The Periodic Table: Crash Course Chemistry #4


Hank gives us a tour of the most important table ever, including the life story of the obsessive man who championed it, Dmitri Mendeleev. The periodic table of elements is a concise, information-dense catalog of all of the different sorts of atoms in the universe, and it has a wealth of information to tell us if we can learn to read it.


Transcript Provided by YouTube:

00:00
Hello, I’m Hank Green; welcome to Crash Course Chemistry.
00:03
Today, we’re talking about the most important table ever.
00:07
Not the table where they signed the Declaration of Independence,
00:10
nor any table of contents, nor this table right here, nor the stone table of Aslan,
00:16
NAY!
00:17
It is the periodic table of elements, a concise,
00:20
information-dense catalog of all of the different sorts of atoms in the universe.
00:25
Today I want to talk a little bit about the creation of this table,
00:28
which is, to be clear, one of the crowning achievements of human thought.
00:32
To start out, though, let’s close our eyes and pretend.
00:38
[Theme Music]
00:45
Imagine you’re in Siberia. And you’re a thirteen-year-old boy.
00:49
And your father, who was a professor but had gone blind,
00:52
leaving your family of more than ten brothers and sisters destitute, has just died. I know, downer.
00:58
Your mom, to support the family, has re-opened an abandoned glassmaking factory in the small town where you live,
01:04
largely because she wants to make enough money to send you to school someday.
01:08
A year passes – the factory burns down.
01:12
But your mom, she sees your potential;
01:14
she knows that you have a keen scientific mind and will not see that squandered.
01:18
So, with your siblings out of the house and on their own, she packs up your belongings, straps them to a horse,
01:25
and with you in tow, rides 1200 miles through the Ural Mountains on horseback to a university in Moscow.
01:32
There, on your behalf, she pleads earnestly and effectively, and they reject you.
01:37
So together, you ride another 400 miles to St. Petersburg,
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to the school where your father had graduated as a scientist,
01:44
and as luck, or extreme, insane, undeniably Russian persistence, would have it, they accept you,
01:52
and your saddle-worn butt, as a pupil. Your mother, having completed her mission, promptly dies.
01:58
If you’re doing your imagining as I told you, you might feel a tremendous debt to your mother,
02:02
and a very deep desire to ensure that you achieve something on par with the sacrifices she made for you.
02:08
And maybe that’s one reason why Dmitri Ivanovich Mendeleev became the crown jewel of Russian science,
02:14
and a theorist who revolutionized how we see the world.
02:18
Mendeleev spent a great deal of time in laboratories as a student, studying the burgeoning new field of chemistry.
02:23
He worked with all the elements that you could work with at the time,
02:27
and his knowledge gave him unique insights into their properties.
02:30
Those insights would come in handy.
02:32
Let’s all imagine we’re Mendeleev again – I like doing that – and that we know a bunch of stuff about chemistry –
02:37
which, you know, we don’t, yet – but we’re imagining.
02:40
So it’s the 1860s, and about 60 elements are known to mankind,
02:44
and their atomic weights are mostly known as well.
02:46
So the simplest thing was just to sort them in order of their atomic weights.
02:50
But interestingly, you, because you’re a cleverpants,
02:53
realized that the most significant relationships seem to have nothing to do with the atomic weight.
02:57
Lithium, sodium, potassium, and rubidium were all extremely prone to reacting with chlorine,
03:03
fluorine, iodine, and bromine;
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beryllium, magnesium, calcium, and strontium were all similar, but less reactive.
03:09
But with a quick inspection, you, and to be fair, a number of other chemists,
03:13
realize that there was a relationship between atomic weights, but it’s periodic.
03:17
At the beginning of the list of elements, characteristics repeat every seven elements.
03:21
On the side here, we now know that it’s every eight elements,
03:23
but in the 1860s, elements were studies based on their reactivity,
03:26
so the non-reactive noble gases had not yet been discovered, so the period occurred every
03:32
As the mass of the elements increases, the repetition starts to get a little less periodic,
03:36
although it’s certainly still there; it just isn’t perfect.
03:38
Some of your colleagues, they’re saying: “Well, such is life.”
03:42
