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Video transcript

in this video we're going to think about how ions will arrange themselves when they form solid crystals when they form these lattice structures so just in very broad brush terms let's say that we have a bunch of this white cation and we have a bunch of this green or this Bluegreen anions and let's say there are in a one to one ratio how will they how will that look how will the solid look if we were to take a two-dimensional slice of it to imagine that we can draw what we could call particulate models were just imagining a two-dimensional slice of the solid and we're just drawing these ions as particles when it looks something like this where maybe the positive ion is all on one side and then the negative ion is on the other side is on the bottom if we were to take a slice would something like this make sense or maybe it's random maybe you have a positive there and then you have some negatives right over there and then maybe you have a positive and a positive and then a positive right over there and then maybe you have some negatives right over there would would this be a reasonable configuration as they form these ionic bonds well when we think about Coulomb forces we know that like charges repel each other and unlike charges or opposite charges attract each other and so when these ionic solids form they're unlikely to form in this way or even in this way because they're going to form in a way that maximizes the attractive forces and minimizes the repulsive the repelling forces and so what would be an arrangement that that does that pause this video and think about it well all the positive charges are going to try to get as close as possible to the negative charges and as far as possible from other positive charges and the same thing is going to be true of negative charges they're going to try to get as far away from other negative charges as possible and as close to other positive charges as possible so the arrangement that you are likely to see is going to look something more like a checkerboard pattern so maybe a positive they're positive they're a positive they're a positive there and a positive there I'm these are all the same ion I'm not drawing it perfectly they'd be the same size and when you do these two-dimensional representations these particulate models it is important to get the size right because we're gonna think about that in a second and then the negative charges would be in between so notice and this in this configuration every negative is surrounded by positives and every positive is surrounded by negatives so it's maximizing the attractive forces and it's minimizing the repulsive forces and if you were think about it in three dimensions you would have a lattice structure that looks something like that and we've seen this in other videos now another interesting thing to think about is the size of the ions that form that ionic solid let's say we wanted to deal with rubidium bromide rubidium bromide what would this look like if I were to draw it in a two-dimensional particulate model like this and I wanted to make the size roughly comparable to what we would see between the rubidium and the bromide pause this video and think about that and I'll give you a little bit of a hint it might be useful to look at this periodic table of elements all right if we were to separate this out into its ions it is a rubidium rubidium cation and a bromide anion now a rubidium cation it has lost an electron so even though it still has 37 protons its electron configuration now looks like that of Krypton now the bromide anion even though it only has 35 protons it's going to gain an electron to become a bromide anion and it also has an electron configuration of Krypton so both of these have the same number of electrons but rubidium has two more protons than bromide us and so the rubidium is going to attract that outer shell of electrons that fourth shell of electrons more than the bromide nucleus is going to and so the rubidium in this example is going to be smaller than the broma and so if I were to draw one of these diagrams it would look something like this let me draw the room let me draw the bromide first so I have a bromide anion I have another bromide anion another bromide anion maybe I have a bromide anion right over here bromide anion right over there let me do a few more make up a little bit if I was doing this with a computer I would make them all the same size so these are our bromide anions and then your rubidium cations would be a bit smaller and so our particulate model right over here our part would look might look something like this we want to make it clear that the cation is a bit smaller than the anion it would arrange it would likely arranged in a pattern that looks like this and notice I'm trying to make the sizes roughly roughly at roughly accurate to show that the cation is indeed smaller than the anion although it wouldn't be dramatically smaller remember they had the same number of electrons and they don't have that dramatically different number of protons so this is just a very rough drawing if they're dramatically different you might show that in the sizes on this diagram
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