[Music] hey everybody welcome to chapter 5's video lecture we're going to be going through all things uh molecules and compounds so that's going to be formulas we're going to talk about molecular compounds ionic compounds there's a lot to this chapter it's almost like picking up an introductory lesson in a new language so buckle up let's start out by thinking of two different elements we're going to start out by thinking of elemental sodium which just means sodium by itself and chlorine elemental chlorine now chlorine is a poisonous very reactive gas it shows up on the upper right side of the table we're looking at a non-metal now if we look at sodium it looks by these little marks that they cut this sodium brick with a butter knife which you'd be surprised even though sodium is a metal you can actually cut sodium with a butter knife it's relatively soft um but it does retain all those properties of metals and it's very reactive now you take this metal sodium you take this poisonous gas cl which we'll learn later shows up as a diatomic meaning there are actually two per molecule you take those and you put them together we get something radically different we get table salt so this table salt isn't poisonous we eat it on a daily basis it's a combination of n a and c l in a ratio of one to one it's a compound meaning that if it's a compound it's got two plus or two or more elements and those elements are bonded those atoms are going to be bonded together now the big take-home very different properties in the ingredients the reactants that went into making nacl when we get nacl though something completely different when we form the compound so big take home here the properties of the compound are going to be different from the properties of the elements that made it up all right just a few general guidelines here so most of what we come across in nature not elements we're looking at compounds we're going to be focusing on reactions going into the next three to four chapters what we excuse me what we don't see a lot are free atoms that are just off on their own in nature we see reactivity uh the exception to their that reactivity would be our noble gases let's pretend i spelled noble right there a compound is different from a mixture so when we looked at nacl this is an n a bound to a c l so they're connected this isn't like a mixture of sugar and rice if we had a mixture of sugar and rice we'd have these little rice particles in here and we would have some sugar they're not really connected they're all independent little particles that can flow when i look at nacl the nas are connected to the cls they are bonded so a mixture does contain two or more substances but a mixture doesn't have those independent substances bonded to one another chemically bonded now over here nacl two or more elements it is chemically bonded that's going to be where we start to talk about compounds okay so compounds are unique they're different from a mixture of elements because we're going to have to start to look at fixed definite ratios or fixed definite proportions when i say h2o you think water if i said h4o you're not thinking that same association because it's not actually a real compound but it's also not what we would associate generally with water so we have to look at fixed or determined regular proportions for a particular compound so in a mixture we can have any proportion if i had sugar water i could have a solution of 10 sugar water that would be bojangle's iced tea i can have a solution of 40 sugar water and that would be my grandma's iced tea so the sugar in the water because it's a mixture it's not a compound i can vary the proportions i can vary the relative amounts but if i looked at nacl it's always 1n a for 1cl i can't change the proportion like i can for mixture okay so let's look at a mixture of gases so this gas or this balloon is going to be filled with a mixture of hydrogen and oxygen gas so i'm going to have h2 in here i'm going to have o2 in here because i've got hydrogen gas and oxygen gas now this is a form of the element oxygen this is a form of the element hydrogen now i could change the amounts of hydrogen and oxygen that show up in this balloon i could have 50 percent of each or i could change that and i could go 60 h2 and 40 percent o2 and that's because these are two different substances we're not looking at a compound we're looking at two different substances being mixed up in this balloon so i can change the proportions see how it has two separate formulas completely and because i have these two complete separate formulas and two separate substances we can vary the amounts now in the balloon that's over here on the right i've got h2o it's a water balloon now we said that compounds contain two or more elements in this case i've got hydrogen and oxygen there's my two or more elements and if we take one of these guys in here see how there are a lot of independent little groups in here well what we've got this white sphere is going to be hydrogen that's attached chemically it's bonded to an oxygen which is also attached to another white little sphere so that's another hydrogen and that's our h2 two hydrogens o so there's our water molecule now we can't change the ratio here of oxygen to hydrogen each one of these if you look at it in the picture each one of these is the same each one of those molecules has one oxygen two hydrogens you change the ratio you change the compound the substance that we would be looking at okay so now we've got a law for this so we've got the law of constant composition so it says all