 Next write down the arrangement of atoms packing also we call this packing of atoms or arrangement of atoms okay so what type of arrangement we prefer the arrangement yes the arrangement which gives the maximum efficiency packing efficiency that is correct right so two mainly types of arrangement we have the first one write down a square close packing two types of arrangement we have the first one is a square close packing in the square close packing in a square close packing the atoms are arranged like this all atoms are in contact one above the other like this the atoms are arranged given above the other this vacant space is what okay the other arrangement is not important that much the other arrangement is hexagonal close packing hexagonal close packing in hexagonal close packing will arrange the atom this way this is the most difficult part of this because this this diagram is important and it's very difficult to draw now in the second layer we'll place the atoms in the space between the two atoms okay so first atom will place you know which one is the better packing this one this one type because it occupies more space okay just want to see which is required like what I have to know make you understand that should be visible this diagram should be like this this diagram is given in NCRT okay you can go and check okay this is the first layer where we place the atoms in between the space of the two atoms in the second first layer okay now here what happens this is suppose layer A okay and if this if you want to put in another layer of this atom so we can place the atoms at the you know void of this okay we'll place the atom here here also if you plan to put get an atom here now you see this space here I'm not drawing the entire thing because nothing you understand okay it's completely best see this one this space is vacant here this one which forms by one this orange one two and this three three is here in the first layer and four which is this yellow one fifth yellow one and six this yellow one so you'll have a space here vacant space here this blue this one can you see this this three is here in the first layer A and this three is here in the best two triangle like this as a first layer man and then this diagram is given in the book when you go to home this NCRT your watch is it you can go and check okay you understand this so one more thing I just tell you that these two sphere are in contact you don't touch me right too wide you consider can you see this wide that is wide one and this is another wide which is above this sphere and one two three this is the second type of what is the difference in these two wide this wide forms by the combination of six sphere one two three in the first layer one two three in the second okay this wide forms by the combination of six can you see that six sphere right so this is wide one and it is an octahedral because it forms by combination of six this wide forms by one two three and one layer in the bottom this one for a sphere this is tetrahedral void so tetrahedral void is on this octahedral void tetrahedral void is something like this you have a three sphere and when you place another sphere on this so which means your space butchega vacant that is tetrahedral void okay so now you see one thing here the layer is a and the second layer is b okay you have two options here when you try to put the third layer on this you have two options here you can fill this void or this wide okay now if you want to fill this void you'll get another layer c different one another layer c but you want to fill this void the void of sphere come exactly above the this sphere is it possible can you see this if you want to fill this void you put in a sphere like this so this sphere comes exactly above the sphere of the first layer so many you have this sphere now the sphere of a layer in this sphere you want to fill this void so you put in a sphere like this okay this comes exactly above the layer in the sphere of the layer in similarly here there will be a tetrahedral void over if you want to fill this steeper layer okay so when you try to fill this void too which is a tetrahedral void you get which layer layer a then again you get layer b a b a b right so if you want to fill this void you'll get a layer again that arrangement is a b a b a b kind of a layer okay but if you want to fill this void octahedral void you'll get a layer c right layer c that is a b c a b c kind of a layer okay one more thing I will tell you you just look at this structure this to center what is this right hexagon with a hope all cornered atoms are in touch you can take face center you look at the structure of that hexagon whether except you will have structure you see what I'm going to get a cornered atoms are in touch ticket a face center or you do things here do a degree of layer b man the yellow one one two and three it is in between this because you have when you put the layer a b a b a b like that you'll get a hexagon pattern in which this three atoms is present in that is H c p arrangement excellent or you have this three sphere one two and three or a second we got the atoms at the corner in touch for a case so this one two three four five and six at the corner or your face this is in touch with all six that is what hexagon I have given you so when you put another layer you'll get a hexagon pattern like this okay or which height over that height is at that we have done the reservation okay to these three atoms these three atoms is in between the two a b layer but it this is where a the first one layer b the three atoms in between is the atoms of layer b then again we have layer a then again we have layer b three atoms that is this one thing you write down case one if you place the layer place the sphere in such a way in such a way that it covers void two which is to try to avoid you place the sphere in such a way it covers void two we observe that we observe that each sphere each sphere of the third layer of the third layer lies exactly above lies exactly above the sphere of the first layer sphere the first layer if we continue with this arrangement if we continue with this arrangement we'll get a b a b a b type packing catch the right now like this a b a b a b type packing a b a b a b type packing okay and this is nothing but h cp hexagonal close packing reason I have just discussed now okay we are getting a hexagon here right then three the sphere of layer b again you'll get layer a hexagon then again layer b again layer a okay so this is h cp that we are discussing okay three this arrangement is known as h cp arrangement okay now you see here coordination number we were discussing correct so this is where you see this sphere is surrounded by how many atoms six of this in the same layer three in the above layer right so this three of b six of a and then again three of three that's where the coordination number is 12 right so it's difficult to understand okay you understand why it is a b a b and hence it is h cp and how coordination number is six six in the same layer three above it and three below it right that's why the coordination number is 12 okay next to write down case two if you place the atom to fill the void one we get a b c a b c type arrangement we get a b c a b c type arrangement a b c a b c type arrangement and this is known as f cc or ccp arrangement f cc or ccp both are same thing cubic close packing face centered cubic arrangement okay it is known as f cc or ccp arrangement okay in this also the coordination number is 12 f cc though we have discussed all the coordination number is 12 okay coordination number is 12 write down next line case three there's a third type of arrangement of packing there's third type of arrangement of spheres and in this the sphere in the first layer is not in contact last one first layer is not in contact so mainly we have two types square close packing x another close packing h cp is the better packing better method we have when we were where we get what more packing efficiency the third type we have like this when the atoms are not in contact in the first layer so we got same parking efficiency for x1 or x2 for x1 and for f ccp and f cc has same uh packing efficiency then how does f cc work? why is f cc working? both you can have like f cp if you talk about the packing efficiency f cp and f cc is equally good because both gives equal packing efficiency okay generally we won't get f cp we have in the slivers we have cubic close packing so mainly we deal with f cc but which one is better we can compare f cc and b cc we can compare f cc and simple cubic unit cell b cc and simple cubic unit cell but overall f cp and f cc has equally good in terms of packing because their packing efficiency is same okay so this is the next arrangement we have the atoms are not in touch like this and when you place the another layer to build a void like this which is also not in touch this arrangement is b cc arrangement this arrangement that we know k does not equal to 2 r right so this arrangement is b cc arrangement which has the not much good packing efficiency obviously the atoms are not in touch so their packing efficiency is not uh very good right so this is the third type of arrangement here the best one is h cp okay and in this we are getting only f cc and h cp arrangement so today we understood the various unit cell various type of packing and packing efficiency okay the location of void is very important okay where we get octahedral void where we get tetrahedral void okay all these things are very important today when you go home this structure is given in n crt in clear book media world ticket just give a reading to it okay arrangement of atoms packing of an engine fake or they don't have a structure for a while you can have that yes this one here is it is given tetrahedral octahedral you can see easily the voids how it is octahedral how it is tetrahedral all these uh you know this structure we have discussed just now so page number 13 is given page number 12 to page number 12 to 13 this two three page you must read okay we'll discuss next class questions on it okay void location minimum distance and all that is very important okay first you know that okay bye thank you