Hi. It's Mr. Andersen and this is chemistry essentials video 25. It's on covalent network solids
which are solids made of these vast networks of interconnected atoms. And those connections
are made with covalent bonds. Now what's interesting is the 3-dimensional structure tells us a
lot about the properties. And so a great example would be this, diamond and graphite. In other
words graphite that you write with a pencil are made of the same thing. They're just made
up elemental carbon. But depending on how those carbon atoms are hooked together, we can have
one of the hardest things on our planet. A diamond. Or we can have something that is
going to be broken by the paper that you're writing on. And so covalent networks solids
are made up of either one nonmetal or two nonmetals. So an example of one nonmetal could
be graphite. Or silicon that we use to make computer chips. An example of two nonmetals
might be silicon oxide that's going to make up quartz or silicon dioxide. Generally these
covalent network solids are made from the carbon group. And the reason why is that they
have four valence electrons. And it allows them to make all of these vast number of bonds.
Their structure can either be 2-dimensional. So graphite will have these two dimensional
sheets. Or it can be these vast 3-dimensional lattices. And depending on what kind of structure
they have, it's going to give them different properties. So for example, graphite is going
to be incredibly soft. That's because it's going to have these soft layers. But it's
going to have a high melting point because you're going to have these covalent bonds.
Silicon is going to have a pretty good structure. It's going to have a higher melting point.
But as we increase the temperature, we start to vibrate those atoms. And it can actually
become a conductor. So it's something we call a semiconductor. And so we can vary the amount
of electricity that is conducted through it. And one of the ways we can do this is we can
dope it. We can add different elements to it, different other atoms. We can either n-type
or p-type dope it. And we can add either electrons or the lack of electrons. And so it allows
us to create things like transistors. And so this is an example of a covalent network.
This is silicon dioxide. These are the crystals in quartz. And you can see that it has this
regular structure. Now the picture that I'm showing you right here doesn't show that 3-dimensional
structure. We're just looking at one layer. But in general they'll form these massive
crystals. And so you can't really say that it's a molecule per se, because it's going
to be more of a ratio of all the atoms that we have in this giant network. It's almost
like an ionic solid in that sense. They're going to be made from the carbon group. And
so carbon, silicon, germanium. And the reason why remember is they have 4 valence electrons.
So you can make these vast connections. They could have a 2-dimensional shape or 3-dimensional.
And as a result of all of the covalent bonding, they're going to have a really high melting
point. So here's a picture of graphite. This is what it looks like if we were to zoom down
to the level of the atoms. And what you see is that there are going to be covalent bonds
that are connecting all of the carbon in each of these different layers of graphite. But
what's missing is that fourth bond. In other words I can only see 1, 2, 3 bonds to each
of those carbons. That means there are free electrons. And what those electrons are going
to do are interacting with electrons from the next layer. And so you get these London
dispersion forces that are holding those layers together. And so those are going to be relatively
weak. And so it's easy for these layers to slide past one another. And that's why we
can make pencil lead out of graphite. Silicon is going to have much more regular kind of
3-dimensional shape. This diamond lattice. And what's amazing, this is what silicon would
look like, if we could model it. You can see that there are four bonds now around each
of those silicon. What we get is one massive crystal of silicon. That's all it is is silicon
in here. And what we can do is we can cut chips out of this. And so we could make the
chips that are in your phone or in your computer. And so what's interesting about it is as we
increase the temperature, we increase conductivities. So as we make it hotter, more electrons can
flow through it. Now that's not the only way that we can regulate the amount of electron
flow. We can also dope it. And what does doping mean? Well, if you look right here we've got
carbon and silicon. So they're each going to have 4 valence electrons. But what we can
do is we can take atoms from the next group or the group below it and we can either increase
the number of electrons or decrease. And so let me show what that looks like. We could
dope it with phosphorus. That's called n-type doping. And the reason it's n-type doping
is now phosphorus fits right into the lattice. It's going to bond to these 1, 2, 3, 4 atoms
around it. But it's going to bring with it an extra electron. And the reason we call
in n-type doping is it's increasing the negative charge that's going to be in that silicon.
So it's easier for that to flow. So let me put that back for a second. We could also
grab boron. So if we grab boron it's going to have 3 valence electrons. It's going to
bond to these three atoms around it. But it's going to bring with it a lack of an electron.
In electronics we call that a hole. And so that's a lack of an electron. And it's an
area where electrons can flow into that. And really they talk about the holes actually moving
around. And so p-type doping stands for this positive or lack of that negative charge.
And so where would we ever apply this? One of the greatest inventions of the 20th century
is the transistor. And I'll put a link down below to a Veritasium video that explains
how transistors work. It's totally fascinating. But the three parts of any transistor, they
used to be much bigger then they are today. There are going to be three leads that go
to it. And so essentially you're going to have a source where electricity can flow in. A
drain where electricity can flow out. And then we're going to have a gate. And what
you have is n-type on either side. So this is n-type silicon on either side. And then
you're going to have p-type in the middle. And there's going to be no electrical flow.
And so what we can do is we can add a little bit of voltage at the gate. And what you get
is now electrons are able to flow. So we've got all these excess electrons here. Flows
through the holes, out the drain on the other side. And so what's amazing is we can make
these things incredibly small. We call them solid state transistors because they're made
of solids. And so on a typical integrated circuit like you might have in a computer,
there might be billions of transistors just on that one circuit. And so this is just application
of that covalent network solid. So did you learn the following? To create a covalent
network solid representation? Remember it's going to be these three dimensional lattices
of either one or two elements, nonmetals. And then, could you relate that structure
to its properties. Remember, if it's 2-dimensional, if they're not connected, it's going to be
really soft. If it's 3-dimensional it's going to be really rigid. And also since there's
covalent bonds in here they're going to have a high melting point. And I hope that was
helpful.