Hi. It's Mr. Andersen and this is chemistry essentials video 52. It's on bond length and
bond energy. Some of the first bond lengths ever measured were measured by scientist Kathleen
Lonsdale who was an x-ray crystallographer. She would put a solid crystal in front of
x-rays. Measure the diffraction. It told her a lot about the structure of that molecule
and she could even measure the distance between atoms. And that allowed her to unlock the
structure of hexamethylbenzene, that famous benzene ring. She was able to determine that
it was a hexagon. That it was flat. And she even measured the aromatic bonds. In other
words the single, double, single, double, single double bonds between the carbons. And
by measuring the length of that, that eventually led to this whole idea of resonance. So not
only was she an amazing chemist, but she broke down a lot of walls. She was one of the first
two female fellows elected to the royal society. First tenured professor. She was an amazing
scientist. But she really got us down this road of bond length and bond energy. And so
if we have two atoms, they're both going to be attracted to one another. And where is
that attraction come from? It's the electrons of one atom being attracted to the protons
of another. And so that's going to pull atoms together. But if they get too close to one
another, then there's going to be repulsion. And we can measure these forces just using
an energy distance graph where we've got the energy that's pulling those or pushing those
apart and then how far they are physically separated. And using a graph like that we're
able to determine the bond energy, the energy holding them together. And also measure the
bond length. How far those atoms are going to be apart. And so the bond energy, if we
were to define it, is simply the energy required to break those atoms apart. Now likewise when
we form a bond between the two we're going to release energy. And that's just simply
the negative bond energy. Now the strength of that bond, or the bond strength is going
to be built upon the charges of the atoms themselves. The bigger the charges are the
greater that bond energy is going to be. And as we increase the number of electrons in
a bond, so as we move from a single to a double to a triple bond, we're actually increasing
the charges. And therefore increasing that bond strength. And one interesting thing that
you should know is that as we increase that strength we're going to pull those atoms together
and we're going to actually decrease the bond length. And so if we look at these two representative
atoms right here, as we bring them close together there's going to be attraction. In other words
as we shorten the distance between the atoms, as they get closer and closer together, there's
going to be a greater attractive force. Now as they start to get really close together,
there's actually going to be repulsion between those two atoms. It's going to push them apart.
And so we can find what's called that Goldilock's. That point where they're not too close, not
too far away and we're going to have the highest amount of energy right here. And they're just
going to kind of vibrate at that space. So if we were to look at two atoms, like two
atoms of hydrogen we would find that that energy to distance graph would look something
like this. There's going to be attractive force between these two hydrogen atoms out
here, but as we bring them closer and closer together, that energy of attraction is going
to get greater. As we move them even closer then we're going to move into this whole idea
of repulsion. And so this is going to be that Goldilock's area right here. And so what we
can measure then is the bond energy. The energy holding those atoms together. And likewise,
since we've got distance here on the x-axis, we can measure the bond length. How far those
atoms are apart. And so if we define what is bond energy it's just two atoms connected
together. And so bond energy is the energy absorbed when we break those atoms apart.
Now likewise there's going to be energy that's released as we bond those together. And that's
going to be the negative bond energy. It's going to be exactly the same. The amount of
energy to break it apart is the same amount of energy that we get when those two atoms
are going to be attracted together. Now what contributes to that bond energy? It's going
to be the electronegativity of all the atoms involved in that molecule. And so let's say
we were to look at boron. Boron is going to have an atomic radius of 83.0 picometers.
What does that mean? The distance from the center of that nucleus to the outside is going
to be 83.0 picometers. And so if we were to connect a boron to a boron, we would anticipate
that that bond length, the distance between the two atoms is going to be twice that. It's
going to be 166 picometers. But if we were to measure the actual bond length in a molecule,
let's throw one up here, and it ends up being 175 picometers. In other words they're farther
apart then we would expect. That means that we're going to have a very weak bond between
these two atoms. And so basically as we increase that bond length, that distance between the
atoms, we're decreasing the bond strength. Now if we were to use another example, let's
say we're looking a rhenium which has a radius of 137.5. If we had two rhenium atoms we would
expect that that bond length is going to be 275 picometers. If we actually measure it
in a molecule like this and it's less than that, that means that they're really overlapping.
There's greater charges holding it together and so we would call that a strong bond. Again,
the higher your bond energy is the closer those atoms are going to be and the shorter
that bond length is going to be. Now what happens as we increase the bond number? Let's
say we're looking at two molecules that look essentially the same. We've got ethane which
is carbon attached to carbon and then hydrogen around the outside and then acetylene. And
so this is the same thing. Carbon attached to carbon with hydrogen around the outside.
But in this case we've got a triple bond right here. How is that going to affect the bond
strength and the bond length? And so if we look at these two carbons, we've got one that's
a single bond and one that's a triple bond. If we were to measure their bond length we
would find that as we increase the number of bonds we're actually decreasing that bond
length. They're getting closer and closer together. Well what does that tell us about
the bond energy? The bond energy is increasing over time. So again, the more electrons that
we're sharing, the more charges there are, the greater that bond energy and therefore
the shorter the bond length is. If we were to measure the same things with nitrogen,
single, double, triple bond, we'd find that we're decreasing the bond length again. So
what's going to happen to our bond energy over here? It's going to increase over time.
And so did you learn to describe the energies involved in both breaking and forming chemical
bonds? I hope so. And I hope that was helpful.