samples of a given compound have the same proportions of their constituent elements so what we would say here is same compound so all samples of a given compound that's what this part means right here has the same proportions has the same ratio of elements okay so that's one way we can start to define chemical compounds now let's look at mass ratios this is one way that we can quantify this or put numbers to it just for a moment okay so we're going to take an 18 gram sample of water we said water is h2o and we're going to decompose it when we learn our reaction takes later this is going to mean break down so we're going to break down 18 grams of water and we're going to get 16 grams of oxygen and 2 grams of hydrogen now that makes sense 16 and 2 grams makes up that 18 grams of water that we started with now if we're trying to calculate an oxygen to hydrogen mass ratio that means that we're going to put the mass of oxygen on top so it's the mass of oxygen to whatever goes on the bottom in this case oxygen to hydrogen so we'll put the mass of hydrogen on the bottom so this would give us 16 grams of oxygen which is what they gave us in the problem here and 2 grams of hydrogen would go on the bottom exactly they gave us in the problem here now if they had asked us for a hydrogen to oxygen mass ratio instead what they list first should go on top so if they had asked for this instead we would have put 2 grams on the top we would have put the 16 grams of o on the bottom and 2 over 16 works out to 0.125 now if you get a decimal it's fine if you get a whole number perfectly fine so don't let an odd number throw you off it's okay if they're not pretty okay let's try another one of these if you would like you can pause the video try this on your own and then we can work through it together this is very similar to the problem that we just worked through so go ahead and pause if you'd like and we'll go over it as a team okay now it says we're doing mass ratios we've got a 17 gram sample of ammonia i don't have to know what ammonia is but it says ammonia is made up of two things it's made up of nitrogen and hydrogen and it gives us relative amounts it says that we would get 14 grams of nitrogen if we broke this down so i'm going to say 14 grams and it says we get 3 grams of hydrogen and it says it wants us to find a nitrogen to hydrogen mass ratio so remember what whatever's listed first goes on the top so we would have 14 to hydrogen ratio whatever is listed second goes on the bottom those grams cancel so this is a unitless number and it's going to give us a total of okay about 4.67 get my pin back here which take home here while the atoms are combining in whole number ratios we actually have a formula for ammonia that works out to nh3 these mass ratios that we're calculating we can get decimal values we're not necessarily going to look at whole numbers because atoms combine in whole number ratios but mass can work out not quite as pretty when you work your calculations okay so how to represent these compounds compounds have constant composition we mentioned h2o water always has two h's one o and these chemical formulas are going to tell us what elements are present and it's also going to tell us how many so it's good to keep one that's familiar in your head like water because you can refresh yourself on the rules now when i look at the structure of water i've only got one of these reds that's my oxygen now notice how here i don't have any number that subscript is missing it's not because we forgot it it's because it's one so really the formula for water is h2o1 but nobody says the one if it's just one we just assume that it's there we assume it's one if nothing's present okay these two little guys down here these must be my h's and that's where we look for constant composition every molecule of water is going to have this ratio of two hydrogens for one oxygen okay so let's roll through a few of our common chemical formulas if we start here at the top we've got nacl now for nacl if i was able to just break this cube down bit by bit and it is a cube shape i'm going to trace the different faces here we're going to learn later in the lecture this is a salt or an ionic compound that forms this type of cubic structure now in nacl we mentioned in the formula that it's a one-to-one ratio of na to cl we don't have any numbers down here because when we don't see numbers we assume that it's one for co2 we've got a one to two ratio meaning for every one carbon that i have i've got two oxygens that show up in the molecule now it's important that we look at constant composition a small shift and the chemical formula has a very big difference in the chemical formula that we look at so co2 is something that we breathe out if but if we're around the second compound any guesses what the second compound is if this is co2 then this would just be co so instead of carbon dioxide with this one c to two o's ratio we would have carbon monoxide which hopefully you have a detector at the house for we're looking at a poisonous gas so very big change in the properties just by one very gentle shift in the ratio okay now at the bottom here we're looking at a molecule of sucrose this is table sugar and these aren't always small simple ratios here we've got a 12 to 22 to 11 ratio much more complex formula when we get into biologically active biomolecules proteins we can look at very wild ratios these are huge molecules in some cases so they're not always simple ratios now what order do we list them now there's a reason why we have co2 and not o2c and that's because