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Hank brings us the story of the electron and describes how reality is a kind of music, discussing electron shells and orbitals, electron configurations, ionization and electron affinities, and how all these things can be understood via the periodic table.
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Transcript Provided by YouTube:
00:00
Hello, I’m Hank Green! Welcome to Crash Course Chemistry.
00:03
Last time we left off with Mendeleev believing he had discovered a cosmic, mystic reality about the world,
00:08
but in fact he had discovered the effects of his worst enemy: tiny invisible particles.
00:14
Electrons, which are so marvelous and peculiar that it wasn’t until 80 years after Mendeleev’s 1st periodic table
00:22
that they were really understood by anyone and to this day still very, very few people understand them.
00:27
But in like 10 minutes from now, if all goes according to plan, you will be one of those people. So let’s do this.
00:34
[Theme Music]
00:43
In 1865, before Mendeleev published his first periodic table,
00:48
a young chemist, and activist, John Newlands published a paper on the periodicity of elements,
00:54
comparing their repetition, at least the first two rows of it, to a musical scale.
00:58
All do re me fa so la ti do and stuff.
01:01
Maybe, he theorized, lithium was just sodium but an octave higher.
01:05
Maybe they were, in a sense, the same note.
01:08
He delivered this idea to the Royal Academy, the most prestigious group of scientists in the world,
01:12
and they basically laughed him off the stage.
01:15
“Music is art and chemistry is science.
01:17
Now describing science in an artistic way might be a fine parlor trick for helping little
01:21
babies or women understand the work you do but they have no place in the Royal Academy.”
01:26
That’s my impression.
01:28
But there was no way of course of knowing that it turns out John Newlands,
01:31
when it came to the actual, functional, physical reality behind the periodicity of elements,
01:36
was more right than any of the scientists who laughed him off the stage that day.
01:40
And he never got to find out how right he was.
01:42
We didn’t discover that his analogies were barely analogies until long after his death,
01:48
but it turns out that reality is like a kind of music.
01:53
And maybe you want to laugh me off the stage right now but bear with me.
01:57
Before quantum mechanics, scientists envisioned the atomic world as just a miniature macroscopic world.
02:02
Electrons seemed to just be particles orbiting around a nucleus.
02:06
In fact, there was a great Dane…no Nick…other kind of Dane… Niels Bohr, yes.
02:12
Like certain other people I might name,
02:13
he sometimes felt like he was in the shadow of his older, more successful brother, an Olympic soccer player,
02:19
while Niels’ handwriting was so poor that he had to dictate his PhD thesis to his mom
02:24
Nevertheless, he was an ingenious physicist.
02:27
Now, you might remember a couple of episodes back,
02:29
when John Dalton determined that elements only exist in discrete packets of matter.
02:33
Well, by Bohr’s time the same thing was known for the energy given off by electrons.
02:38
That energy only came off in what ended up being called “quanta” which is the root of the term “quantum mechanics.”
02:44
In 1913, Niels Bohr came up with a simple model for describing these energy levels
02:47
for a single electron in hydrogen merely assuming circular orbits.
02:52
So there is some truth to the framework of thinking of electrons as particles.
02:55
However, when he or anyone else tried to apply this to more complicated atoms, they failed miserably.
03:01
Long story short, electrons don’t really behave like particles; they’re better described as waves.
03:07
So we’ve known for, like, fifty years that this is a an entirely inaccurate way of visualizing an atom.
03:14
Nuclei, yeah, you can think of them as solid particles but not electrons. Electrons are wave-particle dualities.
03:20
I think of them like resonance in the universe.
03:22
And just like a single string producing multiple notes on a guitar,
03:26
an electron can exist in a number of different harmonics.
03:29
This isn’t an analogy either, quantum physicists actually talk about the harmonics of electrons.
03:35
After a few years of trying to figure this stuff out,
03:37
a couple of very smart physicists started to look at electrons as waves, standing waves.
03:41
This makes a lot of sense, even to us lay guys.
03:44
When you swing a telephone chord in a straight line there are a discrete numbers of nodes,
03:49
depending on the tension and the frequency, or in better physics terms, “energy”, you put into the system.
03:55
The same thing happens to an electron around a nucleus;
03:57
a standing wave is produced only at certain energy levels.
04:01
Anything in between is not allowed.
04:03
An Austrian physicist, Erwin Schrodinger, who you may have heard of because of his cat,
04:08
is the first guy who developed a mathematical model where the electron was assumed to be a standing wave.
