in this video we'll be looking at nomenclature which is writing formulas and names for compounds so when we talk about nomenclature it's kind of a big word but what it really just is is it's a systematic way of writing names and formulas for compounds we think about all the elements on the periodic table in different ways they can bond and combine you get a very very large number of different compounds to exist we have to have a way to communicate information that goes for every discipline chemistry is no different so for us nomenclature is our system to clearly communicate chemical information and it's important just like any other communication tool that you use are trying to relay information everybody has to be on the same page everyone has to understand the same words mean the same things without that it's very tough to communicate effectively whether just in regular spoken language or to kind of convey scientific knowledge so in this course we'll learn the rules for writing formulas and names for basic chemical compounds so there's two different types of compounds that we'll be dealing with we'll be dealing with ionic and molecular compounds we'll start with the ionic compounds ionic compounds get formed from positive and negative ions it's that attraction between the positive ion and the negative ion that keeps the different components together to form that compound when we think about what type of elements form each type of ion well what we saw before is that our positive ions usually come from metals our negative ions usually come from non-metals so when we're looking at an ionic compound if you're seeing a metal and a non-metal that's your clue that you're dealing with an ionic compound later on we'll deal with covalent compounds which are not formed from ions here you get a sharing of electrons keeping the pieces together and your hint for dealing with these is you'll have different types of elements involved you'll have two non-metals that are bonded together so let's start with kind of our rules for naming ionic compounds you'll see this slide quite a bit in this presentation as we kind of go through the rules but first couple of examples we'll do just deals with the first two rules we'll always name the positive ion first and you're going to see that we keep the name of the element then we'll name the negative ion second what you'll see is we'll end up replacing the ending of that negative ion with ide we'll chop off the name of the element replace it with ide we'll have some special rules for number three for dealing with transition metals we're going to see we're going to use roman numerals to indicate the charge because remember those have multiple different charges and then lastly we'll deal with the polyatomic ions polyatomic ions are going to take some special rules we'll see some parentheses having to be used in order to effectively group them before we can kind of get into nomenclature we need to do a little bit of a review not necessarily with the names of the specific groups but the charges so we've seen this slide before but here we're not going to focus on the names here we have to focus on the charges so we want to do this quick review to go through those group 1a positive 1 ions group 2a positive 2 ions all of these b's in the middle those are those transition metals remember we're going to have a special rule for those because they form positive ions of various charges things in group 3a we're going to see form plus 3 ions group 4a we won't see a lot of ions coming from that group group 5a though remember now we're kind of in the nonmetals we're going to start seeing negative ions negative three ions for group 5a group 6a negative 2 ions group 7a the halogens negative 1 ions group 8a noble gases they're not going to form ionic compounds because they don't form ions very easily so we're going to use this information quite a bit today you have to have memorized which ions are going to get formed from which groups again you see kind of on the periodic table again things in group 1a plus 1 2a plus 2. all of these in the middle our transition metals plus something we'll have to figure that out then when we get into group three a plus three four a usually not a lot of ions get formed group five a minus three group six a minus two group seven a minus one and lastly group eight a not a lot of ions are going to be formed there so again here are rules our first couple examples will just deal with rule one and rule two we won't have to get into worrying about rules three or rules 4 just yet so for our first example to kind of illustrate this i wanted to use an example that you're already familiar with many of us already know the name of this compound we already know what this is called but we want to go through the full rules so you can kind of see where that comes from how does this apply to the rule then even things you're not as familiar with you're just going to be able to use those rules in order to correctly name those compounds so first nacl first are we dealing with an ionic compound remember you can tell by the elements that are involved when you look at sodium on the periodic table you'd recognize right away that's a metal sodium is metal chlorine is a non-metal so right away you're definitely sure you're dealing with an ionic compound so being we have an ionic compound we want to think about ions now even for the simple examples i can't stress this enough or encourage you to do this any more than the great amount i can this is really really important write out the ions that you have anytime you have an ionic compound i want you writing out the ions that you have it makes the whole process of nomenclature so much easier even if you think you can see it right away without doing it you really want to do this for yourselves nomenclature is one of the more challenging topics that we do and writing out the ions is a way to make it a little bit easier and take a little bit of