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Video transcript

this video we're going to look at atomic and ionic radii and first we'll start with the atomic radius so if you think about an atom as a sphere the idea of atomic radius is simple you would just take this as a sphere here and then a sphere of course would have a fixed and defined radius and so that would be one way of thinking about it the problem is that an atom doesn't really have a fixed defined radius like the sphere example because there's a nucleus right and then there's this electron cloud or this probability of finding your electron and so there's no real clear defined boundary there and so it's difficult to have a fixed and defined radius so what chemists do is they take two identical atoms so these are two atoms bonded together the same element and if you find their nuclei so let's say that that's their nuclei here and you measure the distance between those two nuclei so this would be our distance D between our two nuclei if you take half of that distance right that would be a good approximation of the atomic radius of one of those atoms and so that's that's the idea behind the definition of atomic radius let's look at the trends for atomic radius and first we'll start with group trends and so here we have two elements found in Group one so hydrogen and lithium and let's go ahead and sketch out the atoms first and so we start with hydrogen which has atomic number of one which means that it has one proton in the nucleus so here's our nucleus for hydrogen okay so one proton in a neutral atom right the number of protons equals the number of electrons and so therefore there must be one electron so I'll go ahead and sketch in our electron here and we'll make things really simple and just show this this simple version of the of the of the atom even though we know it doesn't really exactly look like this and when we do lithium right atomic number of 3 so that means three protons in the nucleus of lithium so this is this is representative of lithium's nucleus with three protons and three electrons two of those electrons are in the inner shell so let me go ahead and show two of lithium's electrons right in the inner shell so that would be in the first energy level and then we need to account for one more so lithium's third electron is in the second energy level or the outer shell in this example and so here we have here we have our two atoms and you can see as you go down a group right as you go down a group you're going to get an increase in the atomic radius and that's because as you go down a group you're adding electrons in higher energy levels that are farther away right so in this case we added we added this electron to a higher energy level which is farther away from the nucleus which means that the atoms of course would get larger and larger so you're adding more stuff to it so it's kind of a simple idea let's look at period trends next so as you're going as you're going across a period this way so as you're going this way you're actually going to get a decrease in the atomic radius and let's see if we can figure out why but once again drawing some simple pictures of our atoms and so lithium right with atomic number of three so we've already talked about that right so there are three protons in the nucleus of lithium so I'm going to go ahead and write that in here so three positive charge for the nucleus of lithium and then we have to account for the three electrons right so once again two of those electrons were in an inner shell so there we go and then we had one electron in an outer shell okay so the picture is something like this now let's let's think about let's think about what's going to happen to that outer electron as a result of where it is so this outer electron this one right here in magenta right would be pulled closer to the nucleus right the nucleus is positively charged that electron is negatively charged and so the positively charged nucleus is going to pull that electron in closer to it at the same time those negatively charged inner shell electrons are going to repel it all right so let me go ahead and highlight these these guys right here right these are our inner shell electrons right like charges repel and so you could think about this electron right here wanting to push this electron outer electron that way and this electron right wanting to push this this electron that way and so the nucleus attracts the negative charge and the inner shell electrons repel the outer electron and we call this shielding because the inner shell electrons are shielding that magenta electron from the pole of the nucleus so this is called electron shielding or electron screening now that's going to be important concepts when so now let's go ahead and draw the atom for brilliance Oh atomic number four and so here's our nucleus for beryllium with an atomic number of four that means there are four protons in the nucleus so a charge of four plus in our nucleus and we have four electrons to worry about this time so I'll go ahead and put in the two electrons in my inner orbital right and our first energy level and then we have two electrons in our outer orbital or our second energy level and so again this is just a rough approximation for for an idea of what beryllium might look like and so when we think about when we think about what's happening right we're moving from a charge of three plus with lithium to a charge of four plus with beryllium and the more positive your charges right the more it's going to attract those outer electrons and when you think about the idea of electron screening right so once again we we have these electrons in