry book, is just one possible arrangement of elements. Many of our grandfathers grew up with quite a different table, one just eight columns wide all the way down. It looked more like a calendar, with all the rows of the transition metals triangled off into half boxes, like those unfortunate 30s and 31s in awkwardly arranged months. Even more dubiously, a few people shoved the lanthanides into the main body of the table, creating a crowded mess.
No one thought to give the transition metals a little more space until Glenn Seaborg and his colleagues at (wait for it) the University of California at Berkeley made over the entire periodic table between the late 1930s and early 1960s. It wasn't just that they added elements. They also realized that elements like actinium didn't fit into the scheme they'd grown up with. Again, it sounds odd to say, but chemists before this didn't take periodicity seriously enough. They thought the lanthanides and their annoying chemistry were exceptions to the normal periodic table rules--that no elements below the lanthanides would ever bury electrons and deviate from transition-metal chemistry in the same way. But the lanthanide chemistry does repeat. It has to: that's the categorical imperative of chemistry, the property of elements the aliens would recognize. And they'd recognize as surely as Seaborg did that the elements diverge into something new and strange right after actinium, element eighty-nine.
Actinium was the key element in giving the modern periodic table its shape, since Seaborg and his colleagues decided to cleave all the heavy elements known at the time--now called the actinides, after their first brother--and cordon them off at the bottom of the table. As long as they were moving those elements, they decided to give the transition metals more elbow room, too, and instead of cramming them into triangles, they added ten columns to the table. This blueprint made so much sense that many people copied Seaborg. It took a while for the hard-liners who preferred the old table to die off, but in the 1970s the periodic calendar finally shifted to become the periodic castle, the bulwark of modern chemistry.
But who says that's the ideal shape? The columnar form has dominated since Mendeleev's day, but Mendeleev himself designed thirty different periodic tables, and by the 1970s scientists had designed more than seven hundred variations. Some chemists like to snap off the turret on one side and attach it to the other, so the periodic table looks like an awkward staircase. Others fuss with hydrogen and helium, dropping them into different columns to emphasize that those two non-octet elements get themselves into strange situations chemically.
Really, though, once you start playing around with the periodic table's form, there's no reason to limit yourself to rectilinear shapes. One clever modern periodic table looks like a honeycomb, with each hexagonal box spiralling outward in wider and wider arms from the hydrogen core. Astronomers and astrophysicists might like the version where a hydrogen "sun" sits at the centre of the table, and 11 the other elements orbit it like planets with moons. Biologists have mapped the periodic table onto helixes, like our DNA, and geeks have sketched out periodic tables where rows and columns double back on themselves and wrap around the paper like the board game Parcheesi. Someone even holds a U.S. patent (#6361324) for a pyramidal Rubik's Cube toy whose twistable faces contain elements.
Musically inclined people have graphed elements onto musical staffs, and our old friend William Crookes, the spiritualist seeker, designed two fittingly fanciful periodic tables, one that looked like a lute and another like a pretzel. My own favourite tables are a pyramid-shaped one--which very sensibly gets wider row by row and demonstrates graphically where new orbitals arise and how many more elements fit themselves into the overall system--and a cutout one with twists in the middle, which I can't quite figure out but enjoy because it looks like a Mobius strip.
We don't even have to limit periodic tables to two dimensions anymore. The negatively charged antiprotons that Segre discovered in 1955 pair very nicely with antielectrons (i.e. positrons) to form anti-hydrogen atoms. In theory, every other anti-element on the anti-periodic table might exist, too. And beyond just that looking-glass version of the regular periodic table, chemists are exploring new forms of matter that could multiply the number of known "elements" into the hundreds if not thousands.
First are superatoms. These clusters--between eight and one hundred atoms of one element--have the eerie ability to mimic single atoms of different elements. For instance, thirteen aluminum atoms grouped together in the right way do a killer bromine: the two entities are indistinguishable in chemical reactions. This happens despite the cluster being thirteen times larger than a single bromine atom and despite aluminum being nothing like the lacrimatory poison-gas staple. Other combinations of aluminum can mimic noble gases, semiconductors, bone materials like calcium, or elements from pretty much any other region of the periodic table.
