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let's think a little bit about the notion of atomic size or atomic radius in this video and at first first thought you might think well this might be a fairly straightforward thing if I'm trying to if I'm trying to calculate the radius of some type of circular object I'm just thinking about well what's the distance between the center of that circular object and the edge of it so that this the length of this line right over here that would be the radius and so a lot of people when they conceptualize an atom they imagine a positive nucleus with the protons in the center right over here and then they imagine the electrons on these fixed orbits around that nucleus so they might imagine some electrons in this orbit right over here just kind of orbiting around and then there might be a few more on this orbit out here orbiting around orbiting around out here and you might say okay well that's easy to figure out the atomic radius I just figure out the distance between the nucleus and the outermost electron and we could call that the radius and that would work except for the fact that this is not the right way to conceptualize how electrons or how they move or how they are or how they are distributed around a nucleus the electrons are not in orbits the way that planets are in orbit around the Sun and we've talked about this in previous videos they are in orbit holes which are really just probability distributions of where the electrons can be but they're not that well defined so you might have an orbit tall and I'm just showing you in two dimensions it would actually be in three dimensions where maybe there's a high probability that the electrons where I'm drawing it in kind of this more shaded in green but there's some probability the electrons are in this area right over here and some probability that the electrons are in this area over here and let's say even a lower probability that the electrons are over this look like this over here and so you might say well okay at a moment the electrons they're the outermost electrons is there you might say well that's the radius but in the next moment there's some probability it might be more likely to dense up here but there's some probability that it's going to be over there and then the radius could be there and so electrons these orbitals these problem of these diffuse probability distributions they don't have a hard edge so how can you how can you say what the size of an atom actually is and there are several techniques for thinking about this one way one technique for thinking about this is saying okay if you have two two of the same atom that are of the two atoms of the same element that are not connected to each other that are not bonded to each other that are not part of the same molecule and you were able to determine somehow the closest that you could get to them to each other so without without them bonding so you were kind of see what's the closest that they can they can kind of get to each other so let's say that's one of them and then this is the other one right over here and if you could figure out that distance that closest that minimum distance without some type of you know really it I guess you know strong influence happening here but just the minimum distance that is that you might see between these two and then you could take half of that so that's one notion and that's actually called the Vander Waals radius another way is well what about if you have two atoms two of the two atoms of the same element that are bonded to each other they're bonded to each other through a covalent bond so a covalent bond we've already we've seen this in the past the most famous of covalent bonds is is well a covalent bond you essentially have two of two atoms so that's the nucleus of one that's the nucleus of the other and they're sharing electrons so they're their electron clouds actually their electron clouds actually overlap with each other actually overlap with each other so in a covalent bond there the electrons in that bond could spend some of their time on this on this atom and some of their bonds some other time on this atom right over here and so when you have a covalent bond like this you can then find the distance between the two nuclei and take half of that and call that call that the atomic radius so these are all different ways of thinking about not with that out of the way let's think about how what the trends for atomic size or atomic radii would be in the periodic table so the first thing to think about is what you think will be the trend for atomic radii as we move as we move through a period so let's say we're in the fourth period and we were to go from potassium to to Krypton what do you think is going to be the trend here and if you want to think about the extremes why do you think potassium is going to compare to Krypton in terms of atomic radius encourage you to pause this video and think about that on your own well when you're in the fourth period the outermost electrons are going to be in your fourth in your fourth in your fourth shell here you're filling out 4s1 4s2 then you start back filling into the 3d subshell and then you start filling again in 4p 1 4 and so forth and so you start filling out the p subshell so as you go from potassium to Krypton you're filling out that fourth that outermost fourth shell now what's going on there well when you're at when you're at potassium you have 19 you know 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 you have 19 protons and you have 19 electrons well you know I'll just drew as you know 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 but you only have one electron in that outermost in that fourth shell and so let's just say that's that electron it at a moment just just for visual it doesn't necessarily have to be there but just to visualize that so that one electron right over there you have 19 yeah and you have 19 protons so there's going to you know you have some you have some as you say Coulomb force that is attracting it that is keeping it to there but if you go to Krypton all of a sudden you have much more positive charge in the in the in the in the nucleus so you have 1 2 3 4 5 6 7 8 I don't have to do them all you have 36 you have a positive charge of 36 let me write that youth there plus 36 here you had plus 19 and you have 36 electrons you have 36 electrons and I'm not I don't even I've lost track of them but in your outermost shell in your 4 3 have the 2s and then you're going to have the 6p so you have eight in your outermost shell so maybe one two three four five six seven eight so one way to think about it you have more positive charge in the center and you have more negative charge on that outer shell so that's going to bring that outer shell inward it's going to have more I guess you could you could imagine one way you know more Coulomb with traction right over there and because of that that outermost shell is going to be drawn in Krypton is going to be smaller is going to have a smaller atomic radius than potassium and so the trend as you go to the right is that you are getting and the general trend I would say is that you are getting smaller as you go to the right in a period and that's the reason why the smallest atom in of all the element with the smallest atom is not hydrogen its helium helium is actually smaller than hydrogen depending on how you depending on what technique you use to measure it and that's because if we take the simplest case hydrogen you have one proton you have one proton in the nucleus and then you have one electron in that 1s shell and in helium you have two two protons in the nucleus and you and I'm not drawing the neutrons and obviously there's different isotopes different numbers of neutrons but the but you have two electrons now in your outer in your outermost shell and so you have more I guess you could say you could have more Coulomb attraction here this is plus two and then these two combined are negative two they are going to be they are going to be drawn inward so that's the trend as we go to the right as we will from the left to the right of the periodic table we're getting smaller now what do you think is going to happen as we go down the periodic table as we go down the periodic table and in a given group well as we go as we go down a group each each new element to down the group we're adding we're in a new period we're adding a new shell so you're adding more and more and more shells here you have just the first shell now the second shell and each shell is getting further and further and further away so as you go down in the periodic table you are getting you are getting larger you're having a larger atomic radius on depending on how you are measuring it so what's the general trend well if you're going to go down that means you're getting smaller as you go up you get smaller smaller as you go up so what what are what are going to be this what's going to be the smallest ones well we've already said helium is the smallest so we're going to be some of the largest what are going to be some of the largest atoms what's going to be the atoms down here in the bottom left so these are going to be large these are going to be small so large over here small over here and the general trend is you go from the bottom left to the top right you are getting you are getting smaller

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