We now have a respectable understanding of the periodic
table itself and the atoms in them.
And now we're ready to deal with molecules themselves.
And to deal with molecules, we have to have some way of
And you represent them with formulas.
And there's two major-- actually, three major ways to
represent a molecule.
One is the molecular formula.
And the other is the empirical formula.
And I'll do it in a different color to differentiate it.
And the difference is, well, let's just talk about what the
word empirical means.
I remember when I first took chemistry the teacher kept
using the word empirical.
Well, what does empirical really mean?
I clearly did not have a very deep vocabulary.
I forgot what age I was.
But it means achieved through observation or experiment, or
based on experience.
So if someone said that they empirically figured out x, y,
or z, it means that they figured it out through an
experiment or they observed it.
The molecular formula is essentially the actual number
of atoms in that molecule.
Let me show you what I'm talking about.
So the empirical formula tells you what people have observed.
Maybe before they even knew that there was such a thing as
atoms, what they would have observed is the ratio of the
atoms to one another in a molecule without knowing in
the exact molecule how many of that atom there are.
Let me show you what I'm talking about.
So if I were to give you, I don't know, a benzene, the
molecular formula of benzene, you have 6 carbon atoms and
you have 6 hydrogen atoms. Now, if you were some chemist
in the 1800's and you didn't know about the actual atoms,
but if you had a big bag of benzene and you were to
measure the ratio of the carbon to the hydrogen that
you had in that bag, you would find out that for every one
carbon you have one hydrogen.
So your empirical formula is the ratio of the two.
You don't know that each atom actually has 6 of these, but
you know that for every carbon, there's a hydrogen.
And for every hydrogen there's a carbon.
And the way to go back, you can go from the molecular
formula to the empirical formula very easily.
You just find the greatest common divisor of the number
of atoms in the molecule.
So the greatest common divisor is six, and six is obviously
six, you divide both of these by six and you get the
It's not easy.
You pretty much can't go back from the empirical formula to
the molecular formula.
You've lost information.
I don't know whether this was C6H6.
Was it C2H2?
You just don't know.
And I mentioned right at the beginning of the video that
there's a third way to represent molecules.
And that's the structural formula.
And we'll do that, off and on, and we've already done it a
Let me show you.
The structural formula for benzene would actually say how
the molecular formula atoms are configured.
So benzene in particular is very interesting.
It looks like this.
It's often drawn like this.
And you'll see this a lot when you take organic chemistry.
But it looks like a little hexagon, where the vertices of
the hexagon are carbon atoms. So let me draw the carbon
atoms in yellow.
So this is carbon, carbon, carbon,
carbon, carbon, carbon.
They have double bonds, every other carbon.
And then they have single bonds to hydrogen.
Let me just do the hydrogen in another color.
Let me do it in magenta.
And obviously, the structural formula gives you the most
Then you can start to think about, gee, how will this
interact with other things?
While the molecular formula just tells you what's in the
molecule, the empirical formula really gives you the
It just tells you the ratio of the different
items in the molecule.
Let's do a couple more.
OK, what if we're dealing with, let's say we're dealing
I think you know the molecular formula for water.
Now what would be the empirical formula for this?
Well we want to know the ratio, so for every oxygen
there's two hydrogens.
Or I guess you could say for every hydrogen
there's a half of oxygen.
So you can't reduce this.
If I wrote this as H2O1, what's the greatest common
divisor of 2 and 1?
Well, it's 1, so you just have to divide them by 1.
So in this case, the empirical and the molecular formula are
the exact same thing.
What about sulfur?
And an interesting molecule, because obviously,
it's just one atom.
Sorry, no, I'm spelling it wrong.
No, it's not a p h, it's f.
Clearly, I shouldn't be making spelling videos.
So the molecular formula S8.
So it forms this neat kind of
octagon-looking chain of sulfurs.
And if I were to draw that, you would see that.
And you could look it up on Wikipedia, if you like.
But its empirical formula, if you had just a bag of sulfur,
you don't know that each atom has eight sulfurs.
You just have a big bag of sulfur.
So, in the empirical formula, there's only one
atom in this molecule.
You divide by eight and you get S.
So you just know that all you've got there is sulfur.
So let's just do one more.
I'll pick a new color.
The molecular formula is C6H12O6.
So for every carbon, there are how many?
For every 6 carbons, there's 12 hydrogen and 1 oxygen.
So if you kind of reduce this formula to its empirical form,
what do you get?
Let's see, you can divide all these numbers by 6, so we get
1 carbon, 2 hydrogens, and 1 oxygen.
So this just tells you the ratio that they exist in a big
bag of this molecule.
This tells you the exact number of
atoms in that molecule.
So now we know a little bit of the difference between
molecular formula, and empirical formula, and
Now let's see if we can use what we know about the
formulas and the periodic table to think a little bit
about the composition, the mass composition, of some of
So the first thing to even think about is, how do you
figure out the molecular mass?
I have my little periodic table down there.
So molecular mass.
So the first question is, how do you figure out-- I mean,
the molecular mass is going to be the sum of all of the atoms
in that molecule, right?
So if you wanted to know the molecular mass of-- let's say
you wanted to know how much does one
molecule of benzene mass?
I don't want to say weight, because it should be
independent of what planet you're on.
So what is the mass of one molecule of benzene?
Well, all you do is you add up the masses of the different
So you have 6 carbons and 6 hydrogens.
So let's do benzene.
You have 6 carbons and 6 hydrogens.
So what's the mass of each carbon?
So let's go back down to the periodic table.
Just to give proper credit, I got this off of the Los Alamos
National Laboratories website.
