 So let's have a look at a couple of lattice structures. Here's an example of an ionic structure. This substance is cesium chloride. So the ratio is one cesium ion to one chloride ion. And you can see that we've got the chloride ion in the center here and it's surrounded in a cubic arrangement by four cesiums. You can twist and turn it around like that. Now this, what I've got showing on the screen here is what's called the unit cell. It's the simplest or the smallest repeating unit of a lattice structure. But I can get this redrawn so that it now shows two by two unit cells. So four unit cells altogether. You can see the original unit cells still marked out there in black. And there are one, two, three, four, five, six, seven other unit cells, eight altogether. So this should be two by two by two in fact. So eight unit cells all stacked together. And then we can redraw it again for three by three by three which looks like this. Now at the moment I've got the ions small, I can make them large which gives you a better idea perhaps of how they pack together like that. And you can see that we can essentially keep adding on unit cells to this structure to make the cesium chloride crystal as big as we like. So the only thing that determines the size of the lattice is how many ions we've got available. Alright if we now look at a metallic lattice, this structure here is called hexagonal close packed and there are quite a few metals that take this particular lattice. If I twist it around this way you can see that within each layer, this shows three layers here, within each layer you can see that the atoms arrange themselves into a sort of hexagonal pattern so that's where the name comes from. I can extend this a little, there we go you can see a bit more of it and again the only thing that's limiting the size of this is the number of atoms that we've got available. Alright so now let's look at a covalent network solid, I'll try a diamond. So here's a small section of diamond and if you look carefully you can see if we pick out a carbon atom here you can see that that carbon atom is forming four bonds to four other carbon atoms and we could pick out another, move it around a little bit if we look at this one here you'd see 1, 2, 3, 4 here as well. So each carbon is bonding to four other carbons and if we move it around to different perspectives you can see the patterns that are involved in this particular lattice and once again the only thing that's limiting the size of this is the number of atoms that we've got. We could just keep on adding carbon atoms to the edge of this to make it bigger and bigger. We have a quick look at graphite as well, here we go so this shows four layers, each layer unfortunately in this particular representation is only very small, it's not showing you very much of the layer but you can see how they're effectively separate the layers, the only thing that holds them together are those weak van der Waals forces. So the point of all this is to distinguish between compounds that are formed of individual molecules and compounds that form lattices. With ionic or metallic substances you know already that they form lattices so that's easy. With covalent compounds it's a bit trickier, they could be either. However, the three most common covalent network substances that you need to know about are diamond and graphite which you've already met and silica or silicon dioxide, SiO2. This is the main component of sand and it forms a tetrahedral structure much like diamond except that each silicon atom is separated from the next by an oxygen atom. Pretty much all of the other covalent compounds that you come across you can assume are molecular unless you're told otherwise.