Naming Ionic Compounds Binary and Polyatomic

Added: 2026-05-10

[00:00:00]breaking in progress all right guys so we are going to go ahead and start on unit five this is our nomenclature unit this is looking at how we name compounds and write formulas for compounds um there are going to be kind of sets of rules that we have to follow and this might be different depending on if it is an ionic compound or a molecular compound so that's one thing that you're going to have to pay attention to as you go through the unit is what type of compound are we dealing with so we're going to start off with ionic compounds these are what we tend to deal with the most um simply because remember ionic compounds do contain a metal and a non-metal and most of our elements are metals so we tend to deal with ionic more often than anything else so before the science of chemistry was developed how they would name compounds was based off of their properties or what they did so for example n2o would make people laugh whenever they breathed it in so it was referred to as laughing gas um there might be a gas that would make you fall asleep so it might be considered sleeping gas things like that however with performing experiments to show how different compounds can be formed we figured out that there's too many different names and too many different things to memorize so it was almost impossible to memorize all of those common names of compounds so the French chemists developed a new method to name these compounds which is what we use today so we actually name them based off of what they're made of versus what they do so we first want to look at how we name individual ions and we're going to put the ions together to name compounds so we are going to start off with monatomic ions first mono means one so this is a single atom that has a positive or negative charge from either the loss or gain of a valence electron so these are going to be how we name our cations and how we name our anions and remember valence electrons the ones that are bonding are the electrons in that outermost energy level so let's look at the cations first remember these are going to be our metals are going to be our cations and remember these are going to lose electrons to have that positive charge elements in groups 1A 2A and 3A are going to lose electrons to have a positive charge equal to their group number so for example sodium is in that group 1A in our alkali metals it has one valence electron so it will lose one valence electron to have a positive one charge so everything in that First Column will end up with a positive one charge as an ion the column next to it that group 2A where calcium is they have two valence electrons they will lose two electrons to have a positive two charge so everything in that column we'll have a positive two charge on that group 3A is on the other side of the transition metals it has Boron at the top which is kind of a metalloid or non-metal but then underneath it we have aluminum so the metals here are going to lose three electrons to have a positive three charge and that goes for the metals that are in that column so to name cations you simply have the element name written as is and then add ion or cation after it so like the sodium the na plus would simply be a sodium ion or sodium cation calcium would just be a calcium ion so four metals we do not actually change the element name we name the element as is and simply add on or cation after it all right and then our non-metals our anions get to be a little bit more complicated so remember anions are the atoms that gain electrons to have a negative charge these are going to be our non-metals and usually elements in groups eight a and group 4A tendonaut form ions so that's where carbon is and then that is our noble gases um to find the charge of a group a non-metal you can subtract 8 from the group number um select group 7ar halogens you can do 7 minus eight and that whole column will have a negative one charge if you're looking from the right so noble gases we said would not have a charge that would be a charge of like zero the next column would be a negative one the column before that would be a negative two one before that will be negative three and like I said that group 4A where carbon is tends to not form ions because it can be positive or negative um so you can look at which column the non-metal is in to find the charge as well now to name the anion the anion name is not identical to the element name whenever we name an anion we take the stem of the element names the first part of the element name chop off the ending and we change the ending to IDE to show that it's an ion it's like an oxygen ion would have that Ox from oxygen and then the ending of IDE so it would become oxide fluorine would chop off the ending and become fluoride nitrogen chop off the ending and become nitride so cations we named as is anions we chop off the ending and change the ending to IDE all right so let's look at putting this together and actually naming binary compounds so binary compounds by means two so this is simply made of two elements and we have binary ionic compounds and binary molecular compounds so we're just going to look at the ionic ones today so whenever you're naming always double check and make sure the compound is made of one metal and one nonmetal that way you can use your ionic rules and to name any binary ionic compound you have the cation name first so remember the metal named as is followed by the anion name where we change the ending to IDE so for example if we have cs2o CS is cesium so we name that as is o is oxygen then a compound we change it to oxide with that IDE ending so that would be cesium oxide um also for example we have nabr n a is sodium we name it as is BR is bromine so we change the ending to IDE to become bromide so let's look at these two examples together we have srf2 Sr is number 38 on the periodic table so you can locate it it is strontium and remember we named that as is and then f is fluorine that's number nine on the periodic table that way you can find it so fluorine we change the ending to IDE so that would be strontium fluoride and that's the name of that compound I'm looking at cacl2 CA is calcium which is number 20 on the periodic table CL is chlorine so we would change that to chloride so that compound's name is cesium chloride so what tends to be more difficult for people instead of naming tends to be writing the formulas tends to be more difficult um so when we're writing formulas for the binaryonic compounds if you know the name of an ion you can write its formula so its formula is going to be its symbol with its charge for that ion so we're first going to write the symbol of the cation and then the symbol of the anion and what we're going to do is we're going to end up adding subscripts to balance the formula because remember compounds have an overall neutral charge so how we can find those subscripts is we can use the crisscross method so the charge the number of the cation becomes the subscript of the anion the charge number of the anion is going to become the subscript of the cation and so you can see this in the picture on the left you see that the one from lithium goes down to be the subscript of oxygen and you don't have to write the one subscript you can just leave it blank and it's kind of assumed that it's one the 2 from oxide now becomes the subscript for lithium and one thing to note is subscript should be written in the lowest whole number ratio so it should be a one to one ratio instead of like a two to two so if subscripts are divisible by the same number you need to reduce it so you can see this from the picture on the right where we have lead that's a positive four charge oxide that's a negative 2 when they crisscross these we have pb2o4 both of those are divisible by two so we can reduce them divide by two so our actual formula is going to be pv02 what's up so let's do a couple examples and we're not going to do all of these um four times sake but let's do um let's do barium bromide potassium nitride calcium nitride and barium sulfide we'll do those middle four so barium bromide if you find barium it is in the second column it's number 56.