It was perfect repetition early on, but later in the list it gets a little fuzzier.
03:46
But not you; you become obsessed. Obsessed with the perfection of the periodicity.
03:52
You write out the names and weights and properties of elements on cards;
03:55
you lay them across your desk, shuffle them, tear them to pieces in frustration, until one day, you realize:
04:02
that you’re simply missing cards.
04:04
The numbers aren’t working, not because there’s something wrong with your ideas,
04:07
but because some elements simply haven’t been discovered yet.
04:10
Armed with this insight, you insert gaps into the table, and things suddenly fall perfectly into place.
04:16
Seven-element periods for the first two rows, with hydrogen in its own category,
04:20
eighteen-element periods for the next two rows.
04:22
You’re so certain that you predict the properties of these missing elements.
04:26
And when a French scientist comes along and says that he has, in fact, discovered one of them,
04:31
you argue with him, saying that you discovered it first in your mind.
04:36
And when you see his data, and it doesn’t match yours,
04:39
you publish a paper saying his data for the new element he discovered is wrong.
04:44
That’s how certain you are of yourself of this beautiful new theoretical framework you’ve created.
04:48
And you know what the really crazy thing is? You’re right! That French guy’s data was wrong!
04:53
You, never having examined the element he discovered, knew more about it than he did,
04:59
because you are Mendeleev, Master of the Elements.
05:03
Okay, we’re done imagining for the episode; that was fun though.
05:05
Different groups Mendeleev had identified are a lot of the same groups that we study today.
05:09
Starting at the left, we have the soft, shiny, extremely reactive alkali metals,
05:14
so reactive, in fact, that they have to be stored in inert gases or oil, to prevent them
05:19
from reacting with the atmosphere.
05:21
Alkali metals want nothing more than to dump off an electron and form a positive ion, or cation.
05:25
And they’re always jonesing to hook up with a hottie from the other side of the table.
05:29
So of course, seeing as they’re so reactive, you don’t find hunks of them lying around in nature;
05:34
instead, chemists must extract them from compounds containing them.
05:38
Next, you have the alkaline earth metals – reactive metals, but not as reactive as the alkali metals,
05:43
forming cations with two positive charges instead of just one.
05:47
Calcium, shown here, undergoes a very similar reaction to sodium with water,
05:51
just a little more slowly, producing a little less heat.
05:54
The middle body area of the table is made up of a nice, solid rectangle of transition metals,
05:59
these are the metals you think of as metal, with iron, and nickel, and gold, and platinum.
06:04
The majority of elements are metals – they’re fairly unreactive, great conductors of heat,
06:08
but more importantly for us, good conductors of electricity.
06:11
They’re malleable, and can be bent and formed and hammered into sheets,
06:15
and they’re extremely important in chemistry but overall surprisingly similar to each other.
06:20
On the far right, just over from the noble gases,
06:22
the halogens make up a set of extremely reactive gases that form negative ions, or anions,
06:27
with one negative charge, and love to react with the alkali and alkaline earth metals.
06:31
The rectangle between the halogens and the transition metals contain a peculiar scatter
06:35
shot of metals, metalloids, gases, and nonmetals;
06:39
these guys don’t end up as ions unless you take extreme action and start shooting other ions at them,
06:44
so generally a bit boring over here, though lots of interesting covalent organic chemistry
06:50
(we’ll get to that).
06:51
Down below, in their own little island, are the lanthanides and actinides,
06:55
metals that were largely undiscovered in Mendeleev’s day
06:57
because they’re so similar that it’s next to impossible to separate them from each other.
07:01
And finally, on the far, far right, also undiscovered when Mendeleev built his chart,
07:06
the completely unreactive noble gases.
07:09
Like a lot of other obsessive scientists, Mendeleev never thought he was done with his table,
07:14
so he held it back for quite a while, only publishing it as part of a new chemistry textbook
07:18
he was working on as a way to make some quick cash that he needed.
07:21
And, as with many other scientific revelations, there were a number of other people hot on this discovery’s trail.
07:27
As many as six people published on the periodicity of elements at roughly the same time as Mendeleev,
07:32
but a few things set him apart.
07:34