that challenge away so all of our examples for ionic compounds we're not just going to write out the name we're going to write out the ions that we have when you're able to look at the ions things get so much easier let's look at these ions we know these are going to form ions because uh we have metal and a non-metal so let's start with the sodium we look at sodium on the periodic table sodium is in group 1a that's going to make a plus 1 charge we look at chlorine on the periodic table when you look at chlorine on the periodic table it's in group 7a we said things in group 7a form a charge of negative one so now we have the ions that we need these are the ions that we have now we can name this first rule says write the name of the positive ion first keep the name of the element so na is sodium name the negative ion second negative ion you replace the ending with ide so instead of chlorine we're going to take off the ending of chlorine instead of using the ine ending like the name of the element we replace that ending with the ide so this is our name of our element our name of our compound sorry sodium chloride just what you're familiar with a couple things to notice about this one and we'll point this out notice the idea of charge balance in this we have the same amount of positive charge that we have of negative charge plus one minus one also notice that our name doesn't contain when we just say sodium chloride for the name it doesn't contain any of the prefixes or any over way of telling how many of each compound of each element are present in that so we'll see that again in the next couple of examples that there's no overt you know prefix or anything that tells us the number of each ion and that idea of charge balance that's a recurring theme in these ionic compounds let's name another one so patel k2o is what we're looking at and first we want to say is this an ionic compound well we look at potassium on the periodic table and potassium on the periodic tables in group 1a that's a metal oxygens in group 6a that's definitely a non-metal and so we have metal non-metal we're dealing with niantic compound notice the subscripted two that you see between the potassium and the oxygen highlighted in green we use subscripts in our formulas in order to indicate how many of each element that we have and the subscript always belongs to the element that came before it so what this is saying is that we have two potassiums one oxygen we see subscripts over and over again they're always going to have the same meaning how many of what came before so now just like before we want to write out what ions we have in place we can look at them directly when we're going through this naming here remember what that 2 tells us that 2 tells us we actually have 2 potassium potassium is located in group 1a things in group 1a we saw had a plus 1 charge so each potassium would have a plus 1 charge we have one oxygen oxygen is located in group 6a things in group 6a have a negative two charge notice that idea of charge balance again my sum total of positives is plus two two potassium's at one h sum total of negatives is negative two so now when i write the name for this i always start with the positive ion positive mine keeps the name of the element that would be for potassium negative ion we chop off the ending and replace that ending with the ide ending so this is going to become oxide and our name of our compound we're looking at is potassium oxide again a couple things to notice in this in the name we're not using any prefixes there's no overt indication that you're dealing with two potassiums and one oxygen if you just look at the name right away the other thing you want to notice is that idea of charge balance the same amount of positives as negatives and we can see that in here plus one and plus one make plus two for a total that balances out with the negative two from the negative ions so you're going to see that charge balance again when we look at our next example mgbr2 so now we want to name this compound first are we dealing with an ionic compound well you look at magnesium that's definitely a metal when you locate that on the periodic table bromine is a non-metal so we do have metal and non-metal present in this so we're dealing with an ionic compound and just like every other ionic compound we're going to name we're going to write out which ions we have and how many of them so we can directly see them it makes this process of nomenclature much much much more straightforward so i have one magnesium ion and when we look at magnesium on the periodic table magnesium is in group 2a things in group 2a have a charge of plus 2. i have two bromide ions i have two bromines here bromine lives in group seven a on the periodic table that means each bromine will have a charge of negative one notice that charge balance happening yet again plus two minus two same amount of positives as negatives so now i'm ready to name this compound positive ion first so we get our magnesium in negative ion second chop off the ending and replace it with ide so instead of bromine that's going to become magnesium [Music] bromide you're going to get that ide ending in here so the name of our compound magnesium bromide in this example we're going the opposite way of the way we went with the previous ones in the previous ones we started with a formula that we had to write the name for this way we're going to do almost the exact opposite we're starting with a name and we're going to write the formula and you want to make sure you're able to do this both ways you want to be equally good going each way so i want to show you an example going this opposite way you're still using these same rules and here's where writing out the ions really pays off you can really see why that's such a good idea to do because it just takes a lot of the kind of guesswork and other things out of it you're able to look at what you have well first are we dealing with an ionic compound