just looking at our environment around us,

that there are different substances.

And these different substances tend

to have different properties.

And not only do they have different properties,

one might reflect light in a certain way,

or not reflect light, or be a certain color, or at

a certain temperature, be liquid or gas, or be a solid.

But we also start to observe how they

react with each other in certain circumstances.

And here's pictures of some of these substances.

This right here is carbon.

And this is in its graphite form.

This right here is lead.

This right here is gold.

And all of the ones that I've shown pictures of, here--

and I got them all from this website, right over there--

all of these are in their solid form.

But we also know that it looks like there's

certain types of air, and certain types of air particles.

And depending on what type of air particles

you're looking at, whether it is carbon or oxygen or nitrogen,

that seems to have different types of properties.

Or there are other things that can be liquid.

Or even if you raise the temperature high enough

on these things.

You could, if you raise the temperature high enough

on gold or lead, you could get a liquid.

Or if you, kind of, if you burn this carbon,

you can get it to a gaseous state.

You can release it into the atmosphere.

You can break its structure.

So these are things that we've all, kind of, that humanity

has observed for thousands of years.

But it leads to a natural question

that used to be a philosophical question.

But now we can answer it a little bit better.

And that question is, if you keep breaking down this carbon,

into smaller and smaller chunks, is there

some smallest chunk, some smallest unit, of this stuff,

of this substance, that still has the properties of carbon?

And if you were to, somehow, break

that even further, somehow, you would

lose the properties of the carbon.

And the answer is, there is.

And so just to get our terminology,

we call these different substances--

these pure substances that have these specific properties

at certain temperatures and react

in certain ways-- we call them elements.

Carbon is an element.

Lead is an element.

Gold is an element.

You might say that water is an element.

And in history, people have referred to water

as an element.

But now we know that water is made up of more basic elements.

It's made of oxygen and of hydrogen.

And all of our elements are listed here

in the Periodic Table of Elements.

C stands for carbon-- I'm just going through the ones that

are very relevant to humanity, but over time, you'll

probably familiarize yourself with all of these.

This is oxygen.

This is nitrogen.

This is silicon.

Au is gold.

This is lead.

And that most basic unit, of any of these elements, is the atom.

So if you were to keep digging in, and keep

taking smaller and smaller chunks of this,

eventually, you would get to a carbon atom.

Do the same thing over here, eventually you

would get to a gold atom.

You did the same thing over here, eventually,

you would get some-- this little,

small, for lack of a better word, particle,

that you would call a lead atom.

And you wouldn't be able to break that down

anymore and still call that lead,

for it to still have the properties of lead.

And just to give you an idea-- this is really something

that I have trouble imagining-- is

that atoms are unbelievably small, really

unimaginably small.

So for example, carbon.

My hair is also made out of carbon.

In fact, most of me is made out of carbon.

In fact, most of all living things are made out of carbon.

And so if you took my hair-- and so my hair is carbon,

my hair is mostly carbon.

So if you took my hair-- right over here,

my hair isn't yellow, but it contrasts nicely

with the black.

My hair is black, but if I did that,

you wouldn't be able to see it on the screen.

But if you took my hair, here, and I

were to ask you, how many carbon atoms wide is my hair?

So, if you took a cross section of my hair, not

the length, the width of my hair,

and said, how many carbon atoms wide is that?

And you might guess, oh, you know,

Sal already told me they're very small.

So maybe there's 1,000 carbon atoms there, or 10,000,

or 100,000.

I would say, no.

There are 1 million carbon atoms,

or you could string 1 million carbon atoms

across the width of the average human hair.

That's obviously an approximation.

It's not exactly 1 million.

But that gives you a sense of how small an atom is.

You know, pluck a hair out of your head,

and just imagine putting a million things

next to each other, across the hair.

Not the length of the hair, the width of the hair.

It's even hard to see the width of a hair,

and there would be a million carbon atoms,

just going along it.

Now it would be pretty cool, in and of itself,

that we do know that there is this most basic building

block of carbon, this most basic building block of any element.

But what's even neater is that, those basic building

blocks are related to each other.

