becomes larger and larger,
what we're seeing is an increase in the amount of ionic bonding.
So when see the large electronegativity difference which is what we would
anticipate between sodium and chlorine, we then expect to see an ionic bond.
But even at that, it doesn't go completely to 100.
So there's still an element of covalency that can be associated with that bond.
So now, if we look at the elements silicon and oxygen, and
we consider what their values for an electronegativity are,
we look at electronegativity of 1.54, the difference Difference between those two.
When we do that we come up from the x axis and we go over to the y axis and
what we wind up seeing is that the bonding characteristic associated with silica
is about 45% ionic and 55% covalent.
So we're seeing this gradual change as we begin to have alloying
elements coming together that form these compounds.
Now, another thing that becomes important when we're discussing
the formation of compounds, is that we can form compounds in
which there is a metallic covalent or a metallic ionic.
And these are illustrated by the compounds that I've described on the slide, and
I've underlined a couple of these because they're really quite important.
In the first case, when we look at the underline of aluminum lithium,
one of the things that we're particularly interested in a compound like aluminum
lithium is that this Al3Li, it will have a particular stoichiometric, that is,
a correspondence that is associated with three aluminums to one lithium.
It turns out that this happens to be a very important
phase that forms in aluminum alloys and we see more about that later on.
If we go over to the next compound that we see, Ni3Al,
which is a phase that forms in nickel aluminium alloys, and
these alloys are are used for high temperature applications.
So, we're going to be seeing these types of structures that
are going to appear in commercially important materials.
So, what we see, then, is that as we begin to form different compounds,
we can begin to have different types of bonding associated with those.
As I say, metallic covalent, or metallic ionic.
Now when we look across the periodic chart, there's one thing before
we leave it that is very important that I want to illustrate and
that is the behavior or density as we move across the chart.
Again, here is our period chart.
And as we look across a row, so
what I've done is to highlight scandium, titanium across to copper and zinc.
And those little blue bars on there indicate the relative densities of
those materials.
So as we go from scandium across the transition metals we see that
the maximum density occurs in the vicinity of iron, cobalt and nickel.
And similarly behaved is what happens in the below, where we go from vanadium,
zirconium, niobium, and ultimately across to silver and copper.
So, we see a maximum in the densities toward the middle, associated with
the change in the d electrons as we add more electrons.
One of the things that becomes very important, and
we're going to see this a little bit later on as the reason that I have brought this
up, is the density of certain elements and
how important they are to the development of new materials and structures.
I've indicated Magnesium on the diagram.
Magnesium is a very low density material.
1.74 grams per centimeter cube.
What's important about this is when we look at components that are made out of
Magnesium, they're going to be characteristically light weight.
So, if weight becomes an essential feature of your structure,
then you need to pay attention to the density.
And again when you come cross the diagram to aluminum, you see that there is
a corresponding increase in density, and so now aluminum is 2.7 grams per cc.
As we move across the diagram, one of the things we begin to see is that increase.
And so what I've done is I've actually indicated the densities that
we are looking at as we move from titanium, ultimately to iron and cobalt.
And we see an increase in the density, and
it peaks in the vicinity of cobalt and nickel.
And what becomes very important about this behavior is the fact
that we can begin to think in terms of specific properties, and
specific properties are those properties that are normalized by the density.
So we can talk about specific strength where we are taking the yield
of the material, the yield strength of the material or
the tensile strength of the material, and we divide it by the density.
And so, if we consider something like iron,
even though iron is typically associated with alloys that are high strength,
often times we can use a replacement like aluminum if we can
improve the strength of the aluminum and modify it with the density of aluminum.
In the next lesson we're going to be dealing with
what we refer to now as weak bonds.
Thank you.
Thomas H. Sanders, Jr.Regents' Professor
