- [Instructor] In many videos
we have already talked about metals and metallic bonds. And in this video we're going
to dig a little bit deeper and in particular, we're
going to talk about alloys, which are mixtures of elements, but still have metallic properties. So first of all, what
are metallic properties? Well those tend to be
things like they're shiny, they reflect light. This is actually a pure
iron sample right over here. You can see that it reflects light. It tends to be malleable, which means you can bend
it without breaking it. And it tends to conduct electricity. And alloys are when you can mix multiple elements together and still have most of these properties. And just as a review of where
these properties come from, we can imagine metallic bonds. And there's a whole video on this, but in metallic bonds, let's say we were to take a bunch of iron and
you can see right over here, iron, Fe, it is a transition metal. And what happens with metals is, is when they form bonds with each other, they're valence electrons, because each of the atoms
aren't that electronegative, they don't want to hog the electrons. They don't want them, just for themselves. They're willing to share
their valence electrons into a bit of a communal
pool of electrons. And so even though you
have a bunch of neutral, let's say iron atoms, you
could actually view them as positively charged ions
in a sea of electrons. And so you have a bunch of electrons here. And where did these electrons come from? Well these are the valence electrons from the neutral atoms that
get contributed to this sea. And this is why most metals are good at conducting electricity. This is why they are malleable. And depending on the metal, if you're talking about a Group one metal, you could imagine that the charge of these ions right over
here would be a plus one. But if we're talking
about a Group two metal or a transition metal, they
have more valence electrons that they might be able to
contribute to this pool. And so if you're thinking
about these ions, they can even have a positive two charge or a positive three charge. But as promised in this
video we're gonna talk about the notion of alloys. And we're going to do
these particulate diagrams that we have seen in other videos. And in the particulate diagrams, we're not going to show
this sea of electrons, but they're going to help us visualize the structure of the alloys. So let's imagine what
iron could look like. And we're just going to look
at a two-dimensional slice of a solid of iron,
where all the iron atoms have formed metallic bonds. And as I said, we're not going to draw this sea of electrons, but they might form a
pretty regular structure, something like this. And so each of these circles
represent an iron atom. But as promised, this
video is about alloys. So let's imagine what
steel might look like. This is a steel blade and
steel is a bunch of iron, so once again, we can
visualize each of these as an iron atom, but
mixed in with that iron is a little bit of carbon. And when you look at the
periodic table of elements you can see that carbon
is a good bit higher on the periodic table of elements and to the right of iron. Neutral iron has 26
protons and 26 electrons, and neutral carbon only has
six protons and six electrons. The valence electrons in carbon
are in their second shell. The valence electrons of
iron are in the fourth shell. So carbon is a good bit smaller. And so, when you mix that carbon in, because it is smaller, it's able to fit in the
gaps between the irons. So you might have, I'll
draw this right here. You might have a little
bit of carbon there. You might have a little
bit of carbon there. You might have a little
bit of carbon there. And so when you form an alloy, where one atom has a larger radius or a significantly larger
radius than the other, you tend to form things like this, which are known as interstitial alloys, and basic carbon steel
is a good example of it. Now you have other situations
where you have alloys between atoms of similar size. And this right over here, this is a brass, I don't know if this is
a clock or an astrolabe, or something like this, but brass is made up of
a mix of copper and zinc. And so when you have an alloy like this, that's between atoms of similar radius, this is called a substitutional alloy. You can imagine that some of the copper has been substituted with zinc. So this is substitutional alloy. Now the last thing you
might be wondering about is can you have a combination of both? And you indeed can. This is over here are panels on the International Space Station, and it's made out of stainless steel. You're likely to have stainless
steel in your kitchen. And stainless steel, you could view it as it's basic steel but instead
of just iron and carbon, It also has a little bit
of chromium mixed in. And so we can visualize this. If this is stainless steel, maybe the blue ones, we say are iron, but it has a little bit of chromium. I'll do that with red. Chromium has a similar radius to iron. It's not exactly the
same, but it is close. So maybe a little chromium there, a little bit of chromium right over there, a little bit of chromium right over there. And if it was just iron chromium we would call it substitutional, but it also has carbon and
carbon has a smaller radius. So maybe a little bit of carbon fitting in the gaps between
the larger atoms there. A little bit of carbon there. A little bit of carbon right over here. And so this is an example of an alloy, that is both interstitial
and substitutional. Now one final question, you're like okay this is all interesting, but why have we decided to put
things like carbon in iron? Well it turns out that even by putting a little bit of carbon in or mixing in with other metals, you're able to change the properties and for
example, steel as an alloy, is much stronger than iron, by itself. And stainless steel once
you mix that chromium in, it's much more resistant to
corrosion, than basic steel. So I'll leave you there. You just learned a little bit
more about metals and alloys.