Name: 週期表。化學速成班#4 (The Periodic Table: Crash Course Chemistry #4)
Uploaded: 2021-01-14T06:03:00.000Z
Duration: 11 min 22 s
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Hello, I'm Hank Green, welcome to Crash Course Chemistry.

Today we're talking about the most important table ever.

Not the table where they signed the Declaration of Independence, nor any table of contents,

nor this table right here, nor the stone table of Aslan, NAY!

It is the periodic table of elements, a concise, information-dense catalogue

of all of the different sorts of atoms in the Universe.

Today I want to talk a little bit about the creation of this table, which is, to be clear, one of the

crowning achievements of human thought. To start out, though, let's close our eyes and pretend.

Imagine you're in Siberia. And you're a thirteen-year-old boy. And your father, who was a professor

but had gone blind, leaving your family of more than ten brothers and sisters destitute, has just died.

Your mom, to support the family, has re-opened an abandoned glassmaking factory

in the small town where you live, largely because she wants to make enough money to

send you to school someday. A year passes - the factory burns down.

But your mom, she sees your potential; she knows that you have a keen scientific mind and will not see it squandered.

So, with your siblings out of the house and on their own, she packs up your belongings,

straps them to a horse, and with you in tow, rides 1200 miles through the Ural Mountains

There, on your behalf, she pleads earnestly and effectively, and they reject you.

So together, you ride another 400 miles to St. Petersburg, to the school where your father

had graduated as a scientist, and as luck, or extreme, insane, undeniably Russian persistance would have it,

they accept you, and your saddle-worn butt, as a pupil.

Your mother, having completed her mission, promptly dies.

If you're doing your imagining as I told you, you might feel a tremendous debt to your mother,

and a very deep desire to ensure that you achieve something on par with the sacrifices she made for you.

And maybe that's one of the reasons why Dmitri Ivanovich Menedeleev became the crown jewel

of Russian science, and a theorist who revolutionized how we see the world.

Mendeleev spent a great deal of time in laboratories as a student,

studying the burgeoning new field of chemistry. He worked with all the elements that you could work with

at the time, and his knowledge gave him unique insights into their properties.

Let's all imagine we're Mendeleev again - I like doing that - and that we know a bunch of stuff about chemistry

- which, you know, we don't, yet - YET - but we're imagining.

So it's the 1860s, and about sixty elements are known to mankind,

and their atomic weights are mostly known as well.

So the simplest thing was to just sort them in order of their atomic weights.

But interestingly, you, because you're a cleverpants, realized that the most significant relationships

seem to have nothing to do with the atomic weight.

Lithium, sodium, potassium, and rubidium were all extremely prone to reacting with

chlorine, fluorine, iodine, and bromine; beryllim, magnesium, calcium and strontium were all similar,

But with a quick inspection, you, and to be fair, a number of other chemists, realize that

there was a relationship between atomic weights, but it's periodic.

At the beginning of the list of elements, characteristics repeat every seven elements.

On the side here, we now know that it's every eight elements, but in the 1860s, elements were studied

based on their reactivity, so the non-reactive noble gases had not yet been discovered

so the period occurred every seven elements.

As the mass of the elements increases, the repetition starts to look a little less periodic,

though it's certainly still there, it just isn't perfect.

Some of your colleagues, they're saying:"Well, such is life.

It was perfect repetition early on, but later in the list it gets a little fuzzier."

But not you; you become obsessed. Obsessed with the perfection of the periodicity.

You write out names and weights and properties of elements on cards;

you lay them across your desk, shuffle them, tear them to pieces in frustration, until one day, you realize -

The numbers aren't working, not because there's something wrong with your ideas,

but because some elements simply haven't been discovered yet.

Armed with this insight you insert gaps into the table, and things suddenly fall perfectly into place.

Seven-element periods for the first two rows, with hydrogen in its own category,

eighteen-element periods for the next two rows.

You're so certain that you predict the properties of these missing elements.

And when a French scientist comes along and says that he has, in fact, discovered one of them,

you argue with him, saying that you discovered it first in your mind.

And when you see his data, and it doesn't match yours, you publish a paper saying his data for the new element

he discovered is wrong. That's how certain you are of yourself and this beautiful new theoretical framework

you've created. You know what the really crazy thing is?

You're right! That French guy's data was wrong!

You, never having examined the element he discovered, knew more about it than he did,

because you are Mendeleev, Master of the Elements.

Okay, we're done imagining for the episode; that was fun though.

The different groups Mendeleev had identified are a lot of the same groups that we study today.

Starting at the left we have the soft, shiny, extremely reactive alkali metals,

so reactive in fact, that they have to be stored in inert gases or oil,

to prevent them from reacting with the atmosphere.

Alkali metals want nothing more than to dump off an electron and form a positive ion, or cation.

And they're always jonesing to hook up with a hottie from the other side of the table.

So of course, seeing as they're so reactive, you don't find hunks of them lying around in nature;

instead, chemists must extract them from compounds containing them.

Next, you have the alkaline earth metals - reactive metals, but not as reactive as the alkali metals,

forming cations with two positive charges instead of just one.

Calcium, shown here, undergoes a very similar reaction to sodium with water, just a little more slowly,

The middle body are of the table is made up of a nice, solid rectangle of transition medals -

these are the metals you think of as metals, with iron and nickel and gold and platinum.

The majority of elements are metals - they're fairly unreactive, great conductors of heat,

but more importantly for us good conductors of electricity, they're malleable,

and can be bent and formed and hammered into sheets, and they're extremely important in chemistry,

but overall surprisingly similar to each other.

On the far right, just over from the noble gases, the halogens make up a set of extremely reactive gases

that form negative ions, or anions, with one negative charge,

and love to react with the alkali and alkaline earth metals.

The rectangle between the halogens and the transition metals contain a peculiar scatter shot of metals,

metalloids, gases, and nonmetals; these guys don't end up as ions unless you take extreme action

and start shooting other ions at them, so generally a bit boring over here,

though lots of interesting covalent organic chemistry (we'll get to that).

Down below, in their own little island, are the lanthanides and actinides,

metals that were largely undiscovered in Mendeleev's day because they're so similar

that it's next to impossible to seperate them from each other.

And finally, on the far, far right, also undiscovered when Mendeleev built his chart,