So let's see, the atomic mass of carbon.
The reason why I used this one instead of my previous one is
my previous periodic table that I got off Wikipedia only
had atomic numbers on them.
But now that we're actually going to start talking about
the mass composition of different atoms
or different molecules.
We're going to have to start looking at the
atomic mass, right?
Remember, the atomic mass, when you think about atomic
mass units, it's just the number of protons plus the
number of neutrons.
So you have six protons in carbon
and roughly six neutrons.
And why is there this decimal?
Because, as we said before, this is an average of all of
the masses of the isotopes you'll find of carbon.
So there's a little bit of carbon 14 on the planet, very
little, but most of the carbon is carbon 12.
When you proportionately average them, you get 12.01.
But let's say we're dealing with carbon 12, just because
that's the most common element.
Carbon is 12 atomic mass units.
And atomic mass units is a unit of mass.
And we'll talk about how small it is.
It's a very, very, very, very small fraction
of a gram or kilogram.
And we'll talk about that, probably in the next video.
So carbon is 12 atomic mass units.
What about hydrogen?
We go to our periodic table.
Hydrogen is here in this dark blue.
And I don't know if you can read it, but this is
interesting, the atomic number of hydrogen is 1.
The atomic mass of hydrogen is 1.0008.
So that tells us that most of the hydrogen on this planet
has an atomic mass of 1.
Which tells us that it essentially has no neutrons.
That hydrogen is a kind of an interesting nucleus there,
where there is really just a proton.
Just a proton sitting in that nucleus.
And so if you were to ionize hydrogen.
If you were to turn into a cation and take one of its
electrons away, what are you left with?
You just have a proton.
A proton sitting by itself, just a single proton, really
is no different than a hydrogen ion.
And that to me is kind of interesting.
That hydrogen is that simple.
It's really just a proton.
So hydrogen has an atomic mass of 1.
If it had any neutrons in it, it would have been at least an
atomic mass of 2.
But hydrogen has atomic mass of 1.
One atomic mass unit.
So what is the mass of one molecule of benzene?
Well it's 6 times the carbon mass.
6 times 12 plus 6 times the mass of hydrogen.
Plus 6 times 1.
So that is 6 times 12, is 72, plus 6 times 1, plus 6, is
equal to 78.
Now, what if someone said, what percent
of benzene is carbon?
Well then you say, OK, this is the piece that's
carbon, right here.
The carbon piece of benzene is 72 atomic mass units.
Right, that's carbon.
So what percentage of benzene is carbon?
Well it's 72 over 78.
The whole thing is 78.
So it's 72 over 78.
And what does that equal?
Let me get a calculator going.
I should've had my calculator open ahead of time.
So 72 divided by 78 is equal to 92.3%.
So benzene is 92.3% carbon by mass.
And of course, the remainder, the 7.7%,
is going to be hydrogen.
Let's do that for a couple of these other guys down here.
So let's say we wanted to know what is the mass of
a molecule of water?
There's enough water on the planet, if you want to know
what that is.
Well we already know what the mass of a hydrogen is.
Hydrogen is 1.
One atomic mass unit.
Oxygen is what?
Oxygen is 16.
Notice, it's exactly 16.
So on most of the planet, you pretty much have, in an oxygen
atom, you have 8 protons and exactly 8 neutrons.
So you get an atomic mass of 16.
So oxygen has an atomic mass of 16 atomic mass units.
So the atomic mass of the entire molecule, you have 2
hydrogens, so you have 2 times the mass of hydrogen plus 1
oxygen-- plus 16-- so that equals 18 atomic
mass units for water.
And once again, if you want to say, what percent by mass of
water is oxygen?
Well it's 16 out of the 18, right?
So if we get the calculator back, you get 16
divided by 18 is water.
So, let's say you round it, 88.9% water.
Sorry, 88.9% oxygen.
So most of water is oxygen.
And this is interesting, even though you have two hydrogens
here, two hydrogens for every one oxygen, oxygen's mass is
so much larger-- it's 16 times larger-- that
most of water is oxygen.
Well, I'm probably running out of time, so the next video I'm
going to talk about how do we go backwards.
If someone gives you the composition, how can you get
the empirical formula.
Actually on a side note, slightly unrelated to what I
just talked about, I was doing some research last night about
metals, because they're actually interesting, about
why some metals conduct more and some conduct less.
Because when I first talked about, you know, these were
obviously the transition metals.
They're backfilling their d orbitals.
And I said, hey, the periodic table that was in-- I think I
was looking in a Princeton Review book that described
these as metals and described these as transition metals.
And I was like, hey, you know, that's kind of not fair,
because I consider iron and copper and gold and silver to
be as metallic as anything.
Why should these be called transition metals and these be
just called regular metals.
And it actually turns out that a common name for these are
Because, to a large degree, they're softer, they have
lower melting points, so the intuition was right.
To a large degree, when we think of metals, these are the
metals I think of.
And when we think of metallic nature in a chemistry sense--
we talked a lot about that, who wants to donate their
electrons the most, that's metallic nature.
They're the guys down here.
And as you go to the top right, these want to donate
their electrons the least. These are the most
They like electrons the most, so they actually have some of
the worst metallic nature, so it actually makes sense to
call them poor metals.
And there's some debate on whether these should even be
called poor metals.
If you look up a bunch of periodic tables, some will
call these metals.
Some will call these poor metals.
But I just wanted to throw that out there just so you're
exposed to it.
And so, you know, for me, it is a little bit more intuitive
to call these poor metals, because they have less
metallic nature than the stuff, especially down here,
the alkali and the alkaline earth metals.
Anyway, see you in the next video.