well calcium is a metal nitride comes from nitrogen we find nitrogen in the periodic table that's in group 5a that's over there it's non-metal we have metal non-metal this is going to be an ionic compound so what do we need to do we need to write out what ions do we have in place we really want to be able to look at and see our ions so calcium we know is in group 2a things in group 2a have a charge of plus 2. nitride well that came from nitrogen that's from group 5a we said things in group 5a have a charge negative three remember whether you write three minus or minus three doesn't matter plus two two plus doesn't matter so now when we look at this do we have charge balance and the answer would be no i have a total of plus two for calcium and negative three for nitrogen i don't have charge bounce i have a unequal amount of positive and negative charge right now well there's a couple different ways that this could be fixed some of which you can do some of which you definitely cannot do so kind of the easiest one is something you can't do the easiest fix would be what if we just change the charges of these things well if we make calcium plus three then we'd have charge balance well we can't make calcium plus three calcium does not form that iron calcium only forms plus two or maybe we could change the charge on the nitrogen if we made the nitrogen minus two instead of minus three well we'd have charge balance but that's not what nitrogen does nitrogen is not going to form a negative two ion nitrogen has to form a negative three ion so we can't just change the charges of these these form specific and set charges but if we can't change the charges we still have to achieve charge bounce somehow the only way to do that is if we can't change the charges we have to change the relative amounts of each that we can do so there's a couple techniques that this you can do this one technique is very mathematical it's least common multiple i'm not going to do at least count multiple i'm going to show this kind of with the you know most simple way doesn't really involve much math it's just the idea of trial and error look at what you're short of and start adding on those things that you're short of so when i first look at this i'm in a total of plus 2 and minus three i'm short positives right now so what am i going to do i'm going to tack on another positive ion now if i look at the total i'm at plus four for my positive charge negative 3 for my negative charge well now i'm short negative charges what am i going to have to do tack on another negative ion now i'm at plus 4 for my positive charges minus 6 for my negative charges now i'm back to being short positives well what can i do i can tack on another positive ion now i look at my total of positives and negatives my total positives are plus six my total negatives are minus six now i have charge balance so now i have equal amounts of positive and negative charge so now i can just write my formula well positive ion comes first that's the calcium how many calcium do i have well i have one two three calciums negative mind comes second that's the nitrogen how many nitrogens do i have two nitrogen so my final formula is ca3n2 notice how in the formula we're not rewriting the charges in the formula that kind of gets messy and sloppy remember nomenclature we're supposed to convey information quickly easily and thoroughly if you start throwing charges in that formula it starts to get messy on the reader to kind of look at us too much to look at just like in the previous examples if you go back we didn't start with any charges in that formula so here's our formula for calcium nitrite so we kind of did the first two rules for dealing with naming of these ionic compounds but one of the things you would have noticed is none of the examples that we have done already involve the transition metal eye now it's kind of time to start dealing with these transition metal ions now we have to get rule three involved and this rule three applies four transition metal ions only what we're going to see we're going to have to do is we're going to have to use a roman numeral to indicate the charge that that transition metal ion has remember when you're looking at the periodic table where your transition metal ions live they live in the middle of the periodic table so you're looking at in here these down here we won't really see any of these down here but these would also be transition metal ions as well but generally you're looking at the ones in the main part of the periodic table so anytime you see any of the elements in here in your ionic compounds you're going to have to follow that transition metal ion rule let's look at some examples that use this transition metal ion rule and i kind of want to just do with these two examples side by side in parallel so you can kind of see the importance of this rule that we're going to have to have so first are we dealing with ionic compounds and if we look at the one on the left i have copper copper is definitely a metal it's the metal we're familiar with bromine is definitely a non-metal so this is an anionic compound what do you want to do with your ionic compound you want to write out what ions you have in place well what ions do we have in place well i have a copper ion well for this copper ion when you look at copper on the periodic table you're going to say ah it's a transition metal we don't know the charge of the copper from looking at the periodic table so i can't put a charge in there just yet i'm going to have to figure out what that charge is when i look at the bromide though i do know something about the bromide bromine lives in group 7a so i know what type of ion the bromine is going to form is going to form a negative 1 charge i have to use that information i still need charge balance i'm using that information from the bromine now to figure out what the charge on the copper is i have one copper and one bromine and if the bromine is