(Intro)

Michael: At its heart, science is about finding patterns in the world and explaining why they happen.  Every field has them, but few patterns are as well-known or as useful as chemistry's periodic table.  In fact, it's so ubiquitous that we often think of a single, universal periodic table.  Really, though, scientists have proposed all sorts of versions over the years, and some of them are stranger than others, but to talk about the periodic tables we don't use, we have to start with the one we do.

There's one version of the periodic table you're probably picturing right now.  Your high school chemistry teacher likely had it on their wall and for good reason.  The standard or longform periodic table is really freaking useful.  It's maintained by the International Union of Pure and Applied Chemistry, the worldwide body in charge of standardizing how chemists work.  

The standard table organizes elements into columns called groups and rows, which are called periods.  The odd blocky shapes that result actually tell us a lot about the properties of elements and their relationships to each other.  As the atomic number of the elements goes up, so does the number of electrons each element has.   Due to their quantum properties, the electrons will occupy discrete positions known as energy levels.  They're like shells that electrons fill in around the atom's nucleus.  

Every element in a period has a similar number of these energy levels, but those levels will be filled with a different number of electrons.  Every element in a group, on the other hand, has the same number of electrons in its highest level but might have a very different number of levels.  That highest level is called the valence shell, and those outermost electrons are the ones that form chemical bonds.  Sodium and potassium, for example, each have a single valence electron and react readily with halogens like flourine and chlorine way over in group 17.  

It turns out that organizing thngs into groups and periods like this reveals a bunch of useful relationships.  Go down a column, for instance, and you'll find the atoms get progressively bigger.  That happens because each element in a group has more energy levels than the last and those levels are located farther from the center, kind of like the suburbs of a city, but slide from left to right and the radius of the atoms tends to get smaller. 

That's because elements in a period have more electrons filling in the same energy shells and more protons pulling on them.  After all, opposites attract.  Most chemists probably don't remember whether gallium is bigger or smaller than indium, and maybe they don't even need to know that often, but it only takes a quick glance at the table to figure it out.  That's the value of good organization.

The standard table does have one weird feature: parts of two rows, called the lanthanide and actinide series, are cut out and placed out of order at the bottom, which looks super awkward but it's just to help the table fit on a standard piece of a paper.

The invention of the periodic table is usually credited to Russian chemist Dmitri Mendeleev.  In fact, the UN has declared 2019 the International Year of the Periodic Table because it marks 150 years since his publication, and although we credit him with developing the table we use today, it's actually changed quite a bit since then.

Mendeleev was struggling to make sense of the different properties of the elements, so he wrote each down on a separate card and ordered them based on their atomic mass, which is calculated based on the average mass of an element's atoms.  Today's table is sorted not by atomic mass but by atomic number, or how many protons an atom has.  Elements can have different numbers of neutrons and electrons, but they always have the same number of protons, but Mendeleev had never heard of protons since they weren't discovered until the 20th century, so atomic mass would have to do.

Mendeleev noticed that there seemed to be a repeating pattern when he arranged his cards by atomic mass.  Elements with similar properties seemed to follow each other again and again, like an alkali metal always following a halogen.  The noble gases hadn't been discovered yet so they were skipped.  He started wrapping these repeating or periodic patterns into rows and columns.  The resulting table might not look like the one we all know and love, but many of its patterns remain today.  

Oxygen, sulfur, selenium, and tellurium, for example, all appear in a row in Mendeleev's notes.  Our modern layout puts them together in a column.  Mendeleev's great breakthrough was to leave space for elements not yet discovered based on where they should be.  He made specific predictions about never before seen substances.  He said, for instance, that chemists would soon find an element neighboring aluminum with an atomic mass of around 68 and a very low melting point.

In 1875, gallium was discovered, a metal with an atomic mass of 69.7 and such a low melting point that pranksters use it to make trick spoons.  Modern chemists have discovered basically all the naturally occurring elements, so this predictive power is less important today, and we've made all sorts of other changes, so side by side, the two tables don't exactly look alike, but Mendeleev's table definitely matters.  Science isn't just about cataloging what we can see today, it's about using that knowledge to predict what else is out there.

An even earlier table was constructed in 1862 by French geologist Alexander-Emile Beguyer De Chancourtois.  Actually, 'table' is kind of a stretch.  His method wrapped the elements around a rotating cylinder.  That might seem like a strange choice, but it reveals that Chancourtois was among the first to recognize the repeating nature of the elements.  Using oxygen and its atomic mass of 16 as a standard, he divided the cylinder into 16 columns and placed each element in a column according to its mass, and he got the order at least somewhat right.  As you turned the cylinder, you can see the familiar progression of lithium, beryllium, boron, carbon, and so on.  His table became known as the telluric screw, because the element tellurium was at its center.

