In this lesson we're going to describe what we mean by the term mixed bonds. When we describe mixed bonds, what we're actually describing is the characteristic of the bond changes progressively as elements combine together moving across the periodic chart. For example, when we start out with taking an element from the first column. So we're looking at sodium potassium and then we go all the way over to the halogen column. We're looking at fluorine chlorine and those remaining halogens. And what happens in this particular case because there is a large difference in electronegativity what we see is predominantly an ionic bond. Now as we begin to form compounds that are associated with moving across the periodic chart, what we begin to see is the interplay of ionic and covalent behavior. And this was first illustrated by calculations attributed to Linus Pauling and what he does is he goes through the calculations showing that the electronegativity difference between the two elements that are forming the bond, will gradually change from ionic to covalent. And what we're going to do is to use Pauling's data, to use an example, to describe silica SiO2, which is an important ceramic material. When we look at the periodic chart, we're going across the periodic chart, so we're looking at the elements as we move. And, the more familiar we become with the periodic chart, and the way it's laid out, the more we will begin to understand these characteristics that are associated with bonding. So when we look at the behavior of electronegativity difference according to Linus Pauling, what we see is as the difference in electronegativity 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.