negatively one charge that means the copper must have a positive one charge that's the only way to achieve charge balance remember i didn't get this plus charge from copper from the periodic table i di figure that out based upon what is happening with those negative ions so now i'm ready to kind of write the name of this positive ion first i have my copper negative ion second the bromine is going to change into bromide so right now before i start using that third rule i'm going to hold off for a moment using that third rule until i do the example on the right hand side i would just have copper bromide now if i look at the example on the right hand side i'm going to get that one to the same point we're at on the left hand side before we start applying that third rule do i have an ionic compound yes copper's metal bromine is a non-metal write out the ions that you have so i have a copper ion and again i don't know the charge of the copper just by looking at the periodic table it's there in the middle i don't know what charge that's going for i'm going to have to figure it out i know i do have two bromines and i do know the charge on the bromine it's negative one so now i have to use that information in order to achieve charge balance my sum total of negatives is negative two i have to have a sum total of plus two from the positives and i only have a single copper that can generate that that means for this compound to have charge balance this copper has to have a charge of plus two so now we're ready to write the name for this positive ion first negative ion second i'd have copper bromide again and here's where you see the problem where just the first two rules that we talked about previously aren't good enough at this point i have the same name for both of these compounds these compounds are very different they have different physical properties things like color melting point they have different chemical properties how they're going to react even though they're both made up of copper and bromine they are very different compounds we need to have a way to distinguish one from the other and that's where that third rule comes in you have to apply this rule when you're dealing with transition metal ions and that rule says that four transition metal ions use a roman numeral to indicate the charge that the transition metal ion has so let's look at the example on the left the copper has a charge of plus one so that means i'm going to have to use a roman numeral one to describe what's happening with the copper if i look at the compound on the right what charge does that copper have that copper has a charge of plus two so i'm going to use the roman numeral 2 in order to indicate that using the roman numerals gives us a way now to differentiate one from the other now we can clearly say which one we're referring to so this transition metals these transition metals and the roman numerals this one starts to add a little bit of complexity and difficulty to dealing with the nomenclature compared to what we are dealing with before the key thing with the roman numerals is knowing what the roman numeral is telling you the roman numeral does not so the roman numeral does not tell you how many coppers there are that's why we don't use regular numbers for that we're using roman numerals what the roman numeral is telling us the roman numeral tells us one thing and one thing only and what the roman numeral tells us is what is the charge of the copper copper's plus one roman numeral one on the right hand side what is this roman numeral telling us it doesn't tell us we have two coppers because we don't it's telling us what is the charge so that's kind of one of the pitfalls of that you can fall into doing these is mistaking the roman numeral for telling you how many of that element you have that's not what it's saying it's not telling you how many is telling you what is the charge of that transition metal ion so we'll do some more examples so you kind of get a chance to see this in action so again we'll kind of do two side by side in parallel so you can kind of see how this ends up working this time i'll just complete the one on the left and then we will complete the one on the right so on the right feo well are we dealing with an ionic compound well we are dealing with an ionic compound iron is a metal oxygen is a non-metal this will definitely be an ionic compound so when you have an ionic compound we want to write out ions so what ions are we needing to write out for this well we need to figure out the we have one iron ion when you look at iron on the periodic table you're going to see it's a transition metal ion just from looking at this position on the periodic table we don't know it's charged yet we're going to have to figure that out from other information let's look at the oxygen oxygen we do know something about oxygen lives in 6a when things in group 6a form ionic compounds they're going to form negative 2 ions we need to use this information to achieve charge balance so the only way we could have charge balance in this compound is if the iron was also plus two in order to achieve that charge bounce that's the only way so now we're able to name this compound on the left positive ion first that's the iron negative ion second take out the ending replace it with ide and now since iron is a transition metal ion we need to use the roman numeral in order to indicate what the charge of that iron is going to be the charge on the iron is plus two so we need roman numeral two here remember that two is not telling us how many irons that's not what the roman numerals say the roman numerals tell us what is that charge the charge on the iron is plus two now let's look at the example on the right fe-203 go through the same procedure on this one do we have an ionic compound yes iron is a metal oxygen is a non-metal metal and non-metal are going to make ionic compounds so let's write out the ions that we have here i have two iron ions