[00:11:52]barium is b a since it is in the second column it does have a positive two charge bromide comes from bromine which is number 35 which has a symbol of br and it's in that group 7A so it has a negative one charge so remember we said we are going to crisscross our charges so that means that the two from barium is going to go down as the subscript of bromine and the one from bromide comes down as the subscript of barium so our formula is ba and you don't have to write the one br and we have that 2 subscript for variant let's look at the next one potassium nitride potassium is number 19 which is K it is in the First Column so it has a positive one charge nitride comes from nitrogen which is number seven it is in group 5A so if you do the 5 minus eight it has a negative three charge and yet again when we crisscross the one from potassium goes down the nitrogen three from nitrogen comes down the potassium so it's going to be K3 n and that balances all of the charges so that it's a neutral compound and for calcium nitride calcium is number 20.

[00:13:32]it is CA it is in the second column so it is a positive two charge nitride we just did it's n and it's in that group 5A so it has an A3 charge so every crisscross C A is going to get the 3 as a subscript from the nitride the nitrogen is going to get the two subscript from calcium let's look at this last one barium sulfide um barium we did up above it's ba and it's in the second concept has a positive two charge sulfide comes from sulfur which is s and it is in group 6A it has a negative 2 charge so when we crisscross we end up with b a 2 S2 notice that our subscripts though are both divisible by two so we would have to divide them by two to reduce it so this would just be b a s for our overall compound and with um ionic compounds we also encounter polyatomic ions we've looked at some of these in unit in our last unit with bonding these were the things that were covalently bonded and had a charge um so polyatomic ions are a tightly bound group of atoms that behave as a unit and carry a charge um so for example sulfate is so4 negative two so it's made of one sulfur and four oxygens that overall has a negative two charge um there actually is a list of these on the back of your reference sheet if you turn to the back page where the Bohr diagram was to the left of the Bohr diagram is a table of common polyatomic ions the names of most of these you'll notice end in either ite or ate and if there's a pair of polyatomic ions that are made up of the same elements the ite is going to Signal the ion that has less oxygen so if you look there's a sulfite and a sulfate the sulfite is the one that has less oxygen and we're going to look at how you name things with polyatomic ions um you're never going to change the name of the polyatomic ion so you would just write the name of that as is so let's do some examples of compounds that have polyatomic ions um naming is pretty straightforward it's just recognizing if there are more than two capital letters you know you have a polyatomic ion in there somewhere but let's look at writing the formulas for these because that's the one that's a little more difficult so we're going to write the formula for the cation and then the formula for the polyatomic ion and crisscross the balance the charges just like we did anything else um so lithium carbonate lithium is number three on the periodic table it is Li is in the First Column so it has a positive one charge carbonate anytime you see that ite or ate look at your polyatomic list carbonate is CO3 with a negative two charge and just like we do with other ionic compounds we are going to crisscross so the two from carbonate is going to go down as the subscript from lithium the one from lithium goes down as the subscript for carbonate now that does not change the formula for carbonate it just tells us that we have one carbonate so we don't do anything to the subscript of that polyatomic we write it as is so that is our final formula for lithium carbonate let's look at potassium sulfate this is going to be treated the same way potassium is number 19 