in this video we're focusing on writing formulas of ionic compounds so let's take a look at the steps that we do for that the first thing that you want to do when you have a a name and you're trying to write the formula of that is to write the the symbol or the formula for the cation and note its charge when I say cation I'm talking about the positively charged part of that that always is the first part of the compound that's the one that's named first so we'll we'll take a look at examples of that the second thing that you do is you focus on the anion that's the negative part of the compound you write the symbol or the formula of that anion that's the negatively charged part of the compound it's always written last and you note it's charge as well then you're going to look at the charges themselves if the absolute values of the two charges are equal then you just stick the two ion formulas together and you're done if those two charges cancel out that's it however if they do not cancel out if they're not equal you want to swap the charges using the absolute values of the charges as subscripts so we'll see examples of that here shortly and if you are adding a subscript onto a polyatomic ion you want to place parentheses around that polyatomic ion to show that you have multiple units of that polyatomic ion and not just one of the elements there now when I say ions and formulas and charges and all that where do we get this well there are lots of of ions lots of charges this is a chart if you're working along with my AP chemistry workbook then you have a copy of this in your workbook if you'd like to obtain one then I have a link in here where you can purchase the workbook if not I have a link to it also where you can download a copy of this ion chart this has not all but most of the ions and formulas that you would run across in AP Chemistry so these are the the cations here on the left side anions on the right side so we're going to do a bunch of examples here the best way to get good at this is to do lots and lots of examples so let's try the first one here magnesium sulfate well magnesium has the symbol mg and if you look at the ion chart or the periodic table you know that magnesium has a positive two charge now sulfate has the formula so4 and it has a negative two charge so once again we look at those charges do they cancel out yes they do plus two and a minus 2 cancel out so we just put those two formulas together it's mgso4 magnesium sulfate here's another one ammonium sulfide so ammonium has the formula nh4 with a charge of positive one and once again if you're wondering where we're getting these it's right off of that ion chart these polyatomic ions have names and charges and formulas that you just have to learn you need to know those sulfide of course comes from sulfur that's s it has a charge of negative two so once again we look at those charges do they cancel out nope they don't do they plus one and minus two don't cancel out so we have to swap them that means that this this 2 right here will go down here and the one would go on here but of course we don't write ones so it's just going to go like this now notice that we are putting a subscript onto a polyatomic ion so we need parentheses around this don't we and there's a practical reason most the time too we don't want this to look like a 42 so by putting parentheses around this it becomes obvious that we have two units of that ammonium polyatomic ion there how about Iron II nitrate well iron is Fe and the 2 tells us what the charge is it's it's it's plus two and by the way that Roman numeral is there because iron can have multiple charges it can sometimes be plus two sometimes it's plus three so this is just telling us which form of iron we have we'll talk more about that in the next video now nitrate is a polyatomic ion NO3 has a negative one charge so once again we look at those charges do they cancel out no they don't so that means we have to swap them so the 2 is going to become our subscript on the nitrate and of course one well we don't write the one but it would become the understood subscript on the iron so it's going to be like that and of course since we're sticking a a new subscript onto a polyatomic ion we need those parentheses just like that let's take a look at manganese IV chlorite well manganese is MN and once again the 4 tells us it's a plus 4 charge and chlorite is clo2 with a negative one charge that's right off of the ion chart and once again do the charges cancel nope they don't so we have to swap them so the 4 is going to go down here on the subscript of the chloride and of course we need parentheses on that don't we since we're sticking a subscript onto a polyatomic ion how about cesium dihydrogen phosphate that looks really complicated doesn't it just once again think about cesium CCM is CS it's got a plus one charge if we look on the ion chart we notice that dihydrogen phosphate is on there it has a it has a formula of h2po4 with a negative one charge so do the charges cancel yes they do don't they plus one minus one cancel out so let's just stick these together CS h2po4 let's take a look at lead 4 oxide so lead is PB and of course it's got a plus four charge that Roman numeral four there tells us that oxide is O with a negative 2 charge so do the charges cancel nope they don't so we have to swap them