so the subscript two is telling me i don't know their charge yet i'm going to have to figure that out i have three oxygen ions oxygen lives in 6a we know something about this everything in group 6a will have a charge of negative 2. so now i need to use this information in order to figure out what charge the iron has remember my sum total of positives and my sum total of negatives have to balance each other out i have to have that charge balance so where am i at if i count through all my negatives negative two negative four negative six i'm at minus six that means my sum total of positives has got to be plus six so how can i get a sum total of plus six well if my sum total needs to be plus six that means each individual iron has to be plus three plus three and plus three would give me plus six that would balance out with the minus six that are coming from the oxygens so that's how we determine that the charge on this iron is plus three by using charge balance and what we knew about the negative ions so now we can write the name of this positive ion first that's iron negative ion second replace the ending with the ide ending and now since iron is a transition metal we need to make sure that we use a roman numeral to indicate what charge it has and here you might be tempted to put plus six well that's not the charge that the iron has plus six is a total charge you just list what type of iron we're dealing with the type of iron that we're dealing with is iron that has a charge of plus three remember what your roman numerals are telling you kind of back on the left what's our roman numeral telling us that roman numeral 2 told us we had a charge of plus 2. what's that roman numeral on the right telling us well that plus 3 matches that roman numeral remember that's the definition of what the roman numerals mean what they're telling us it's not how many it's what is the charge that is the question that they answer the roman numerals are always tied into the charge of those transition metal ions we want to do an example going the opposite way so the previous ones we did we had the formula we had to write the name for this one we're going to go the opposite way we're going to go from name to formula for these so still ionic compounds still have transition metals in them how do we know we're dealing with an ionic compound well we look at what we have well manganese is a metal sulfide comes from sulfur sulfur is a non-metal so we're definitely dealing with an ionic compound still remember what do we want to do every time we have an ionic compound every time we have an ionic compound we want to write out what charges are we dealing with what charges do we have in place well start with the manganese ion when you look at manganese's mn write that a little bit more clearly so we'll rewrite that so manganese is m when you look at manganese on the periodic table you don't know what charge it's going to have based upon its position on the periodic table we have to figure out that charge well this time it's telling us what that charge is look at that roman numeral in the middle i'm circling in red here v i well v i is the roman numeral for six now that roman numeral six is telling us what that charge on that manganese has to be this manganese is plus six so now we can deal with the sulfide well sulfide comes from sulfur sulfur's s we look at sulfur on the periodic table sulfur on the periodic table resides in group 6a things in group 6a we said form a charge of negative two so now i have my ions in place i got the charge from the manganese from the roman numeral i got the charge from the sulfur based upon its position on the periodic table what do i still have to do i have to achieve charge balance i'm at plus six minus two how can i achieve charge balance well i'm clearly short negative charges so i'm at plus six minus two i need to start adding in negative ions so i'm going to add in another sulfur what am i at now well now i'm at plus six minus four well closer but i still haven't seen charge balance yet i'm still short negative ions what will i have to do i will have to add in another negative ion now i have charge balance my sum total of positive charge is plus six my sum total of negative charge is minus six so now i can write the formula for my compound positive mind goes first how many manganeses do i have i just have one negative ion second that's the sulfur how many sulfurs do i have i have three so my final formula is mns3 for this name remember you want to be comfortable going in both directions formula to names and the opposite names to formulas our next set of examples involve ionic compounds that contain polyatomic ions so a little bit before we start getting into some examples of the polyatomic ions it is absolutely critical that you memorize the names and the formulas of the polyatomic ions there's no way of getting around knowing them if there's an alternate to just kind of memorizing these i would let you know but you really just have to get these memorized there's no way of kind of getting around that fact and generally the sooner that you know them the easier the material becomes because reason why that is is you have to know what they are these are the factoids before you start using them in order to answer questions and solve problems let's look at some ionic compounds that contain these polyatomic ions so here's our example nano3 how do we know this has that polyatomic ion well you memorized it you recognize that hey when i see that no3 i want to be thinking i have a polyatomic ion that i need to deal with there's our polyatomic ion so when we deal with these polyatomic ions the key thing for dealing with a polyatomic ion is that we keep it together that's what it means polyatomic means many atoms this is a single ion that as a group of atoms has a charge so it's a group of atoms that as a group have a charge so we always want to think about keeping those pieces of a polyatomic ion together so