Placeholder Image

Subtitles section Play video

  • - [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.

- [Instructor] In many videos we have already talked about

Subtitles and vocabulary

Click the word to look it up Click the word to find further inforamtion about it