04:14
Now, it is time to move a little bit into the realm of metaphor here
04:17
because I’m gonna tell you how I think, how I think of electron shells and orbitals, not really how they are.
04:23
The music of electrons is not simple music. It’s no three chord song;
04:27
it’s like Beethoven but with more rules, hard fast rules that can’t be broken.
04:33
In that way, at least, it’s more science than art.
04:35
Electrons exist in orbitals a bit like the individual notes on a keyboard.
04:39
But the orbital’s tone isn’t complete until it has two electrons in it, and orbitals exist in shells.
04:45
The first shell just has a single orbital, an s-orbital, which can only fit two electrons.
04:51
That’s why the row of our periodic table only has two elements.
04:54
They play simple song those two and a song that every other element will build upon.
04:58
The second electron shell is physically larger and thus can include more than just the s-orbital.
05:04
A second sort of orbital with three different configurations is added, the p-orbital.
05:08
Instead of just being a single tone, the p-orbital is more like a 3 part harmony with 2 electrons
05:14
in each part for a total of 6.
05:16
Those 6 electrons of the p-orbital plus the 2 s-electrons are the eight electrons referred to in the octet rule:
05:22
the desire for most of the lighter elements to have 8 electrons in their outer shell, just like a musical scale.
05:27
This is often described in terms of fullness or satiation, as if the atoms are devouring electrons.
05:32
But I prefer to think of an incomplete electron configuration as a cacophonous symphony
05:37
playing in different keys and at different tempos.
05:39
And the closer you are to harmony the worse it is, until one final note chimes in, that eighth note
05:44
and everything crisps into full resolution a deep, complex tone emerges,
05:49
and the atom settles into complete harmony, the harmony of the noble gases.
05:53
That’s just how I think of it.
05:55
And just like with music, there’s a bit of notation to learn as well.
05:58
It’s important to know how to write out what we call “electron configurations,”
06:02
a condensed way of showing exactly where all of an atoms electrons are.
06:06
First, we write the number of the shell, then the letter of the orbital, then the number of electrons in that orbital,
06:11
and repeat until we run out of electrons.
06:14
So for hydrogen, with just one electron it’s 1s1. For fluorine it’s 1s2 2s2 2p5.
06:19
Now as we move to the third row, an interesting thing happens;
06:22
the third shell adds a third kind of orbital: the five part harmony, with ten electrons, of the d-orbital.
06:28
But you might be saying, “There’s still only eight elements in the third row.
06:31
What’s up with that, Hank? It’s all lies, I’m leaving!”
06:34
OK calm down, the atomic symphony composes itself in peculiar ways.
06:38
Because building the 3d orbital requires a lot of energy,
06:42
electrons actually go into the s-orbital of the fourth shell, 4s, before going into the third shell’s d-orbital.
06:48
This is actually a trend that continues, and to remember it just write out the following on a piece of paper —
06:53
these are the orbitals we know and all the shells that we’ve seen exist.
06:58
To figure out what order to fill them in, you just draw a diagonal line from the top right to the bottom left as you go.
07:04
So 1s first, then 2s, then 2p 3s, then 3p 4s, 3d 4p 5s, 4d 5p 6s, and so on.
07:13
Armed with this knowledge you could write out the electron configuration for pretty much any element on the table.
07:18
Iron, number twenty-six, would be 1s2 2s2 2p6 3s2 3p6 4s2 3d6.
07:26
Now there are a couple of elements that have weird electron shells, but you can just look it up on Google.
07:32
An interesting thing about d-orbitals, and the even bigger, more electron rich f-orbitals,
07:37
is they don’t really need to be filled quite as much as the s and p’s,
07:43
because they’re literally shielded beneath the s-orbitals of the next shell.
07:47
The s and p-orbitals I think of kinda like the trumpets and violins: it’s really terrible when they sound bad,
07:53
but the base notes, deep and rich, hide a bit underneath the rest of the orchestra,
07:58
just like the d-orbitals literally hide underneath the s-orbitals that have already filled above them.
08:02
Yes these incomplete orbitals affect them,
08:05
but because their shielded, these middle of the chart elements are generally less reactive
08:09
and happier to bump electrons along from atom to atom making them conductive,
08:14
or just hanging out together in big masses of electron-sharing lumps of metal.
08:18
So why are orbitals useful when it comes to understanding how an atom is likely to react?