when we name this ionic compound we're still going to write our ions well first ion is pretty easy to write out it's the sodium ion we recognize that group 1a going to have a charge of plus one now for nitrate we have to recognize that that no3 is that polyatomic ion we keep it together we're not splitting it into a nitrogen and three oxygens it stays together and as a group this collection of atoms no3 has a charge of negative one we need to keep that together we're not splitting it up if you split it up you'll never achieve charge bounce and there's no way of really fixing that you have to absolutely keep that together so now when we name this we follow the kind of same procedure we did before positive line comes first keep the name of the element that's sodium then we name the negative ion second now usually you would take the name of the element and chop off the ending replace it with ide well a little bit different procedure for the polyatomic ions for these polyatomic ions each polyatomic ion has a specific name you just use that name you use its name you say what it actually is no3 minus you memorize has a charge of nitrate this compound is sodium nitrate that's our name of our compound no more no less so the key thing in being able to deal with these polyatomic ions is just have to spend the time memorizing them again probably a three by five index card to make a flash card for yourself name on one side formula in charge on the opposite side if we do a another example of an ionic compound that has polyatomic ions we'd be looking at k2so4 well how do you know you have that polyatomic ion in there well you're memorizing the polyatomic ions such that you can recognize them and know what to do with them in formulas and compounds so we see that polyatomic ion in there so let's write out our ions again just as we've done before i have two potassium ions when we look at potassium on the periodic table potassium is in group 1a for a charge of plus one at so4 we recognize that as a polyatomic ion and you would have memorized this so4 will have a charge of negative two now we're able to write the name of this compound positive ion first that would be the potassium negative ion second here for the polyatomic ions you're just going to use the name for that polyatomic ion so4 2 minus is sulfate you would have had that memorized so the name of our compound here is potassium sulfate we have to be very very careful with these polyatomic ions so i know i've stressed it a lot already but you need to take your time to memorize them and memorize them very very well you have to be very very careful with these the reason why you have to be very very careful with these is small subtle differences mean big changes so just one example there's other ones that i could have pointed out but just kind of one idea to kind of warn you about making sure you're being careful and knowing these well are the eights versus the ites these are different by a single letter they have similar formulas but they're a little bit different an example is nitrate is no3 minus but nitrite is no2 minus that's how careful you have to be when you are memorizing these also some polyatomic ions have names that are close to the name of a monoatomic eye well you compare nitrite versus nitride well the difference in those two words is by a single letter it's a d versus a t very very similar sounding names however the formulas couldn't be more different nitrite polyatomic ion no2 minus nitride monoatomic ion and three minus so you just have to be very very careful with these you really need to spend time and memorize these well otherwise it's very very easy to make mistakes on these [Music] so we didn't quite get at the last rule yet that's coming up in our next example that's rule four so we dealt with rule one rule two we did transition middles for rule three rule four applies to the polyatomic ions and it's a if rule if there's more than one of the polyatomic ion present so far previous examples you only saw one of the polyatomic ion if you have more than once you need to use parentheses around the polyatomic ion and place the subscript outside the parentheses that way you're very clear to the person looking at it what you actually have in place and we're going to do an example of that so an example of this also gives us a chance to practice going the opposite way for some of these starting with the name writing out that formula so calcium hydroxide how are we going to write the formula for this well we're dealing with an ionic compound metal polyatomic ions so we're definitely dealing with an ionic compound we want to write out what ions we have well calcium we know calcium calcium we look at on the periodic table it's in group 2a things in group 2a have a charge of plus two hydroxide well hydroxide is one of those polyatomic ions you just have to know the formula in charge for that you have to memorize it there's no way to deal with this except to have that memorized the formula in charge for hydroxide is o h with a negative charge now just like before all ionic compounds have to have charge balance these are not an exception you still have to have charge balance and when you look at what you have right now do you have charge balance the answer is no i'm at plus two and minus one how can i fix that i need to add another one of these negative ions so i add another one the hydroxide ions now i'm at plus two and minus two now i have that charge balance now i can write out what my formula is going to be well positive ion first negative ion second here's where that parenthesis rule gets used at if you just kind of randomly put a2 there thinking oh the reader is going to understand that i'm really referring to two units of hydroxide well that's not being clear so we don't want to do that instead whenever we have more than one of our polyatomic ions instead what we need to do is start off the same way calcium we still have our hydroxide but now to