>> host: Welcome back
to Chemistry Matters,
and the third video
on our Unit 4 playlist:
Chemical Bonding.
We've investigated some
of the characteristics of
intramolecular bonds,
specifically ionic
and covalent bonds,
which sit at either end
of the bond spectrum
we discussed in
the second video
for this unit.
Now let's go back out
to our classroom where
our students are learning
about types of bonds
that fall more in the middle
of the bonding spectrum.
>> professor: Remember earlier
how we said it's more accurate
to think of different types
of bonds as falling along
a spectrum, as opposed to
being one specific type?
Well, let's explore
a type of bond that falls
right in the middle
of that bond spectrum--
a polar covalent bond.
The reason polar covalent bonds
fall into the center
of the bond spectrum
in between ionic
and covalent
is because the atoms
that participate
in polar covalent bonding
have electronegativities
that are different
from each other.
But there just isn't
a large enough difference
for an electron to be
completely removed
from the atom with
the lower electronegativity.
The compound you're most
familiar with that has
polar covalent bonds is water.
In a water molecule,
we say the electrons that are
being shared between
the hydrogen atoms
and the oxygen atoms
are being shared unequally.
What we mean is that
the oxygen has
the greater electronegativity,
so it tends to keep
the electrons near itself.
Since the electrons
tend to stay near the oxygen
more than they do the hydrogen,
we say the oxygen end
of the molecule develops
a partially negative charge.
Meanwhile, think about
the hydrogen atom.
What did it start off with
in terms of electrons
and protons?
>> student: one proton
and one electron, right?
>> professor: Exactly,
but if its one electron
is spending most of its time
around the oxygen atom,
then what do you have left?
>> student: Just a proton?
So does that mean
the hydrogen ends of
the water molecules
are partially positive?
>> student: Yeah,
it's basically just
an exposed proton?
>> professor: That's exactly
right! And that's what makes
the water molecule polar.
It's similar to the way
a bar magnet has a north
and south pole.
Polar molecules have
a partially positive
and a partially negative end.
And these types of molecules
are characterized by chemists
as containing dipoles,
which simply means
that the molecule
has two poles,
similar to a bar magnet.
And in the same way
that two bar magnets
stick together
when brought close together
polar molecules
will also stick together,
with the partially negative end
of one molecule attracting to
the partially positive end
of the other molecule.
Now, let's compare
the polar covalent bond
to a nonpolar covalent bond,
like we explored earlier
with the hydrogen molecule.
Take a look at this image
of a chlorine molecule,
which is two chlorine atoms
bonded together.
Their electronegativities
are identical,
so there's no way there can be
a buildup of electrons
on either side
of the structure
like we saw with
the polar water molecule.
It's kind of like
a tug of war battle
in which the opponents
are evenly matched.
The rope stays taut
in the middle--
keeping both sides together.
That's how a nonpolar
covalent bond works.
The electrons spend equal time
with both atoms,
so equal attraction
to each atom.
>> host: By now,
you have a solid,
general understanding
of how intramolecular
bonding works.
You should also have
a better sense of
the various types,
or ranges of bonds
that hold matter together
to make up
our physical world.
So now we're going to learn
how to predict what kind of
intramolecular bond
a substance has,
and where that bond would lie
along the bond spectrum.
This activity will help you
visualize the types of bonds
that form as atoms
come together to make
molecules and compounds.
For this lab, our students
will use a simulation
of bonding that you can find
in our Chemistry Matters
Toolkit.
There you'll find instructions
on how to form bonds
between two or three atoms
and how to use atoms
with a wide range
of electronegativities
to form different kinds
of bonds.
The simulation will allow you
to conduct your own experiments
and collect data
about the types of bonds
you're forming.
So let's return to
our classroom
where our teacher and students
are getting some practice
with this bonding
simulation activity.
>> professor: Look at
the atoms, A and B.
Above them you can see
slider boxes that show
whether the atoms have high
or low electronegativity.
Notice that I have set
the slider bars
so that atom A and Atom B
both have the same
high electronegativity value.
Below the atoms
you can see a color-coded bar,
which is kind of like
the legend you find
on map.
This color bar is a key
for understanding the meaning
of colors in the electron
cloud drawing above.
Blue indicates an area
with strong positive charge.
Red indicates an area
with strong negative charge.
And the white bar color
indicates no strong charge.
Which is why the electron cloud
in this image is white.
Your job right now
is to look at some images
I've created by adjusting
the slider bars, like this one,
and use the data you see
to figure out what slider bar
combinations produce
ionic bonds,
which ones produce
polar covalent bonds,
which ones produce
mostly covalent bonds,
and which ones produce
non-polar covalent bonds.
So let's start with
this bond image.
I want to see if you can tell
what kind of bond it is.
Look at the two atoms,
A and B.
Does this image show
a partial positive
or negative charge
in this molecule?
>> student: It looks like
it doesn't have a charge.
The circles around
the atoms are white.
>> student: So what does
that mean?
>> professor: Remember,
the white electron cloud
around the molecule
indicates there's
no strong charge
on the molecule.
>> student: Yeah,
under the slider bars,
the molecule is identified
as being "more covalent."
That makes sense because
both atoms are pulling hard
on the shared electrons.
>> professor: You're correct.
The bar labeled bond character
tells us that it's a more
covalent bond.
A molecule like this,
remember, with no strong
partial positive
or negative charge,
is a nonpolar molecule.
So this is an example
of a nonpolar covalent bond.
>> host: Our students
have gotten a nice start
identifying and predicting
types of bonds.
Now they're going to continue
this activity by moving
the electronegativity
sliders themselves to create
different types of bonds.
This will help them
gather data and identify
patterns in the formation
of intramolecular bonds.
A lot of students
have found this to be
a very helpful exercise
for solidifying
their understanding
of bond types.
So before continuing
with the rest of this unit,
now is an excellent time
for you to get
some practice too.
In our Chemistry Matters
Toolkit, you'll find
the information needed
to perform this exercise.
You can use simulations
just like our students used,
and even manipulate
the properties of the bond types
by adjusting
the electronegativity sliders.
When you start the simulation,
make sure you click on
the Bond Character button
in the View box
and the Electrostatic Potential
in the Surface box.
This activity lets you
explore how the nature
of bonds that form
between atoms
is directly impacted
by the electronegativity
of the atoms.
You'll get to choose
how many new molecules
you form.
No matter how many
new molecules you choose
to form, make sure
you write down
the electronegativity
and the bond character
for each bond you create.
Then you'll be able to look
for patterns in your data.
Once you've practiced
identifying and predicting
types of bonds, rejoin me
for the next video
on the Unit 4 playlist
where we'll take a look
at the results our students
found with this activity.
I'll see you there!
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