don't we so we stick these together the two and the four and as we look at that we might realize that you know a two and a four can be simplified can't they so if we have a case like this where we have uh you know two subscripts that we are placing on the compound that can be simplified yeah simplify the two and the four down to a a one and a two just like this so lead 4 oxide is actually pbo2 now let's try another example sodium peroxide so sodium is n a with a positive one charge peroxide is O2 with a negative two charge so do the charges cancel they don't do they so we have to swap the the charges it's going to be like this and can we simplify that down well it might seem we can but actually we can't we can't and the reason for that is that peroxide is O2 we are not allowed to mess around with or simplify down the subscripts of an actual polyatomic ion so we have to leave that peroxide is O2 so that can't be changed we just have to leave this as na2o2 now how about silver dichromate silver is a g it's got a plus one charge and then dichromate is the polyatomic ion cr207 with a negative 2 charge so do the charges cancel nope we have to swap these don't we so the 2 is going to become the subscript on the silver just like that no parentheses here because we're not putting any subscripts on a polyatomic ion this time let's try some more like I said we have a lot of examples it takes a lot of practice to get good at this so barium phosphate well barium is ba and we got A plus two charge there and then we have phosphate which is po4 with a negative three charge the charges don't cancel do they so we have to swap them the 3 is going to go down here on the barium the two is going to go down as the subscript of phosphate just like that of course we can see we need parentheses don't we to make that look appropriate and complete next we have nickel two hydroxide so nickel is in I and that 2 tells us that the charge of nickel is 2 and not something else hydroxide is o h with a negative one charge the charges don't cancel do they so we have to swap the charges and it's going to go like this and we have to realize that o h hydroxide is a polyatomic ion so we have to put parentheses around the o h just like this to show that that 2 applies to both the O and the H in that compound let's try lead 4 carbonate so lead is plus is PB and it's plus four and then carbonate is CO3 with a minus two charge so do they cancel nope they don't so we have to swap the charges just like this and looks like we need parentheses and you might notice that we have a two and a four all right so we can simplify that down just like we did earlier earlier with the lead for oxide so this becomes not a two and a four but a one and a two just like this often whenever you have a plus 4 and a minus two that's the most common case where you'll have to simplify like this and notice it's okay to do this because we're not messing around with the CO3 that's the that's the carbonate formula about calcium oxalate calcium is CA with a charge of plus two and oxalate is c2o4 with a negative two charge so do the charges cancel yes they do so we don't have to swap it's just just put those together c a c2o4 let's do a few more examples and then we'll we'll uh stop for this section here for this video magnesium acetate so magnesium is mg with a plus two charge and acetate this is one of those formulas a lot of students forget it's c2h3o2 with a negative one charge and the charges don't cancel so we have to swap them it becomes just like that and as you can see since we're placing that subscript onto a polyatomic ion we need the parentheses around it just like this and then we have Cobalt 3 cyanide so Cobalt is co and we have a plus three charge because the Roman numeral three tells us that now cyanide is CN with a negative one charge do we have the charges gonna do we have the charges canceling or do we have to swap them we have to swap don't we so it becomes Co cn3 and cyanide is a polyatomic ion so we have to put parentheses around that to show that this 3 applies to the entire ion unit there zinc sulfite so zinc is z in with a plus two charge and sulfite is SO3 with a negative two charge so do the charges cancel or do we have to swap them they cancel don't they plus 2 minus two so we just stick those together and there's your formula ZN SO3 aluminum per iodate so aluminum is Al with a plus three charge per iodate is io4 with a negative one charge so do the charges cancel or do we have to swap them we have to swap don't we so it becomes like this and of course we need parentheses around there now just so you know so it's clear those numbers outside the parentheses are multiplied by what's inside the parentheses to tell us how many atoms of those elements we have so for example this aluminum we we just have one aluminum atom in this unit but there are three iodines because there's one times three making three iodine atoms but oxygens we have 12 oxygens four times three gets us twelve so if you were to be asked how many atoms overall do we have in a formula unit of aluminum periodate well it's 12 oxygens plus three iodines plus one aluminum so that adds up to you know 12 plus 3 plus 1 that's 16 atoms in that formula unit hope you learned something hope you have improved your skills at writing these formulas I hope you're subscribed thanks for watching