[INTRO]
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 “long-form” 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 the
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 the halogens like fluorine and chlorine
way over in group 17.
It turns out that organizing things into groups
and periods like this reveals a bunch of useful
relationships.
Go down a column, for instance, and you’ll
find that 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 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 by 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 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 Alexandre-Émile Béguyer
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 sixteen
columns and placed each element in a column
according to its mass.
And he got the order at least somewhat right
-- as you turn 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 than 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 lanthanides 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 right-most 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 need 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, fluorine,
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 in mass and size to reactivity
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, it gives us some pretty
nifty shapes.
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[OUTRO]