that is K it does have a positive one charge sulfate that ate you know it's on your polyatomic list that is so4 with an a of two charge so this when we crisscross is going to look the same the one from tassium just tells us that we have one sulfate but remember we don't change the subscript but that means we're going to have two potassium so it's going to be K2 so4 where this can be a little bit more difficult is if we should have more than one of those polyatomic ions so our cation or whatever that first one that we're using and we're crisscrossing does not have a charge of one so when we're supposed to have more than one of the polyatomic ions how we signify that is we place parentheses around the polyatomic ion with that subscript on the outside so to show you what this looks like let's look at calcium nitrate calcium is number 20 on the periodic table that's CA it has a positive two charge nitrate the ate you know that's on your polyatomic list that is NO3 and that has negative one charge so whenever we crisscross calcium is the one for nitrate so we don't have to write a subscript there but that 2 from calcium tells us we should have two nitrates if we do not put parentheses and we just put the two there it looks like we have 32 oxygens so we place that nitrate in parentheses showing that we have two of the nitrate ion all together and whenever you are writing these notice that if it is a capital letter make sure that we can tell it's capitalized if it is a lowercase letter in the symbol make sure we can tell it's lowercase you cannot just use all capitals or all lower case pay attention to how it is in the symbol as it's given and this next one ammonium sulfite ammonium is your only positive polyatomic ion it's at the very top of your polyatomic list it has a formula of nh4 plus so that means it has a positive one charge and then sulfite make sure you pick up the right one sulfite is SO3 negative 2.

[00:20:43]so when we crisscross these the ammonium the nh4 gets that subscript of two from thistle oh excuse me that should say SO3 I administered that that gets the subscript of 2 from sulfite and then sulfite the SO3 gets the subscript of one from the ammonium so we don't have to add anything to it but I wanted to bring that one up because that's your only positive polyatomic all of the others are going to be anions looking at the next one magnesium sulfate so magnesium it's number 12 on your periodic table that's Ng it's in the second column so it is a positive two charge sulfate with the ate is so4 with a negative 2 charge so when we crisscross charges magnesium is going to get the 2 from sulfate sulfate is going to get the 2 from magnesium now notice here we crisscross over 2.

[00:22:03]there are two subscripts that are divisible by two now it cannot be the subscript that is part of the polyatomic ion but if we have multiples of that polyatomic ion we can reduce this to just NG so4 so there will end up being one of each of those in that formula so you can reduce it if the subscripted is outside of parentheses but if it is part of the polyatomic ion you cannot reduce it on the flip side if you are naming the compound um you simply State the cation name first then the anion name just as you would for your other ionic compounds but remember you are never going to change the name of a polyatomic so if we had something like um k mno4 notice that there are more than two capital letters so we know there's a polyatomic there so you can look at your polyatomic list and determine that it's going to split kind of down the middle right there so we would name k is potassium so we would name it as is and then we would look at nno4 on our polyatomic list is permanganate so we name it as is and do not change the ending so you are able to get a head start on your homework and start practicing naming these compounds um that have just the metal and the non-metal or they contain polyatomic ions um wherever we get to transition metals we'll deal with those tomorrow we do have to add an extra step in there but I want you to practice with these first before we move on to anything else