clearly show that we have two of these hydroxide ions we would place the polyatomic ion in parentheses and put the subscript outside the parentheses now you're being perfectly clear what you have now we use parentheses here just like you would use parentheses in writing to put things as a group or parentheses in math to group things together now this is the correct way of writing this now you can kind of see okay get rid of that what we had before the incorrect way of doing it now it's very clear what we have we have two hydroxide ions in here so anytime you have more than one of those polyatomic ions you need to use parentheses in order to clearly indicate that the subscript goes outside of the parentheses so the next couple examples that we want to do aren't necessarily anything new or using a rule a new rule it's just combining a couple of things we've already have done we've talked about transition metals we've talked about polyatomic ions now i'm just going to do an example that has both of them present in the same compound how do we deal with that well all the same rules apply but let's look at an example of doing this first we're recognizing we're dealing with ionic compound we recognize that because we have a metal that's going to form a plus positive ion and we recognize we have one of our polyatomic ions this one forms a negative ion so i can see i'm going to generate positive and negative ions out of this so whenever we have an ionic compound we want to write out what are the ions what are those charges so for the positive mind the titanium so ti i don't know the charge right away all i know is that i have one of them so i can't figure out the charge right away because it's the transition metal ions they're in the middle of the periodic table i'm not sure what it's going to do yet i'm going to have to figure it out i'm going to have to figure out based on what's happening with the negative ions so i have two of these co3 polyatomic ions here's one where you gotta know the charge of these polyatomic ions if not you're stuck and you're not going to be able to complete this problem co3 you're memorizing this has a charge of negative two so now i can use the information from the negative ions to figure out what the charge on the titanium has to be i still need charge balance what's my sum total of negative charges well minus two and minus two make negative four that means my titanium has to have a charge of plus four in order to have charge balance so now i'm ready to write my name well positive ion first let's get our titanium in here positive ion first negative ion second co32 minus that's the polyatomic ion that you have to have memorized you have to know the name of that that one is carbonate and now since titanium is a transition metal ion i need to use a roman numeral to indicate the charge titanium has a charge of plus four so i need the roman numeral four here well the roman numeral four is i v remember what that roman numeral is telling you that roman numeral is telling you what is the charge of that transition metal ion that's its meaning so our name for this compound titanium 4 carbonate let's look at another one again another example that just contains both a transition metal ion and a polyatomic eye so this one chromium three chloride how do we know we're dealing with an ionic compound well i see chromium that's a metal chlorite that's one of those polyatomic ions that has a negative charge that i have memorized so i see my positive ions coming from the chromium my negative ions coming from the chloride so i'm still dealing with an ionic compound whenever you're dealing with an ionic compound you want to write out the ions well chromium is cr so chromium cr and when i find it on the pr cable it's the transition metal ion i don't know its charge right away but the name is telling me that charge that roman numeral 3 that you see in the name that is what is telling us that we have a charge of plus three now chlorite chloride's one of those polyatomic ions that you're going to have memorize the formula and charge on chloride is c l o two with a negative one charge so now i have the type of ions that i have now i'm looking at how can i achieve charge balance i'm looking at chromium plus three total positives my total negatives are only negative one so i need to get more negative ions in here in order to achieve that charge balance so i'm at plus three minus one i'm going to add another negative now at plus three minus two i need to add another negative so now i'm at plus three minus three now i'm ready to write my formula out i have one chromium now for my chlorides remember here i have a polyatomic ion i have more than one of them i have more than one of them i need to set it in parentheses and get my subscript outside of the parentheses to clearly show how many of that polyatomic ion i actually have so my final formula for this will be cr clo2 3. so those were the ionic compounds we have a different type of compound that we're going to name we also need to name our covalent compounds sometimes they go by the term molecular compounds a similar set of rules which is both good and bad it's similar because it helps in memorizing them it's a little bit bad in that it's easy to mix up am i dealing with ionic or am i dealing with these covalent molecular compounds so here are the rules we're going to apply we're going to name the first element keep the name of the element we're going to name the second element replace the ending with ide third rule is an always rule we will always use a prefix to describe how many of the second element is present then the fourth rule if there's more than one of the first element we'll end up using a prefix something to notice about this nowhere on here are we talking about ions and the reason we're not talking about ions is because four covalent molecular compounds ions don't exist so we really have to make that clear remember no ions for these we're not looking