Like our modern table, elements in each column of the cylinder shared some common properties, but it was flawed.  It included some compounds rather than just elements and some elements were in more than one place.  Still, the telluric screw could have been nearly as revolutionary as Mendeleev's periodic table, except for the part where it wasn't until later that anyone even noticed.

The chemists of the day couldn't be bothered to read the work of a geologist.  Plus, his paper didn't even include a diagram.

The idea of a spiral table, though, has stuck around.  In 1870, German chemist Heinrich Baumhauer constructed a 2D table that's much easier to read and store than a rotating cylinder.  It was expanded nearly a century later by Theodor Benfey, with more elements and more colorful labels to aid interpretation.  Supporters argue that the key feature of a spiral table is that it's continuous.  

Look at the standard table and you might think that chlorine and argon have much more in common than say, argon and potassium.  After all, they're right next to each other.  In a spiral representation, though, it's easy to see that chlorine, argon, and potassium are three consecutive elements, each with one more proton than the last.  Conversely, you might think that the lack of a gap makes it hard to see where periodicity actually repeats.  It comes down to preference, but not a lot of people prefer the spirals.  

If you're looking for the periodic benefits of the standard table and the continuous nature of a spiral one, then Roy Alexander's 3D design is just for you.  A museum educator from Brooklyn, Alexander in essence took the regular table and folded it back on itself, shaping each row so that it could connect smoothly to the next.  The result is a design that connects every element to its neighbors but also preserves the groupings many chemists find useful.

Alexander didn't know it, but his table was actually very similar to a number of previous designs.  Its most unique feature is that he convinced the US patent office to grant him a patent on the arrangement in 1971.  Not bad for something designed in part by nature.

Odd as they look, these designs do address one other concern.  They bring the poor orphan lanthinides and actinides back into the fold.  

The table that's had the most influence on our modern version was probably the one put together by French engineer Charles Janet around 1928.  It's often called the left-step table, and it's a particular favorite of physicists.  When read from top to bottom and left to right, it gives the exact order in which electrons fill up an atom's available energy shells.  If you have to deal with electron configurations a lot, this is a big deal.  Knowing how many electrons an element has in its energy levels can help predict its chemical properties and some of its physical ones as well, like its magnetic behavior, so actually, having a handy reference for how an element's energy shells fill up is useful for a lot of scientists.

Each shell has one of several basic shapes and every shape can hold a different number of electrons.  Like many things in science, for historical reasons, the shapes have confusing names.  The first four are called S, P, D, and F.  As electrons get added to an atom, they fill up shells with these shapes in a specific order.

The first two go into an S shell, the second two go into another S shell, the next six go into a P shell, and so forth.  Different shells correspond to different energy levels.  You can see how this would all get very confusing very quickly.  You can get this information from the standard periodic table, but it's not the main organizing principle unlike the left-step table.  

Each block represents one of the basic shapes.  The rightmost is the S shell, the next over the P shell and so on.  So if you want to know the electron configuration of say, oxygen, it's easy to find by reading off each subsequent row.  1s2, 2s2, 2p4.  So much easier.  To convert the left-step arrangement to the standard table, all you have to do is shift the S block from the left side to the right and move helium to be next to hydrogen, which makes sense if you're thinking about electron configurations.  Hydrogen is 1s1 and helium is 1s2.  That might seem like a small change, but people will probably tinker with the periodic table forever.

In 2006, chemist Eric Scerri proposed a new table based on the left-step design, but a touch more aesthetic.  His table is based on the idea of chemical triads, or groups of three elements that share similar properties.  First noticed in the early 1800s, triads might be the earliest known indication of periodicity.  What's very cool is that if you average the mass of the lightest and heaviest member of a triad, you almost always get the mass of the middle one.  

For example, lithium has an atomic mass of 6.94 and potassium a mass of 39.1.  Average those together and you get 23.02, just a hair more than the mass of sodium, which also shares some properties with the other two.  These days, triads aren't that important because like, we know the mass of sodium, but before Mendeleev and before the full discovery of periodicity, they had a certain predictive power.  Scerri argued for a return of triads partly because they're elegant.  They make a table that looks very orderly.  It aligns as many triads as possible, including creating a new one of hydrogen, flourine, and chlorine.  

Now, are the aesthetics that important?  Probably not.  After all, the table is just a convenient way of organizing elements, but it does reflect what scientists have been doing for more than a century.  

Before even Mendeleev, physicists and chemists have been trying to make sense of the ways elements are alike and different.  Our modern periodic table manages to convey a bunch of those patterns.  It organizes a tremendous amount of information from trends and mass and size to reactiivty and even states of matter, but once in a while, someone decides to tinker with it, and that's a good thing, because it reveals new patterns and serves different needs, or at the very least, gives us some pretty nifty shapes.

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