in chemical formulas we list the most metallic element first so what that means is on the periodic table the further left we go and the further down we go things become more metallic so more metallic elements get listed first in the formula so if i look at carbon versus oxygen carbon is further left on the periodic table so it's more metallic so it got listed first in co2 so let's try a couple of examples here it says how would we write a compound that formed between one sulfur and one calcium so let's write these down i've got sulfur i've got calcium now if i look these up on the periodic table calcium shows up over here sulfur shows up over here so i've got calcium which is definitely further left and if it's further left that means it's going to be more metallic which makes sense because the whole left side of the periodic table metals so i'm going to list calcium first i'm going to list sulfur next now let's just focus on the order that they show up in for now we'll worry about refining these formulas later but just when we're focusing on order we want to put the more metallic element left the less metallic element on the right so let's try a different one how would we write a compound formed between two oxygen atoms and one nitrogen atom okay so we've got oxygen to play with and we've got nitrogen to play with so let's look at the table if we look at a little snapshot if we blew up the table then i would have the element showing up in this order and we could say the n is further left and being further left n is more metallic it says that i have one nitrogen atom so i'm just going to write n i can assume that one's there it says two oxygen atoms so i'm going to write o and when i write this little subscript that too only applies to what it's directly next to so that means two oxygen atoms and we assume that there's a one there for my nitrogen so that would be our formula all right so sometimes we have polyatomic ions let's break down the word poly means many atoms that have a charge so polyatomic ion just means many atoms that carry a charge together a very common one that you're going to have is nitrate you're going to hear that a lot until the end of the course and we have let's look on our chart we have a common polyatomic ion chart down here you will have this on exams i am going to give you a list to memorize but that's more for your own good you keep going in biology you keep going in chemistry engineering health sciences you will see polyatomics and the list that i've given you on blackboard alongside the notes good list to carry you through to future classes now like i said it will give you this chart on exams though let's read through nitrate right here if i read across is no3 minus now what that means is in a nitrate polyatomic ion i've got it doesn't show it but i've got one nitrogen atom i've got three oxygen atoms and then together by that negative charge it means that this entire collection of bonded atoms are going to carry a negative charge so they all share that negative charge they carry it together what that looks like structurally this blue center is our nitrogen our oxygens are the red spheres on the outside they're all bonded together and that negative charge that collection of electron that is shared between these atoms that show up in the polyatomic ions so generally whoops generally my pen is spazzing out a little bit we'll have that negative charge uh dispersed and shared across those four atoms okay so let's take a look at how this plays into a formula now it's going to turn out that in formulas involving polyatomics or ions period charges we're going to want some charge balance so when i look at mgno32 we'll do a lot more of this later but think about nitrate nitrate it said on our chart is no3 with a minus charge now let's say that i've got two of these if i have two of these then that's going to equal negative 2 total charge from my nitrates so if i drew a little divider here that means that over here because i have two of these nitrates that tells me i have negative two charge over here now when i look at my magnesium i've only got one magnesium so what has to be the charge on magnesium to balance that out well that one magnesium has to carry a positive two charge to be able to balance that out so here i've got a magnesium that has a two plus charge and whenever we want to indicate charge we write it in the upper right so this would be a magnesium two plus and then here because these act as a whole unit they collectively act it's a polyatomic ion we're not going to break this into individual atoms we're just going to say that i have 2 and when i want to say i have 2 of something i'm going to put it out front i have 2 of these no3 minuses now if i wanted to go down to individual atoms i could say that i've got two times one it's going to multiply through and where i have two n atoms or two times three that would tell me i have six o atoms now we're going to have three types of formulas we're going to have empirical formulas molecular formulas and structural formulas empirical formulas just tell you relative numbers it's the simplest ratio so let's leave ourselves a newt simplest ratio is our empirical formula a molecular formula tells you the actual not the ratio but the actual number of atoms of each element that show up in the compound so this is not simplified when we're looking at a molecular formula so let's look at h2o2 if i look at h2o2 it's two h's for every two o's we could simplify that that could be simplified to one to one ratio so my empirical formula would just be ho