1. He was obsessive.
07:36
He knew the data better than anyone else,
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and had spent a ton of time working on a theory that many people thought was just an interesting little quirk.
07:43
And 2. he realized in a way no one else did that the idea of periodicity had far-reaching consequences.
07:49
It seems as if he had a deep belief in the cosmic importance of what he was doing,
07:54
almost of religious fascination.
07:56
Mendeleev believed in God
07:57
but also he believed that organized religions were false paths to the unknowable nature of God.
08:03
I like to believe that he thought he saw some divine pattern in his tables,
08:07
and Mendeleev felt as if he was coming to know God in a way that no other man had.
08:12
To be clear, this is pure conjecture.
08:15
And as we now know, the periodicity of elements is a physical phenomenon.
08:18
It’s a function of electrons, which are in some ways pretty dang peculiar, but certainly not at all mystical.
08:24
But we’ll get to that peculiar physical reality in the next episode.
08:28
The periodic table that we know and love – I love it anyway – if a representation of reality;
08:34
a way of understanding and sorting the universe as it exists.
08:37
But that form of the table is not by any means set in stone;
08:41
indeed, a contemporary of Mendeleev envisioned the table set onto a screw, or cylinder,
08:46
with the elements wrapping around from one side to another.
08:48
While Mendeleev’s table looks more like a map up on a wall,
08:52
de Chancourtois, a geologist, envisioned more of a globe.
08:55
Unfortunately for de Chancourtois, no publisher could figure out how to print his cylindrical 3D table,
09:01
and so he published his paper without a graphical representation of his Periodic Cylinder of the Elements,
09:06
and it was largely ignored.
09:08
I guess they didn’t have paper craft back then.
09:10
I am a huge fan of this cut-and-tape model of the periodic table;
09:14
you can make your own – there’s a link in the description –
09:17
and there are also a ton of other designs for periodic tables
09:20
that have various advantages over the one that we’re all familiar with.
09:23
Our periodic table, as it stands, it really a little bit unhappy with itself, frankly;
09:27
the lanthanides and actinides really should be part of the table, but we separate them out,
09:31
because it’s hard to fit that on a piece of paper; really, this is what it should look like.
09:35
And really, it would be best if it wrapped around into a circle,
09:38
so that fluorine, and neon, and sodium were all next to each other, instead of being on opposite sides of the map,
09:44
because they’re just one proton away!
09:45
Mendeleev’s contribution, nonetheless, is more powerful than at first it seemed.
09:49
He ended up forming a guide to help future chemists understand things that wouldn’t be
09:53
discovered for 25, 50, even 100 years.
09:56
Indeed, after Mendeleev’s theories were published and accepted,
10:00
the overwhelming cry form the scientific community was “Why? Why? Why?”
10:06
And although Mendeleev was not himself concerned with this stuff,
10:09
he actually denied the existence of atoms, or indeed anything he couldn’t see with his own eyes.
10:14
It turned out that the answer to the first “Why”, was the electron.
10:18
That sneaky little electron; Mendeleev, if he’d been around to see their discovery, he would have hated them.
10:25
But you, you will have a healthy respect for them,
10:28
after you learn all about them on the next episode of Crash Course Chemistry.
10:32
Thank you for watching this episode of Crash Course Chemistry. If you were paying attention, you now know:
10:36
The terrible, beautiful, and wonderful story of Dmitri Mendeleev;
10:40
How he organized the elements into the periodic table;
10:42
Some of the basics of the relationships in that table;
10:45
Why Mendeleev stood out from his colleagues;
10:48
and how the table as we know it today could stand some improvement.
10:51
This episode of Crash Course Chemistry was written by myself,
10:54
filmed and directed by Caitlin Hofmeister, and edited by Nick Jenkins.
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The script was edited by Blake de Pastino and Dr. Heiko Langner,
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our sound designed is Michael Aranda, and Thought Café is our graphics team.
11:06
If you have any questions, please ask them in the comments below.
11:08
Thank you for learning with us, here in Crash Course Chemistry.


This post was previously published on YouTube.

Photo credit: Screenshot from video

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