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Periodic Table: Periodic Trends: Atomic Size

Added Oct 29, 2015 •

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10/29 lesson

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I miss when you were going to talk about atomic radius we'll talk about what atomic radius is and then we'll learn how to compare elements to each other based on their Tomic radius atomic radius a fancy term for the size of an atom  this picture of the Adams kind of the traditional view of the enemy of the nucleus in the center and then are electrons to be just kind of moving around the side we know those electrons we don't really know exactly where they are we know that there in these clouds of probability and so if you can imagine just bringing it a room roll over here and trying to start measure this atom trying to measure the radius I mean it'd be pretty hard because those electrons are Ashley moving around and trying to find out where the edge of that atoms going to be is pretty tough and so the way we think about atomic radius is this atomic radius is one half the distance between the nucleus of bonding atoms and so traditionally we have to view the items as Bond together and this is called the bonding atomic radius no there is another definition called Van Der waals radius or the nonbonding radius which this is going to be essentially the same it's one half the distance between two nucleus of an atom that are smashing together but we're going to go with this one the bonding atomic radius 1 what's the distance between the nucleus of two bonding atoms so what would this look like well let's just assume our Adams kind of look like spheres here Valle this other stuff out of the way and these are our atoms then we can just put our nucleuses right here in nuclei right Center there and if these atoms came in and bonded with each other they would be able to get  that close are we going to just imagine these things kind of like solid spheres which is not true but it'll work for what we're trying to do here so if we can imagine the distance here between there to nucleus so that much if we just take one half that distance that's what it Sonic radius is that's what we're looking at here okay so let's go ahead and look periodic table so we can see this trend now I've put in these atoms for one of the periods on the periodic table we're starting here with lithium working our way you're right over here to Neon and you can see as we move from left to right across the periodic adding smaller and smaller and smaller which is weird because lithium only has 3 electrons whereas neon has 10 electrons so you'd think since were adding more stuff things should get bigger but that's not what happens here's why if I look at to board diagrams here I'll drive Bohr diagram for lithium and one for neon hopefully we'll be able to see what's going on here okay so the charge in the nucleus of a lithium atom is three positive I'll just write that in there three positive because it has 3 protons and it's also going to have 3 new electrons let's go ahead and put those electrons in here  that's the way the first energy level we can only hold two and then we put the third electron there on the second energy level and then for neon we're going to have a nuclear charge of ten so 10 positive because it has 10 protons and then it has 10 electrons will put those around with do in the first energy level and then the other eight here we'll go around in the second energy level I said that's what these two atoms are going to look so why is neon smaller than lithium well here's the reason is because the charge inside the nucleus of the neon at is a much stronger than the charge inside the nucleus of the lithium atom and so since this three positive charge is trying to attract these electrons it is going to wreck them to themselves and cut it keep these electrons around the nucleus neon has a much greater charge and so it's Force pulling these electrons plane is a whole lot greater and since there's only these two energy levels same as lithium has it's going to pull those energy levels in much closer to itself and so as me go from left to right across the periodic table atoms get smaller  let's go back to the periodic table look at the trend moving down a column so if we look at this First Column here again will start with lithium here and let's bring in sodium and then potassium and cesium and we kind of run out of room because they just keep getting bigger and bigger and bigger so why do they keep getting bigger when we just finish saying that if we add more electrons as we move from left to right across the periodic table they get smaller why are they getting bigger as we go down well let's take a look at it was these elements so we could see why that is  okay here's a Bohr diagram for lithium and sodium there in the same Hall on the periodic table I could see them right here here's lithium and sodium right below it and so let's take a look at these Bohr diagram lithium it's Bohr diagram here it has two energy levels their sodium on the other hand if I look at how many electrons I filled in I've only put 10 in there so but it has 11 protons less 11 electrons and so if I look at that second ring we already have eight electrons in their and so we have to add another energy level  and so we can put that other energy level in there that is where we're going to put that 11th electron and so the elements get bigger As you move from the top to the bottom is because we keep on adding another energy level each time so we could see that at sodium add  300g levels potassium then it's a little bit bigger is going to have for rubidium there is going to have five and so on and so on go on so they keep getting bigger and bigger cuz we're adding more energy levels  and that is atomic radius

Uploaded by Laura Houston.

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