green here shielding our outer shell electrons from the effect of that positively charged nucleus now now you might think that outer shell electrons could shield too right so you might think that oh this electron right here in magenta right could shield the other electron in magenta but the problem is they're both at pretty much the same distance from the nucleus so outer shell electrons don't really shield each other it's more of these inner shell electrons and because you have the same number of inner shell electrons shielding as in the lithium example all right so so let me go ahead and highlight those again right so we have two inner shell electrons shielding and beryllium we also had two inner shell electrons shielding and lithium because you have the same number of shielding but you have a higher positive charge right a higher positive charge those outer electrons are going to feel more of a pull from the nucleus and they're going to be pulled in even tighter than you might imagine so or at least tighter than our previous examples so these electrons are pulled in even more and because of that you going to get the beryllium atom as being smaller than the lithium atom hence the trend hence as you go across a period right you're always going to increase in the number of protons and that increased pull right is going to pull those outer electrons in closer therefore decreasing the size of the atom alright let's look at ionic radius now and ionic radius can be kind of complicated depending on what chemistry you are involved in so this is going to be just a real simple version if I took a neutral lithium atom again so lithium right not so we've drawn this several times we go ahead and draw it once more alright so we have our lithium nucleus which we have three electrons so once again we'll go ahead and sketch on our three electrons real fast two electrons in the inner shell and one electron in the outer shell like that and let's say you were going to form a cation right so you're going to take away an electron from our neutral atom all right so we had let me go ahead and draw this in here we had three protons in the nucleus and three electrons those cancel each other out to be a neutral atom and if we were to take away one of those electrons right so let's go ahead and show lithium losing an electron all right so if lithium loses an electron it's going to lose it's going to lose that outer electron so the nucleus still has a plus three charge right has three protons in it and we still have our two inner shell electrons like that but we took away that outer shell electron right so we took away this electron in magenta so let me go ahead and label this so we lost an electron so that's this electron right here and so you could just show it over here like that and by doing so right now we have three positive charges in our nucleus and only two electrons and so therefore therefore our lithium gets a +1 charge right so it's Li plus it's a cation and so we formed a cation which is smaller than the neutral atom itself and that just makes kind of intuitive sense right if you take away this outer electron right now you have now you have three positive charges in the nucleus and only two electrons here so you're pulling those electrons in you lost that outer electron it's getting smaller and so the cation is smaller than the neutral atom and so we've seen that neutral atoms will shrink when you convert them to cations so it kind of makes sense that if you take a neutral atom and add an electron it's going to get larger and so that's our that's our next concept here so if we took something like chlorine all right so a neutral chlorine atom and we added an electron to chlorine all right so that would give it a negative charge right so we would get chlorine with a negative charge or the chloride anion I should say and so in terms of sizes right let's go ahead and destroy a representative atom here so if this is our chlorine or neutral chlorine atom and we add an electron to it it actually gets a lot bigger all right so the anion is bigger than the neutral atom and let's let's see if we can think about why here so if we were to draw an electron configuration or should write a noble gas electron configuration for the neutral chlorine all right so you should already know how to do this you would just write your noble gas in brackets so neon and then 3s2 3p5 so seven seven electrons in the outer shell for the neutral chlorine atom for the chloride anion right you would start off the same way he would say neon in brackets three s two and you would be adding an electron to it so it would be three P five it be three P 6 like that and so now we would have so this will give us eight electrons in our outer shell and this would give us only seven electrons in our outer shell now the explanation for the larger size of the chloride anion in most textbooks is you'll see people say that the addition of this extra electron here right so that means that those electrons are going to repel each other more you have eight of them instead of seven and so because they repel each other more it gets a little bit bigger and that makes that make sense but you'll see some people disagree with that explanation and I haven't really seen a great alternative offered and so however you want to think about it generally the anion is larger than the neutral Adam but in terms of the explanation for that you could think about it as electrons replenish out if you wanted to despite the fact that people disagree with that you could think about just more stuff as a really simple way of thinking about it but again in general for exams think about the anion of being larger