at ions we're not looking at charges we saw this slide before we dealt with the ionic compounds in the first part of this video now we're looking at the covalent or the molecular compounds key things here no ions you know we spent pretty much almost an hour talking about ionic compounds and ions now you have to get rid of all of that you can't think about ions at all because there are no ions present what's holding these compounds together what's holding the pieces together is a sharing of the electrons you're going to be able able to identify these covalent molecular compounds from the ionic compounds again based upon what elements are in play your key thing that you're looking at here two non-metals bonded together that's your clue that you're dealing with these covalent molecular compounds let's look at some examples i'm going to do these kind of side by side first off do we recognize that we're dealing with molecular compounds now instead of ionic compounds the answer should be yes we can see that based upon the elements that are involved i look at the example on the left carbon and oxygen both non-metals the example on the right carbon and oxygen both non-metals so i kind of want to do these two in parallel so you can see we're always going through the first three rules regardless so let's go ahead and do this first off molecular compound two non-metals what does that mean that means that we are not writing ions do not write ions here because they don't exist there are no ions in these compounds so what's the rule say well the rule says name the first element keep the name of the element so the first element is carbon on the example on the left and if we look at the example on the right the first element is carbon make that a little bit smaller just make sure we have room to get in the second part next rule says name the second element replace the ending with ide well the second element in each case is oxygen so we're going to chop off the ending replace that ending with ide so the oxygen is going to become oxide for the example on the left for the example on the right same thing oxide right now you're going to notice that we have a problem just using the first two rules we have the same name for both of these compounds that's a big big problem we have to have a way of differentiating these these are very very different compounds so different that one of them the compound on the left is required for life co2 the compound on the right is very very deadly and a dangerous poison we definitely want to be able to differentiate between these two and not mix them up that's where rule 3 comes in or step 3 comes in that's not always rule you will always use a prefix for the second element just kind of quickly go through our prefixes prefix for one is mono two die three try four tetra five penta six hexa seven hepta eight octa nine nona ten deca so the example on the left how many oxygens do i have well i have two oxygens on the left the prefix for two is dye this becomes what you're very familiar with compound on the left is carbon dioxide now if we look at the compound on the right compound on the right we have one oxygen the prefix for one is mono and this is just an english sling you end up chopping out one of the o's you leave the other o in there it makes the word sound weird so carbon monoxide your co detector in your home your carbon monoxide detector to alert you if any of this compound is present so we now have a way of differentiating it and we have that way because of that third rule that always rule that rule that says you will always use a prefix to describe how many of that second element you have present we look at another example of a molecular compound p2s5 again we look at the elements involved we look at phosphorus when we're looking at phosphorus it's a non-metal we're looking at sulfur it's a non-metal so we have two non-metals here that's what's telling us that we're dealing with a molecular compound so we're going to go through and we're going to name this remember no ions in place here we're not writing out any ions write the name of the first element keep the name then the first element is phosphorus second element sulfur replace the ending with the ide that's going to make that sulfide second element always gets a prefix so for sulfur we have five of them so the prefix for five is penta now the first element will only get a prefix if there's more than one if you look back at the previous examples we did with the carbon monoxide carbon dioxide we only had one of the first element we didn't use a prefix here for that first element that phosphorus we have more than one of them we have two of the phosphorus there when you have more than one you have to use a prefix so the prefix for two is dye so here you're looking at diphosphorous pentasulfide so the second element always gets a prefix no matter what the first element will only get a prefix if you have more than one of them so in this example just like before the ionic compounds we want to be able to go both ways formula to name name to formula so here's an example going that opposite direction starting with the name having us write out what that formula is so the name we're looking at is dinitrogen tetrabromide first are we really dealing with a molecular compound well when you look at nitrogen on the periodic table it's a non-metal when you look at bromine on the periodic table that's also a non-metal so we have our two non-metals in place so we're definitely dealing with a covalent compound a molecular compound so we're not going to write ions but first element that we have is nitrogen how many of them do we have well we're going to have two of them we figured that out from the dye prefix out in front second element bromide well that's really from bromine how many of them do we have well the tetra in front tells us that tetra is a prefix or four so our formula will be n2 br4 dinitrogen tetrabromide two nitrogens four bromines