That a carbon atom is made up of even more fundamental

particles.

A gold atom is made up even more fundamental particles.

And depending-- and they're actually

defined by the arrangement of those fundamental particles.

And if you were to change the number of fundamental particles

you have, you could change the properties of the element, how

it would react, or you could even change the element itself.

And just to understand it a little bit better,

let's talk about those fundamental elements.

So you have the proton.

And the proton is actually the defining-- the number

of protons in the nucleus of an atom,

and I'll talk about the nucleus in a second-- that

is what defines the element.

So this is what defines an element.

When you look at the periodic table right here,

they're actually written in order of atomic number.

And the atomic number is, literally,

just the number of protons in the element.

So by definition, hydrogen has one proton,

helium has two protons, carbon has six protons.

You cannot have carbon with seven protons.

If you did, it would be nitrogen.

It would not be carbon anymore.

Oxygen has eight protons.

If, somehow, you were to add another proton to there,

it wouldn't be oxygen anymore.

It would be fluorine.

So it defines the element.

And the atomic number, the number

of protons-- and remember, that's

the number that's written right at the top,

here, for each of these elements in the periodic table--

the number of protons is equal to the atomic number.

And they put that number up here,

because that is the defining characteristic of an element.

The other two constituents of an atom-- I

guess we could call it that way--

are the electron and the neutron.

And the model you can start to build

in your head-- and this model, as we go through chemistry,

it'll get a little bit more abstract and really hard

to conceptualize.

But one way to think about it is,

you have the protons and the neutrons that

are at the center of the atom.

They're the nucleus of the atom.

So for example, carbon, we know, has six protons.

So one, two, three, four, five, six.

Carbon-12, which is a version of carbon,

will also have six neutrons.

You can have versions of carbon that

have a different number of neutrons.

So the neutrons can change, the electrons can change,

you can still have the same element.

The protons can't change.

You change the protons, you've got a different element.

So let me draw a carbon-12 nucleus, one, two, three, four,

five, six.

So this right here is the nucleus of carbon-12.

And sometimes, it'll be written like this.

And sometimes, they'll actually write the number of protons,

as well.

And the reason why we write it carbon-12--

you know, I counted out six neutrons--

is that, this is the total, you could

view this as the total number of-- one way to view it.

And we'll get a little bit nuance

in the future-- is that this is the total number of protons

and neutrons inside of its nucleus.

And this carbon, by definition, has an atomic number of six,

but we can rewrite it here, just so

that we can remind ourselves.

So at the center of a carbon atom, we have this nucleus.

And carbon-12 will have six protons and six neutrons.

Another version of carbon, carbon-14,

will still have six protons, but then it

would have eight neutrons.

So the number of neutrons can change.

But this is carbon-12, right over here.

And if carbon-12 is neutral-- and I'll give a little nuance

on this word in a second as well-- if it is neutral,

it'll also have six electrons.

So let me draw those six electrons, one, two, three,

four, five, six.

And one way-- and this is maybe the first-order way

of thinking about the relationship

between the electrons and the nucleus--

is that you can imagine the electrons are, kind of, moving

around, buzzing around this nucleus.

One model is, you could, kind of,

thinking of them as orbiting around the nucleus.

But that's not quite right.

They don't orbit the way that a planet, say,

orbits around the sun.

But that's a good starting point.

Another way is, they're kind of jumping around the nucleus,

or they're buzzing around the nucleus.

And that's just because reality just

gets very strange at this level.

And we'll actually have to go into quantum physics

to really understand what the electron is doing.

But a first mental model in your head

is at the center of this atom, this carbon-12 atom,

you have this nucleus, right over there.

And these electrons are jumping around this nucleus.

And the reason why these electrons don't just

go off, away from this nucleus.

Why they're kind of bound to this nucleus,

and they form part of this atom, is

that protons have a positive charge

and electrons have a negative charge.

And it's one of these properties of these fundamental particles.

And when you start thinking about,

well, what is a charge, fundamentally,

other than a label?

And it starts to get kind of deep.

But the one thing that we know, when

we talk about electromagnetic force,