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let's talk about how to write chemical formulas for ionic compounds so what this means is we're gonna start with a chemical name like magnesium chloride and learn the steps that we have to go through to take this chemical name and use it to write a chemical formula like mgcl2 okay let's start with our first example lithium oxide so when I'm doing these kind of problems the first thing that I want to do is find both these elements on the periodic table I'm using this kind of weird version of the periodic table that I just wrote out I left out a lot of the elements because they're not important for what we're doing here and I thought that are kind of distracting but don't be confused this really is no different from the periodic table that you probably have in your book it's just that it's missing a lot of the elements anyway lithium I want to find that it's right here Li and oxide oxide is just another word for oxygen it's what we call oxygen when oxygen has a charge I'm going to talk a little bit more about that later anyway oxide is just another word for oxygen and oxygen is over here they're on opposite sides of the periodic table and check this out too there's this big thick staircase that separates lithium from oxygen what's a staircase doing well if you remember the staircase separates the metals on this side of the periodic table from the nonmetals on this side of the periodic table so lithium is a metal and oxygen is a non-metal this is important because we have a metal and a nonmetal connected together and that means that we're dealing with an ionic compound ionic compounds are always metals and nonmetals so when we have metals and nonmetals making an ionic compound that means that the atoms in that compound have a charge so I want to find out what the charge of those atoms is and I can do that by looking at where they live what column they live in on the periodic table here's what I mean so lithium lithium lives in this column everything in this column has a plus one charge so I'm gonna write this right here Li one plus everything in this how column has a two plus charge three plus charge you might want to write this on your own periodic table that you have for your reference everything here has a three minus charge and oxygen which lives in this column has a 2 minus charge so I'm gonna write it right here so Li 1 plus O 2 minus ok so now I ask myself does the plus charge and the minus charge balance when I have one atom of both of these the answer is no because I have one plus charge but I have two minus charges so the charges don't balance but I want to figure out how to balance them I need to have them balanced in order to write the chemical formula so what I can do is I can add more lithium atoms I can add more oxygen atoms or I can add both of them until I get the charges to balance out I have two minus here and only one plus here so what I'm going to do is I'm going to add another lithium atom li plus so that now I have two positive to balance out my two negative and now they're balanced so in order to get the charges balanced I have to have two lithium's and one oxide or one oxygen now when I write the chemical formula I'm literally going to say how many of each of these atoms I need in order for the charges to balance and we've said that's two lithium atoms so I'm gonna write Li and then the two after it that indicates that I have two lithium atoms and then one Oh to show that I have one oxygen and I'm not going to write anything after that if you have a letter without anything after it it means that you just have one of them so lithium oxide two lithium's and one oxygen is how we get the charges to balance for this compound here's the next one potassium nitrite just as before I'm going to take out the periodic table and I'm gonna find both of these guys on potassium is K it's right over here and nitride is what we call nitrogen when it has a charge on it and so nitrogen is over here check it out metal nonmetal separated by this big thick staircase and we know we're dealing with an ionic compound and whatever we think I own a compound metals and nonmetals we want to think charges what charges did the atoms have potassium lives in this column here the same column is lithium and so it has a one plus charge so I'm going to write that up here k+ and nitrogen lives in this column where everything has a minus three charge so here we have n 3 minus what I have one atom of k+ and 1 atom of n3 - do the charges balance out here the plus and minuses balance out it doesn't I've got 1 plus I've got 3 - that doesn't balance right but I can add more atoms of either type in order to get the charges to balance so since I don't have as much + as I have - I'm going to add a bunch more potassium here I add another now I have 2 + and I'm gonna add one more now I have 3 plus the 3 + potassium on this side balances out the three - nitride on this side now when I write my chemical formula I want to literally say how many atoms of each type do I need for the charges to balance I'm gonna need 3 potassium so I'm gonna write K 3 and I only need one nitrogen or one nitride so I'm gonna write n with nothing after it if I don't write anything after it means 1 so 3 potassium 1 nitrogen the correct formula for this is K 3 n okay here's another one sodium chloride which is the name of common table salt once again I'm going to find them both on the periodic table sodium right here is in the 1 plus column and chlorine or chloride is in this column right here we've got metal and nonmetal so we're dealing with ionic compounds which means the atoms have charges I want to figure out what the charges are as I said before na is in the one plus column so an atom of sodium has a one plus charge here and then chloride is what we call chlorine when it has a charge and chlorine lives in this column so it has a one minus charge so CL one - okay so now I have one out of a sodium plus one and I have one atom of chloride minus one what I have an atom of both of these this is positive and negative does it balance out it does I don't need to add more of either one of these atoms I'm totally set so to write the chemical formula for sodium chloride I want to say again how many of each I need for the charges to balance out I'm just gonna say na CL I don't put any numbers after them because if I don't put a number it means that I just have one of them so I have one sodium and one chlorine okay you might be getting the hang of this I want to try to convince you to watch this one last example because it's a tricky one that confuses a lot of people aluminum oxide once again periodic table aluminum here oxygen here metal nonmetal its ionic which means charges are important so aluminum is in the three plus column one atom or ion of aluminum anion assess what we call an atom that has a charged anion of aluminum has a three plus charge and an atom of oxygen oxide here has a two minus charge al three plus O 2 minus now I need to figure out how many of each of these atoms I need for the charges to balance out and this example is a little bit trickier than the ones before how are we going to be able to add atoms to get the same number of positive and negative charge let me give you him we're going to start by adding another aluminum atom in here their aluminum ion to be precise so now I have three plus here and three plus here each aluminum atom has a charge of three plus so I have a total of six-plus now I can balance out the six plus by adding more of the oxygen I add one more oxygen I have a total of four minus and I add one more I have a total of six minus so this is how we balance out the charges in aluminum oxide with two aluminum ions with three plus each total of six plus and three oxygen ions with a total of three oxygen ions with a total of 2 minus each which gives us minus six now I'm going to write the chemical the chemical formula to show how many of each I need to have that's going to be al one two and oxygen one two three I have two aluminum's and three oxygens okay so now let me tell you what I'm gonna do if you're still a little bit shaky on this I'm gonna do a few more practice problems in a minute if you feel really good about this you might want to just turn the video off and go and watch the videos that I have on writing formulas for compounds with transition metals and for compounds with polyatomic ions but before I do a few more practice problems I want to talk about two commonly made mistakes so many people make when the writing is foremost and we might just want to stick around to watch that here are the two mistakes people make let's assume that you're asked to write the chemical formula for lithium oxide and you do this you find out that you wanna have two lithium's with one plus each to balance out the charge on oxygen okay here is one mistake that people make they say okay I have two lithium's and one oxygen so I'll write Li 201 they say yeah yeah there's only one oxygen so it can't hurt to just add the oh one here can it it can't hurt it does hurt it's just not right to do you never want to put a 1 after an element so Li 200 1 is wrong instead you want to write Li 2 O without any numbers after this and this is the right way to do it here's another mistake that people often do they don't remember to get rid of the charges when they write the chemical formula so they end up writing something like li+ 202 - where the charges that were on the atoms here stay when they write the chemical formula this also unfortunately is wrong when you write the chemical formula you want to get rid of all of the charges so you're just right again Li - Oh make sure there aren't any charges in the chemical formula when you finally write it okay