B2 中高級 7 分類 收藏
- [Instructor] Let's talk a little bit
about metallic solids.
And here is an example of what
a metallic solid might look like.
They tend to be shiny like this.
Some would say lustrous.
Some of you might be guessing maybe this is some type
of aluminum or silver.
It actually turns out that this is sodium.
Our same friend sodium that we saw bonding with chlorine
to form sodium chloride and form ionic solids,
it can actually bond with itself with metallic bonds.
This right over here,
you might guess is silver or something.
It actually turns out this is calcium.
And I know what you're thinking.
Isn't calcium kind of this chalky white powder?
Well no, those are compounds formed with calcium,
things like calcium oxide.
But this right over here is pure calcium.
And the reason why it has to be in this container,
it is highly reactive with oxygen.
So that's not oxygen that is in this container.
It's some form of inert gas.
But calcium when it just bonds to itself
with metallic bonds, which we'll talk about in a little bit,
it also looks kind of similar.
It's this shiny, metallic, or lustrous look to it.
And what do you think this is?
Well this is something we're used to associating
with metals, this is gold.
But once again, you can see it has this lustrous property.
So what is it about metals or metallic solids
that allow them to be lustrous in this way
and have other properties that we're about to see?
And to understand that, we just have to look at
the periodic table of elements.
And that most of the periodic table of elements
is actually some form of metal.
You have in red right over here, this group one elements,
not including hydrogen.
Those are your alkali metals,
and you have your alkaline earth metals,
your transition metals,
your post-transition metals, your metalloids.
It's really only what you see in yellow and blue here
that are not your metals.
So how do metals form solids when you just have
a pure sample of them?
Well the general idea, you can look at your alkali metals,
they all have that one valence electron.
And to get to that stable outer shell,
it's much easier for them to give away a valence electron.
And that's why we often see these folks are dissipating
in ionic bonds.
They can be ionized quite easily.
But if you have a pure sample of them,
they can contribute electrons
to a sea of electron, one each.
These alkaline earth metals,
they have two valence electrons.
They too can be ionized or if you have a pure sample
like in a calcium, they can contribute two valence electrons
to a sea of electrons.
And the transition metals here have a similar ability
to contribute valence electrons.
And so in general, we can view metallic solids
as having cations, these positively charged cations
in a sea of electrons.
So you have all these electrons here.
I'll just draw all these minus charges that they're in.
Where do those electrons come from?
Well if you're looking at the alkali metals,
each of those atoms could give one electron to that sea
because it doesn't really want that valence electron.
If you're talking about alkaline earth metals,
they could each donate two electrons to that sea.
Now given that you have this positive charge
in this sea of electrons,
what are you think of the properties?
How good do you think this will be
at conducting electricity or heat?
And many of you might guessed, if you looked at a wire,
wires are made out of metals, because they are excellent
at conducting electricity, or they tend to be excellent
at conducting electricity, because you have
all of these electrons that can move around.
And so if you apply a voltage, they will start moving
and conduct electricity.
And those electrons can also be good at conducting
thermal energy or heat.
Now what would be, we already talked about them
having the shiny, lustrous property,
but how easy would it be to bend them?
Ionic solids, we talked about
they can be strong but brittle.
As soon as you try to shift them around a little bit,
they can break.
But what do you think is going to happen here?
If let's say right over here, I were to push really hard
and on the top I would have pushed really hard to the left.
Do you think this will be brittle?
Or do you think it will be malleable?
It's easy to bend.
Well if you have a pure metallic solid,
it's actually quite malleable.
If you just took this top part and pushed it
to the left like this, no big deal.
You have those cations that are still in those
that sea of electrons.
And that's generally true of metallic solids.
They're very malleable.
They are not brittle.
In fact, so much so that often times we want them
to be a little bit more rigid.
We want them to be a little bit harder.
And that's why we might do things like add other elements
into the metallic solid.
For example, pure iron is reasonably malleable.
But if you wanna make it stronger,
you could stick carbon atoms in between.
For example, you could put a carbon atom there,
or carbon atom over there.
And that way, it kind of disrupts
this electron sea a little bit.
So it's not quite as malleable.
It'll be stronger and more rigid.
So I'll leave you here.
This is just an extension of what we've already learned
about metals and metallic bonds.
To just realize why most of the periodic table of elements
that we're familiar with has some of these properties
when they are, when you have pure solids of them.


Metallic solids

7 分類 收藏
林宜悉 發佈於 2020 年 3 月 28 日
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