Hi. It's Mr. Andersen and this is chemistry essentials video 19 on covalent
bonding. When you look at these three crystals over here, they look somewhat similar. But
the bonds that hold them together are going to be really different. We've got metallic
bonding up here. Ionic bonding here. And then we have covalent bonding here. And so what's
going on with the electrons is the best way to tell each of these apart. And when we start
with covalent bonding you really have to start by talking about electronegativity which
is how much an atom wants an electron. And it's going to increase as we go up and across
on the periodic table. And the reason why, it's based on the shell model and Coulomb's
law. And so as we decrease the size of an atom, as they get smaller, it's going to be
closer to the protons that are found in the nucleus. And then as we move over to the right
those positive values are going to increase and we're also going to increase that attractive
value. And so based on those electron negativities we can look at the atoms inside a molecule
and we can figure out is it a covalent bond. And if so what type of covalent bond is it.
Now if those electronegativites are exactly the same, then we call that a nonpolar covalent
bond. An example would be oxygen that you're breathing right now. It's two oxygen atoms.
They have the same electronegativity. So they're going to share the electrons equally. Also
you should know this. If it's a carbon and a hydrogen that are attached together, carbon
is really good at sharing its electrons. And so we generally call that a nonpolar covalent
bond. We can look at these bonds using a graph where we graph the potential energy on one
side and then the distance between the atoms on the other side. And so what becomes apparent
is going to be the bond length. How far those atoms are apart. And then the bond energy.
How much energy would we have to put in to break those atoms apart. Now if the electronegativity
is different, then we call that a polar covalent bond. And so when we do that we have something
called a dipole where there's a partial positive and a partial negative charge. Whichever one
has the highest electronegativity is going to have that negative charge. But even though
we may have differences in charge, we still have conservation of charge over that whole
molecule itself. Also as we move from covalent bond to more and more electronegativity, we
start to move into what are called ionic bonds. And it's a gradient. It's a continuum. And
so we can measure these to tell the difference between them. And so a covalent bond remember
is going to be sharing of the electrons. So a great example would be hydrogen gas. Hydrogen
has 1 electron in its outer shell. It would like to have 2. And so it can share that 1
electron with another hydrogen atom. And so now we have the sharing of these two electrons.
And so we have a covalent bond between the two. Carbon for example has 4 valence electrons.
According to the octet rule it would like to have 8. And so what it can do is it can
share each of those 4 electrons with another hydrogen atom and now we have methane which
is going to be a nonpolar covalent bond. And so here's that electronegativity. Remember
as we go up on the periodic table, it's going to increase. And then as we go across it's
going to increase as well. Now why is it increasing when we go up? It's because the atoms are
becoming smaller. And as the atoms become smaller those electrons are closer to the
nucleus and so there's going to be a greater charge. And as we move across you can see
the electronegativity is increasing as well because we're increasing the amount of positive
charges on the inside. And so we're increasing that pull. And so you can see that fluorine
is going to have the highest electronegativity on the periodic table. That means it wants
electrons more than any other atom. And so if we have two atoms that are sharing electrons,
and the electronegativity differences are zero, we call that nonpolar. So oxygen for
example. If we look at oxygen its electronegativity is 3.44. So if we compare it to itself, it's
going to be no difference between the two. So that would be a nonpolar bond. Same thing
with nitrogen. Remember if we're ever connecting carbon and hydrogen they're going to share
the electrons equally. And so we'd also call that a nonpolar covalent bond like in methane.
Now we can study these using a graph like this. Where we put potential energy on one
side. And then we put internuclear distance between the two. So let's start with hydrogen
or H2. If we have 2 hydrogen atoms together, as we move them really close together, they're
going to want to push on each other. In other words there's going to be repulsion. As we
move them farther apart, eventually they're going to get into an area where there's this
sweet spot between attraction. And then as we move them really far apart they're going
to be so far apart that Coulomb's law says there's not going to be a great enough force
to pull them together. And so if we're below this line, it's attraction. But what we'll
find is when we have the perfect distance apart, we're going to reach what is the bond
energy and the bond length of those two atoms. In other words, bond length is going to be
how far generally hydrogen atoms are going to stay apart. And bond energy is how much
energy would we have to put in to actually break that hydrogen apart. So this graph is
incredibly important. When we're looking at nonpolar covalent bonds what we're going to
find is that their electronegativities are different. So this right here would be hydrochloric
acid. If we look at chlorine, 3.16, and hydrogen 2.20, there's a difference between those electronegativities.
And so the chlorine is actually going to be a negative charge. And the hydrogen is going
to have a positive partial charge. Or if we look at water. Oxygen at 3.44. Hydrogen at
2.20. And so the oxygen is going to pull those electrons closer to it. So you're going to
have a partial negative charge here. And then a positive charge out here. And that really
allows us to have water attracting to hydrochloric acid. There's going to be a dipolar charge.
And the connection between those two would be intermolecular at this point. And so how
do you figure out what type of bond it is? Well there's one quick way to do it. You could
just look at their electronegativity differences on that chart. And if it's less than 0.5 then
it's generally going to be nonpolar. If it's between 0.5 and 1.7 it's going to be polar
covalent. And if it's greater than that, generally it's going to be ionic. And we'll talk about
that in the next video. That's not a great way to do it. Better ways are to look at what's
being connected. Generally in covalent bonds it's going to be 2 nonmetals. In ionic it's
going to be metal and nonmetal. And then more important than that we should look at the
properties. And so if we're looking at covalent bonds, generally we're going to have gases,
liquids, solids. Low melting and boiling point. And they're going to be poor conductors. But
as we move to ionic, it's going to be a crystalline solid. High melting, boiling points. And we're
going to be good conductors, especially when we dissolve them in water. And so did you
learn the following. To rank polarity based on location of atoms in the periodic table?
Remember as we go up and to the right we're going to increase electronegativity based
on Coulomb's law and the shell model. And I hope that was helpful.