The clusters work like this. The atoms arrange themselves into a three-dimensional polyhedron, and each atom in it mimics a proton or neutron in a collective nucleus. The caveat is that electrons can flow around inside this soft nucleic blob, and the atoms share the electrons collectively. Scientists wryly call this state of matter "jellium." Depending on the shape of the polyhedron and the number of corners and edges, the jellium will have more or fewer electrons to farm out and react with other atoms. If it has seven, it acts like bromine or a halogen. If four, it acts like silicon or a semiconductor. Sodium atoms can also become jellium and mimic other elements. And there's no reason to think that still other elements cannot imitate other elements, or even all the elements imitate all the other elements--an utterly Borgesian mess. These discoveries are forcing scientists to construct parallel periodic tables to classify all the new species, tables that, like transparencies in an anatomy textbook, must be layered on top of the periodic skeleton.
Weird as jellium is, the clusters at least resemble normal atoms. Not so with the second way of adding depth to the periodic table. A quantum dot is a sort of holographic, virtual atom that nonetheless obeys the rules of quantum mechanics. Different elements can make quantum dots, but one of the best is indium. It's a silvery metal, a relative of aluminum, and lives just on the borderland between metals and semiconductors.
Scientists start construction of a quantum dot by building a tiny Devils Tower, barely visible to the eye. Like geologic strata, this tower consists of layers--from the bottom up, there's a semiconductor, a thin layer of an insulator (a ceramic), indium, a thicker layer of a ceramic, and a cap of metal on top. A positive charge is applied to the metal cap, which attracts electrons. They race upward until they reach the insulator, which they cannot flow through. However, if the insulator is thin enough, an electron--which at its fundamental level is just a wave--can pull some voodoo quantum mechanical stuff and "tunnel" through to the indium.
At this point, scientists snap off the voltage, trapping the orphan electron. Indium happens to be good at letting electrons flow around between atoms, but not so good that an electron disappears inside the layer. The electron sort of hovers instead, mobile but discrete, and if the indium layer is thin enough and narrow enough, the thousand or so indium atoms band together and act like one collective atom, all of them sharing the trapped electron. It's a superorganism. Put two or more electrons in the quantum dot, and they'll take on opposite spins inside the indium and separate in oversized orbitals and shells. It's hard to overstate how weird this is, like getting the giant atoms of the Bose-Einstein condensate but without all the fuss of cooling things down to billionths of a degree above absolute zero. And it isn't an idle exercise: the dots show enormous potential for next-generation "quantum computers," because scientists can control, and therefore perform calculations with, individual electrons, a much faster and cleaner procedure than channelling billions of electrons through semiconductors in Jack Kilby's fifty-year-old integrated circuits.
Nor will the periodic table be the same after quantum dots. Because the dots, also called pancake atoms, are so fiat, the electron shells are different than usual. In fact, so far the pancake periodic table looks quite different than the periodic table we're used to. It's narrower, for one thing, since the octet rule doesn't hold. Electrons fill up shells more quickly, and nonreactive noble gases are separated by fewer elements. That doesn't stop other, more reactive quantum dots from sharing electrons and bonding with other nearby quantum dots to form ... well, who knows what the hell they are. Unlike with superatoms, there aren't any real-world elements that form tidy analogues to quantum-dot "elements."
In the end, though, there's little doubt that Seaborg's table of rows and turrets, with the lanthanides and actinides like moats along the bottom, will dominate chemistry classes for generations to come. It's a good combination of easy to make and easy to learn. But it's a shame more textbook publishers don't balance Seaborg's table, which appears inside the front cover of every chemistry book, with a few of the more suggestive periodic table arrangements inside the back cover: 3D shapes that pop and buckle on the page and that bend far-distant elements near each other, sparking some link in the imagination when you finally see them side by side. I wish very much that I could donate $1,000 to some nonprofit group to support tinkering with wild new periodic tables based on whatever organizing principles people can imagine. The current periodic table has served us well so far, but reenvisioning and recreating it is important for humans (some of us, at least). Moreover, if aliens ever do descend, I want them to be impressed with our ingenuity. And maybe, just maybe, for them to see some shape they recognize among our collection.
Then again, maybe our good old boxy array of rows and turrets, and its marvellous, clean simplicity, will grab them. And maybe, despite all their alternative arrangements of elements, and despite all they know about superatoms and quantum dots, they'll see something new in this table. Maybe as we explain how to read the table on all its different levels, they'll whistle (or whatever) in real admiration-staggered at all we human beings have managed to pack into our periodic table of the elements.
From the book The Disappearing Spoon by Sam Kean. [c] 2010 by Kean. Reprinted by permission of Little, Brown and Company, New York, NY. All rights reserved.
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|Title Annotation:||CHEMISTRY: PERIODIC TABLE; excerpt from "The Disappearing Spoon: And Other True Tales of Madness, Love, and the History of the World from the Periodic Table of the Elements"|
|Publication:||Canadian Chemical News|
|Date:||Feb 1, 2011|
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