because it's a simplified ratio my molecular formula is h2o2 so this is molecular and this is empirical let's look at glucose for glucose we've got a 6 to 12 to 6 ratio well these are all divisible by 6. so this is the molecular formula because it's not simplified but i could simplify this if i divided them all by six this could be c h2o or one c for every two h's for every one o and that would be our empirical formula now we do run into certain cases where the empirical formula is the same as the molecular formula and water is one of those cases h2o can't be simplified and when we have a case where it can't be simplified then molecular formula is going to equal the empirical now the third type of formula we're not going to spend a lot of time with until we get to organic which is going to be our last module but that's where we start to look at connectivity so h2o for example this was its molecular and empirical formula but when we look at a structural formula it shows these lines and these lines represent chemical bonds that hold it together and what they're really made of if you think about the outside of the atom the outside of the atom was a electron cloud these bonds are in the case of molecules shared electrons so every line you see is going to represent two electrons being shared between two different atoms so just to uh go over it again from a 30 000 foot view empirical formulas simplest ratio molecular formula actual not simplified ratio and structural formulas show connectivity the knee bones connected to the hip bone so it shows how everything is interconnected all right so let's get into different representations when we start to look at 3d so how these would appear in three-dimensional space we start to look at molecular models for this we have two common ways to look at molecules we have a ball-and-stick model where we've got these spheres these spheres are atoms so spheres equal atoms these are color-coded so if you look at the list we have here to the left we've been showing h2o as two white spheres two hydrogen spheres with one red sphere one oxygen these are common color uh combinations that you see for each element when you go into biology or health science you'll see these three-dimensional models and oxygen is generally red nitrogen is blue hydrogen is white and carbon is black it's the most common that you're going to see in biology and they do recur so good to keep in mind in future classes all right now going and connecting these when we see lines just as we did in the structural formulas these lines are bonds we said that each one of these bonds represents two electrons being shared between atoms so i have two electrons being shared between these two carbon atoms when i look at this oxygen though if you see it's a little different there's two lines that means four electrons are being shared between this carbon and this oxygen when i move to a space filling model this is going to show more of what we would think of in terms of atoms being surrounded by electron clouds as the atoms come together and bond we're going to see an overlap in those clouds and the overlap is shown in these space filling models okay so just all the different ways that we have of looking at formulas working our way up to the three-dimensional models we've got the molecular formula ch4 which by the way this is c1 h4 can't be simplified and because it can't be simplified it's not just the molecular formula this is also the empirical formula when we move to the structural formula structural formula tells us connectivity so here it told us that there are one carbon for every four hydrogens the structural formula told us that the carbon was at the center and the hydrogens were all connected to the carbon the ball and stick model doesn't show everything at 90 degrees like it does here the ball and stick model gives us a better idea of what it looks like in 3d space which is actually more like 109.5 degrees and you don't have to know that but it's not how it appears so these structures that we write out on paper these structural formulas don't give us a really clear picture what it looks like in three dimensions and then when we go to a space filling model this is a more realistic picture overall of what we're looking at when the atoms come together for bonding especially when we start to look at enzymes and active sites we get a feel for what these sites actually look like when we scale those atoms up so let's start our way through on classifying elements and compounds we're going to start with a term that we've seen before pure substances made up of only one substance all the way through we can break that into two categories we can look at elements which is where we're looking at a block off the periodic table like if i looked at carbon carbon would be representative of just one element now if i look at a compound i start to look at just reminding you the definition we covered two or more elements and it's not just two or more elements they are bonded okay so let's start on the left here we can take elements and break them down again into atomic elements which these are just single atoms for molecular we would be looking at two or more atoms that are bonded together and our examples here an atomic element would be something like our noble gas neon where we had atoms of an element that doesn't play well with others it's not very reactive so it makes sense that neon a noble gas would be a single atom it would be an atomic element but if we look at something like o2 o2 we're going to learn in a few minutes is more reactive it's going