ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: Molecules, crystals, and diatomic elements | Middle school chemistry | Khan Academy

If you zoom into water,

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If you zoom into water,

you will find a molecule of water,

where you have one oxygen atom attached to two hydrogen atoms.

You will find trillions and trillions of them.

Similarly, if you were to zoom into methane,

which we also call natural gas,

you will now find molecules of methane,

which happens to be one carbon atom attached to four hydrogen atoms.

Again, you will find trillions and trillions of them.

So the big question now is,

what would you get if you were to zoom into table salt, which is NaCl?

Well, our guess could be we'll find molecules of NaCl,

where one Na is attached to one Cl.

Trillions and trillions of them, right?

Wrong.

That's not what we'd get.

We'll get something completely different. But what?

Well, let's find out.

So what will we find if we were to zoom into NaCl?

We'll get something like this,

where you'll find trillions of Na's and trillions of Cl all connected to each other.

We call this a crystal of sodium chloride, crystal of NaCl.

You probably heard of this word crystal,

it means we have an orderly repeating pattern.

This looks very different, right?

So let's compare it with what we saw earlier.

If we were to go back to the molecules of water,

what we find is that an individual oxygen is connected to two other hydrogen atoms.

And that connection is due to a force of attraction,

which we call a chemical bond.

The same thing is happening over here.

There's a chemical bond because of which a carbon atom is attached to four hydrogen atoms,

right but

when it comes to a crystal of NaCl look we don't

have individual molecules like with what we see over here we don't find them instead we find trillions

and trillions of Na's connected to trillions and trillions of Cl in

that chemical bond so there are no individual molecules you only have a crystal

but wait a second wait a second wait a second again if we go back to molecule of water.

The reason why we call it H2O is

because you have two hydrogen atoms connected to one oxygen atom to form a single unit molecule, right?

Same is the case over here.

It's because you have one carbon attached to four hydrogens.

That's why we say the,

you know, this is CH4.

But what about over here?

Why do we call it NaCl?

I mean, if you don't have individual molecules,

what does it mean to say NaCl?

Well, what it means is that if you were to take a chunk of this crystal,

then you will find equal amounts of Na and Cl.

In other words, you will have them in the ratio 1 is to 1.

For every 1 Na, you will find 1 Cl.

That is what that means over here.

It doesn't mean that you'll find a molecule of NaCl where 1 Na is attached to 1 Cl.

No, no, no. You find a crystal,

but the ratio of NaNCl is 1 is to 1.

That's what it means over here.

And we can take another example of the salt.

There are other salts as well.

So for example, if you take potassium oxide,

it also happens to be a salt.

It is K2O, where K is potassium, O is oxygen, okay?

But what does it mean to say K2O?

Does it mean that there are molecules of K2O?

No, this is again a crystal.

It's a crystal of potassium oxide.

But what does it mean for us to say K2O?

Well, what it means now is that if you were to take a chunk of this crystal,

you will find that there are twice as many potassium as oxygen.

You'll have two potassium atoms for every oxygen atoms.

That's what it means to say it is K2O.

So putting it all together,

what we have so far is that we can have two kinds of compounds.

We have compounds that are made of individual molecules and we have compounds which are made of crystals.

Now, of course, these are just two general categories of compounds,

but there are more advanced forms of compounds as well.

We'll not worry too much about them.

But anyways, the compounds that are made of molecules are called molecular compounds

and the ones that are made of crystals are called crystalline compounds.

So look, not all compounds are made of molecules.

That's the important thing.

They can also be crystalline.

They can also be made of crystals.

But here's an interesting question.

Not all compounds are molecules, right?

But what about the other way around?

Are all molecules compounds?

Well, let's see.

Let's take an example.

What if we were to zoom into oxygen?

Well, we'll find that two atoms of oxygen are combined together to form O2,

which is a molecule of oxygen.

And of course, we'll find trillions and trillions of them.

But the big question is, is this a compound?

The answer is no, it's not a compound.

Because remember, compounds are chemical combination of two or more different elements.

Here, there's a chemical combination of the same element.

Oxygen and oxygen is getting combined.

So it's a molecule, but it is not a compound.

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In fact, we call this as diatomic element.

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Di because there are two.

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And there are other examples of diatomic elements.

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Nitrogen, fluorine, chlorine, and many more.

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Well, of course, one thing to remember is

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that these elements are usually diatomic when they're alone like when you have oxygen gas or chlorine gas.

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But of course, when they do form a compound, they can split apart.

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Again, go back to molecule of water.

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You can have one oxygen atom combined to two hydrogen atoms.

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Anyways, if you put it all together,

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we have elements which are the fundamental building blocks of matter.

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Some elements are made of individual atoms.

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We usually call them monoatomic,

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mono for single individual, like gold or mercury.

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And other elements can be formed as diatomic,

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like molecules of nitrogen or oxygen.

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And when two or more different elements chemically combine together, we get compounds.

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We can have molecular compounds where you find individual molecules,

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or we can have crystalline compounds when there are no molecules at all.

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And finally, we categorize this all under what we call pure substances,

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because they're made of purely the same stuff.

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For example, water is purely made of water molecules.

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sodium chloride is purely made of NaCl

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and oxygen for example is purely made of oxygen molecules so

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because they're all made of purely the same stuff these are all pure substances

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ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: Molecules, crystals, and diatomic elements | Middle school chemistry | Khan Academy

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