08:23
Well first, it really matters how much energy is required to remove an electron from an
08:27
atom to form a positively charged ion.
08:30
This energy is called “ionization energy.”
08:33
If there are several electrons being removed this is a step-wise process,
08:36
starting with the electron at the highest energy level, or the outermost one.
08:39
Since the outermost electron has the highest energy, there’s the least energy necessary to remove it.
08:43
More energy is needed to remove the second furthest one out and so on.
08:47
And of course when all the electrons in the outermost shell are removed
08:50
there is a really large energy jump necessary to remove an electron from the next shell down
08:55
because that shell will be isoelectrically analogous to a noble gas.
08:59
Just like how atoms are isotopically the same when they have the same number of protons and neutrons,
09:03
atoms are isoelectrically the same when they have the same number of electrons.
09:07
And just like there’s energy associated with removing an electron to form cations, or positively charged ions,
09:12
there’s energy associated with adding electrons,
09:15
usually to fill an orbital to achieve a stable two or eight electron shell configuration.
09:20
Just like with the ionization energy there’s a discrete energy jump involved with the adding of an electron.
09:25
That energy is called “electron affinity.”
09:27
Now, you ready for the real mind melter? If you’re following along in your periodic table,
09:32
which of course you aren’t,
09:33
you may have noticed a little something interesting.
09:35
On the left-hand side you have your s-orbitals — one, two, three, four, five, six, seven s.
09:40
In the middle, you got your d’s: 3d, 4d, 5d, 6d.
09:43
And on the top right, 2p, 3p, 4p, 5p, 6p.
09:47
And below of course, in the little island of the lanthanides and actinides, your f-orbitals: 4f and 5f.
09:52
And so, with just a glance at your periodic table you can work out electron configurations
09:58
and elemental stability and the fundamental, physical reality of the elements.
10:02
That’s why this thing is so beautiful to me,
10:04
because when you get to know it, you see all those flawed, competing harmonies,
10:09
and the actions and reactions that occur because of them,
10:12
changing their song into something more stable and powerful and eternal together, making everything.
10:19
Now as I’ve gone through today’s episode I’ve described electrons mostly in musical terms as vibrating waves,
10:25
harmonies in the fabric of the universe and that’s, indeed, how I like to think about them,
10:30
but of course, that isn’t a complete story.
10:33
And I have gotten sick of people telling me that the human brain is incapable of imagining the reality of the subatomic.
10:39
So I’m actually just going to serve a big heaping pile of reality on you right now no matter how odd it turns out to be.
10:46
There are, a number of everywhere-permeating fields in our universe.
10:50
One of those, is the electron field.
10:52
In order for an electron to exist, there has to be an excitation of the electron field
10:57
and we can describe those excitations as waves, just as a wave in the ocean is an excitation of the water.
11:03
At any given moment, the electron can be anywhere within the function of the wave.
11:07
But waves are defined not by harsh boundaries, instead they’re strong in some areas and weak in others.
11:13
The strength of the wave at one certain point in space determines
11:16
how likely it is that you will find an electron there at any given time if you measure.
11:20
And so if we’re trying to understand reality,
11:22
we should not think of electrons as circling around the nucleus of an atom, like planets around a star,
11:27
but instead as an excitation around the nucleus, and the shape of that excitation is the orbital.
11:33
Orbitals are precisely the reason that everything exists.
11:37
They are the root, and the key, and the nexus, and the crux, and the keystone,
11:41
and every other metaphor of not just chemistry but existence.
11:45
Thank you for watching this episode of Crash Course Chemistry. I hope it blew your mind.
11:49
If you were paying attention you now know about a poor young man who was laughed out
11:53
of a meeting of snotty scientists because of being far more correct then anyone could ever have imagined.
11:58
About a great Dane whose incorrect model of the atom was pretty amazing anyway.
12:03
About electrons as music, and electron shells and the orbitals they contain.
12:07
How to write out electron configurations, what ionization energies and electron affinities are,
12:12
and how the periodic table ties all of these realities together.
12:16
And with all that knowledge now in your head, you know more then 99.9% of the world about electrons.
12:22
This episode of Crash Course Chemistry was written by myself,
12:25
filmed and directed by Michael Aranda, who is also our sound designer, and edited by Nick Jenkins.
12:30
The script was edited by Blake de Pastino an Dr. Heiko Langner.
12:33
Katherine Green was our script supervisor and our graphics team is Thought Cafe.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video