to form a die or two diatomic molecule two or more atoms two o atoms in this case they're bonded together these are space filling diagrams so we have a molecular element now we're going to get a lot more into molecular and ionic compounds in the next few minutes so first let's focus on the left two examples here so elements being either atomic or molecular we're going to focus on atomic elements atomic elements being atoms that exist as single atoms in nature now most elements so if we look at an aluminum can an aluminum can is a collection aluminum atoms packed next to one another if we look at molecular elements molecular elements exist in nature as diatomic or even more atoms that are bonded together to form molecules now we said that we if we had two atoms bound together then we were going to have a molecule and these molecular elements could be something like s8 or eight atoms of sulfur bonded together to form a molecule it could be something like o2 two atoms of oxygen bound together to form a molecule now the ones that i want you to know off the top of your head know these super important when we start to write reactions your diatomic elements um now you can abbreviate this as h naught and then your halogens so h you look at where this occurs on the periodic table so h is up here and then n o f c l b r and i so these are all within this l shape here say for the h do know these diatomics flash cards there's no trick really other than seeing that they do fall within this l shape of one another on the table okay so atomic elements if we look at mercury it's a liquid at room temperature which is odd for a metal we've got a collection of single atoms that represent the basic units of liquid mercury if we look at chlorine though chlorine gas we addressed this at the beginning of the lecture when we talked about the formation of sodium chloride sodium chloride is formed in part by chlorine gas which is made of cl2 now cl2 tells us that there's not just one cl but there's another they're bonded together and each one of these represents a molecule because it's two or more atoms bound together so the basic unit of that substance isn't single free-floating atoms like it would be for mercury where i just have one mercury atom there i've got multiple atoms showing up in a molecule so if i start to look at compounds not at elements but at compounds we were able to divide those into two different areas let's go back to our flow chart compounds two or more elements bonded together we started to look at two flavors we could either have molecular or we could have ionic so the way we're going to start breaking this up is by what they're composed of all nonmetals i'm going to abbreviate that nm for molecular compounds and frionic we will have metal plus a nonmetal it'll be a combination or we're going to have polyatomic show up in the mix so let's jump in some molecular compounds formed from two or more non-metals now remember these are molecular which it does mean that we need two or more atoms because it's going to be a compound it's got to be of two or more elements and these are going to be bonded so they're going to form molecules in this case we had co2 we had carbon that was our first element we had oxygen that was our second element so a combination of two or more elements and a total of one two three three atoms they're all bound together in the same molecule this would be an example of a molecular compound when i look at dry ice dry ice is made up of co2 so i take these individual little molecules and i pack them in close to one another and then i get out dry ice but the simplest unit the basic unit that builds that dry ice is a molecule of co2 now instead let's switch and let's look at table salt for table salt we said we were looking at nacl and that there was a one-to-one ratio of n a to c l for ionic compounds we've still got to have two or more elements they're still going to be bound together but the difference is we're not going to have these individual molecules that pack together we're going to have our charged particles so for ionic when i think ion i think charges we couldn't have all positives this wouldn't hold together we couldn't have all negatives wouldn't be stable you're going to have a balance of positive and negative charges so we're going to have cations that are positive they're going to balance out with anions that are going to be negative and when you combine them in the right ratio they cancel one another out and we end up with a neutral crystal lattice which is the name of this repeating structure and it's going to be stable because of that charge balance because of that neutrality now we need to know some typical charges we are not memorizing all the charges on the periodic table but memorize these charges these are going to help you while you're figuring out formulas the good news there's a pattern so group one plus one group two plus two notice the noble gases how they're non-reactive no charge group seven minus one minus two minus three these are the trends that i want you to take home the only other two elements you don't really have to know scandium uh zinc is plus two and silver is plus one and really aluminum is plus three these are good charges to have handy they go by group number when we start with the cations cations are positive so one plus for group one two plus for group two and then you can count backwards starting in group 18 is zero 17 is minus one 16 is minus two 15 is minus three and then you've got these three charges that i do want you to know now we're going to use these what