Picture This: The Periodic Table
It looms over every chemistry classroom and lecture theatre, two towers bookending serried ranks of compartments rather like the British Houses of Parliament—and with at least as much authority. The Periodic Table is chemistry’s icon, a codification of the entire chemical universe expressing the relationships between the elements from which all ordinary matter is constituted. There are just 92 or so of these natural elements (occasional oddities like technetium, element 43, barely exist in nature because they are so unstable), but scientists have now appended to the roster a gaggle of extra ones, from neptunium (element 93) to oganesson (118), that are wrought artificially in nuclear reactions, the most massive of them living for just an instant before decaying.
I must once have known the Periodic Table by heart. It’s too long ago for me to be sure, but I have to surmise as much because chemistry students at Oxford University weren’t given the table for their exams. In a gesture of characteristically perverse exceptionalism, we were expected to memorize it. But don’t ask me now where to locate rhenium or iridium: all those obscure transition metals in the long central block of the table are a blur. If, however, you do know where an element goes—if you can assign it to the right row and column of the table—you can deduce a lot about it. You can figure out how the electrons in its atoms are arranged into shells, and make good guesses about the types of compounds it forms, its melting and boiling points and its propensity to react with other elements. In my finals, I wrote an entire essay about niobium—niobium!—on that basis. Goodness knows what it said.
The Periodic Table was conceived as a scheme for bringing order to the elements. When there were deemed to be only four of these—the earth, air, fire, and water of the Greek philosopher Empedocles (it was just one of the elemental systems proposed in ancient times, but enjoyed the weighty advocacy of Plato and Aristotle)—things seemed simple enough. But during the Renaissance, natural philosophers were increasingly forced to accept that the metals then known—copper, iron, lead, tin, mercury, silver and gold—were not as interconvertible as the alchemists believed, but seemed to have an elemental primacy about them, too. More and more of these became recognized—zinc, bismuth, cobalt, and others—along with other new elements such as sulfur, phosphorus, carbon, and, in the late eighteenth century, gaseous elements like nitrogen, hydrogen and oxygen. When the French chemist Antoine Lavoisier (who named those latter two) drew up a list of known elements for his seminal textbook Traité élémentaire de chemie in 1789, he counted 33—including light and heat, which he called caloric.
The list didn’t seem to be arbitrary though. In the early nineteenth century, several scientists noted that some elements seemed to come in families, resembling one another in the kinds of reactions they engaged in and the compounds they formed. Some claimed to see triads: the halogens chlorine, bromine and iodine for example, or the reactive metals sodium, potassium (both discovered by English chemist Humphry Davy in 1807) and lithium (identified in 1817). Was there a hidden pattern to the elements?
The Russian chemist Dmitri Mendeleev, working at Saint Petersburg University, is usually credited with discovering that pattern. A Siberian by birth, with Rasputin-like dishevelled hair and an irascible manner, he published his first Periodic Table in 1869. It is “periodic” because, if you list the elements in order of their mass, certain chemical properties seem to recur periodically along the list. The table is produced by folding that linear list so that elements with shared properties sit in vertical columns (although Mendeleev’s first table had them instead in rows, effectively turning today’s table on its side).
Mendeleev’s insight wasn’t unique; by then the existence of a periodic structure that organized the known elements was clear to others too. In particular, the German chemist Julius Lothar Meyer drew up a table almost identical to Mendeleev’s in 1868, but he didn’t get it published until later—and so missed out on the accolades, to his immense chagrin. Mendeleev, however, had the foresight to see that his table only worked if he left some slots empty: elements presumably yet to be discovered. When some of these were found soon after and had just the properties he predicted, he was vindicated.
Still, it’s a weird kind of periodicity. At first, chemical properties seemed to recur every eight elements. But in the row that starts with potassium, there’s an interlude of ten metals—the transition metals—and so it continues thereafter, creating a periodicity of 18. And after lanthanum (element 57), chemists discovered a whole series of 14 metallic elements with almost identical properties that have to be squeezed in too—frankly, these elements, called the lanthanides after the first of their ilk, all seem a bit redundant. There’s another block like this after radioactive actinium (element 89), called the actinides. In most Periodic Tables, the lanthanide and actinide blocks are left floating freely underneath so the table doesn’t get stretched beyond the confines of the page. (Some insist that this long-form table is the only proper one.) Why this odd structure?
The answer became clear with the invention of quantum mechanics in the early twentieth century. The chemical properties of elements are mostly determined by how the electrons in their atoms are arranged. New Zealander Ernest Rutherford showed that atoms comprise a central, very dense nucleus with a positive electrical charge, surrounded by enough negatively charged electrons to perfectly balance that charge. Rutherford imagined the electrons orbiting the nucleus like moons, but in the quantum-mechanical description they occupy nebulous, smeared-out clouds called orbitals. Using quantum mechanics to describe the disposition of electrons shows that they are arrayed in shells. The first of these can contain just two electrons—this is the only shell possessed by hydrogen and helium, the two lone elements at the tops of the towers—while the next has eight, and then 18. The shape of the periodic table thus encodes the character of the quantum atom.
All clear? Not quite. Even now, there’s no consensus about how to draw the Periodic Table. Hydrogen, the first and lightest element, has always been awkward: it tends to get plonked on top of the first column (the alkali metals), but it doesn’t really fit there—it’s not a metal, after all. Some prefer to see it float freely above the rest, a hydrogen balloon over the edifice of elements. And representing the rather awkward nuances of the quantum shell structure in a two-dimensional diagram involves compromises, which have prompted the invention of all manner of ingenious alternatives to the traditional block format: spiral and circular tables, loops and stadium shapes, tiered ziggurats, three-dimensional models, dizzyingly imaginative cartographies of elements. None has caught on.
Some of the fiercest arguments involve the lanthanides and actinides, which begin in the third column of the Table. Which elements truly belong in those two slots? Older tables put lanthanum (symbol La) and actinium (Ac) there, with the rest of the series relegated to those disconnected basement blocks. Others instead assign those two positions to the last of the lanthanides and actinides: lutetium (Lu) and lawrencium (Lr). Some leave the position undefined, labeled only ‘La–Lu’ and ‘Ac–Lr.’ The problem is that the arguments for one choice are chemical—which elements are chemically most similar to scandium and yttrium higher up in column three?—while the other option is preferred quantum-mechanically, based on how the electrons are configured. In some ways this is a dispute about authority. Which has the final say on the periodic table: chemistry or physics?
Put that way, you can see the potential for acrimony. I was for a time a member of a group tasked by the International Union of Pure and Applied Chemistry—the authority on chemical nomenclature and systematization—to make recommendations that might resolve the matter. But the group couldn’t agree, and so the argument continues. Or to give it a more positive spin: you’re free to choose the Periodic Table you like best. ♦
Subscribe to Broadcast