we know to figure out what we don't know we're memorizing a small subset that we can use to figure out a lot of other charges okay so in a compound we're in an ionic compound that contains cations and anions cations go first anions go last so something we've already looked at was nacl now because of this rule that means that n a must be positive and c l must be negative and if we look at our chart n a shows up in group one so it carries a plus one charge when it forms an ion cl shows in group 17 or 7a and it carries a negative one charge so if n a carries a plus one and cl carries a minus one that's how they balance out their charge that's why the formula of table salt is just nacl because a one-to-one ratio balances it so let's use what we do know to figure out what we don't know let's look at fecl3 we know that overall we want the charge on this to balance iron shows up as a transition metal and transition metals can take on various charges so i'm not sure whether iron is plus two or plus three but what i do know is that cl is dependable cl has a minus one charge so when i come over here draw a little line each one of these cl's is gonna carry a minus one charge so if i take three of those minus one charges it's gonna give me a total of negative three charge on my anion side which was my negative side so over here on my cation side i have to balance that charge with positive so i know that this one iron has to balance out negative three that means this has to be positive 3 to balance out the charge now the charges cancel so this fe each iron is a plus three charge and we would say that each cl has a negative one charge and that's what we'd have to have for this one to three ratio to have a charge balance or a neutral charge overall so let's figure out how to write formulas and these are going to be ionic formulas because we have metals combined with nonmetals so here i'm looking at calcium and oxygen so i want to combine ca and o now there are a couple of different ways to do this some i prefer more than others now if i look at calcium on the table calcium is in group two so it's going to carry a plus two charge oxygen is in group 16 so it's going to carry a minus two charge so if this is plus two and this is -2 then i could say if i just have one of each of these they'll cancel each other out so my formula for a compound between calcium and oxygen would just be cao and that would be correct now the shortcut which you have to be careful of is to write these charges up here and then you can take that number it and bring it down take that number cross it and bring it down so you're crossing and bringing down the magnitude or size of the charge not the sign so that would tell me the formula would be ca2o2 but for ionic compounds these have to be the simplest ratio so seeing this is a two to two ratio we'd have to simplify it and get our ca oh so the way i prefer for you guys to do this is to look at the charges and then and then use logic if it's a single calcium a single oxygen then they're going to neutralize their charges my positive 2 and my negative 2 and then we can just write out our formula no need to cross and bring down charges okay so let's move on to the second example oops all right magnesium and chloride so mg and cl mg is in group two so it has a plus two charge cl is in group 17 has a minus one charge so just thinking about how many we need of each this single mg has a plus two charge so i'm gonna need more of these chlorides to balance the charge i'm gonna need two of these because two of these cls at minus one a piece is going to give me my negative two total charge that i need to balance out my positive charge for my magnesium so this is going to be mg cl2 or ratio of 1 magnesium to two of my chlorides oops okay now again we can do that with the cross and bring down method that would give us let me erase this so we can go through that process we would take our 2 bring it down we take our 1 we'd bring it down we'd have mg1 cl2 same as the answer we just got now let's try the last one here a little more complicated because we're not just looking at elements off the periodic table now aluminum was plus 3 and phosphate whenever you say ate or ite that clues you in that there's a polyatomic ion so let's shoot back up to the polyatomic ions okay so we're looking for phosphate so if i look through it's right here so if i read across phosphate is po4 3 minus so let's go back to our example phosphate is po4 3 minus and if we just look at the charges here they're equal and opposite so i could just have one of each one of my aluminums and one of my phosphates now i have to be careful here because remember these act as a group so when i have a polyatomic show up in a compound we can wrap that in parentheses so either way would be acceptable here i could write it as alpo4 but i'm going to write it out as al po4 and separate that off in parentheses for me that's a good reminder that i'm dealing with a polyatomic ion the plus 3 from the aluminum cancels out the negative 3 and the phosphate so i'm left with a neutral compound so look at all of these and in all of these in the final formulas that we wrote i did not include charges and that's because if the charges cancel out completely you're not excluding the charges and for a properly written ionic compound we aren't doing anything but making sure it's all neutral so i've left you a summary here that helps you go through the process when you're working through homework problems but basically the same process that we just went through for all three of these examples