Atomic Theory of Matter in Chemistry (Atoms & Molecules) - [1-2-1]

0:01 hello welcome back the title of this lesson is called atomic theory of matter and this is part one so in this lesson uh the purpose of it is to kind of take a historical glance back at what we know about the structure of matter and the good news is you already know a lot of what is already in this lesson because you already were taught when you were young that everything around you is made up of atoms and everybody knows this in the modern age but the purpose of this lesson is to travel back in time to the beginning of the 1800s where when people didn't know that the structure of matter was composed of these tiny little things that we now call atoms and we're going to talk about what was put forth as the atomic theory of matter back then which by and large with a couple of exceptions really carries over into the modern day so

0:47 everything we're talking about even though it was discovered a long time ago is applicable today and then as we go on further in the lessons we're going to zoom into the atom itself and talk more about the structure of the actual atom because i know that you also know that atoms are made up of smaller things protons neutrons and electrons and now in the modern modern age we know that the protons and the neutrons are actually composed of even smaller things called quarks so it's like going down the rabbit hole right you know deeper and deeper down the rabbit hole but for the purpose of chemistry we don't really care about what the proton is made of right what we really care about are the elements and the atoms because those are the things that combine together to give us all of the variety of the substances that we know and we see around us okay

1:33 so um again put yourself back back in uh the frame of mind from someone a couple hundred years ago to 250 years ago you know that there's something called water it's all over the planet and it's refreshing and it's cool and you can pour it and it's a liquid okay and you know that if this uh this water if you leave it out in the sun it will start to disappear it'll evaporate and you think it's in the air but i mean you really don't know but but you suspect it's floating in the air somewhere but it's certainly not something you can pour anymore okay and you also know that you build fires and if you take for instance charcoal which they didn't use charcoal too much back then but if we can use the the um the concept of charcoal which is mostly carbon just carbon and if you burn it then what ends up happening after the

2:18 burning process is over is you have ash left behind and then of course the smoke has gone up into the atmosphere right now all of these different processes people were observing all the time right you can see things changing around you if you put iron outside it's going to rust it's going to change in its luster and it's in it and it's in its appearance and things like this if you burn something it's obviously going to change its appearance into something totally different looking if you burn wood then the ash left behind is completely different looking if you were a scientist and doing experiments you might take something that you think is hydrogen and which is a colorless gas and you would react it with something called oxygen which is another colorless gas and even though you start with two colorless gases when you react those

3:04 together what do you get you get water which is not a colorless gas and it pours and it flows and we drink it all the time so even though hydrogen is not something you drink and oxygen is not something you drink once they are into this new thing called water then it's something that we drink and we need so for the longest time millennia people just thought that the universe was a bunch of different uh uh essentially materials right that water was different than we didn't really know what water was made of is what i'm trying to say it was just a new substance of course there were the theories of earth fire wind and water and and those are the fundamental elements and everybody had their own pet theories but by and large until we came up with an atomic theory of matter all the different substances that you see around you they just look totally

3:50 different from one another okay so as experiments were performed and scientists really started combining things and seeing what was created and produced as a result of that we now call that chemical reactions then they started to deduce how this kind of situation would come about and so now what was put forth by a scientist named john dalton in the early 1800s is what we call the atomic theory of matter and it explains the great variety of things we see around us in terms of building blocks that we now call uh elements or atoms same same word or same meaning different word okay so what i'm going to do is i'm going to read to you i'm not going to write it all down dalton's atomic theory of matter and with the exception of one thing at the end i'll talk to you about basically everything on this list is applicable to what we already now know to be factual okay and

4:36 then after that we're going to talk about a couple of important observations that were studied back back in the day in early science scientific discoveries and we're going to learn about those those are called the law of conservation of mass and a few other things that are all directly uh still true and applicable to chemistry today so dalton's atomic theory you can call this a classical atomic theory again these scientists didn't know anything about the structure of an atom but what was put forth as an explanation for all the things that are around us the different variety of things is instead of of there being an infinite variety of things leaves and trees and gases and liquids and solids and and and water and water vapor and and everything being separate we instead uh it was stead

5:22 proposed by dalton that everything every piece of matter that you can touch is actually made up of a relatively small number of of what they called indivisible elements called atoms now we know that atoms are divisible we can split an atom but for now just put that aside of course we can split the atom but you can't do that with chemistry you have to use nuclear physics and that's another another topic for another day the word atom is meant to signify that it's an indivisible thing in other words if you get down to a single oxygen atom if you start cutting your sample of oxy or carbon let's call it carbon you cut your sample in half and half and half and have eventually get to one carbon atom you can't split a carbon atom anymore because if you do that it won't be carbon anymore it'll be something else

6:07 so it'll be some other nuclear decay or something we'll talk about in the future but these things are called indivisible because they retain the character of the elements okay and we now know the periodic table is about 100 elements of course there's some heavier ones it really goes up to about 118 elements but those are the last ones are radioactive we don't talk about them too much because they're not naturally occurring in nature by and large we say the periodic table is on the order of about 100 elements and everything you see around you is just different combinations of those elements that is the fundamental nugget of atomic atomic theory or the atomic theory of matter is put forth by dalton is that instead of there being a million different varieties of substances just happen to be on earth everything is made of of a finite number of ways in which you can combine these these really and truly

6:53 indivisible things which we call atoms so number one uh matter in the universe is made up of elements which are essentially atoms and these atoms are indivisible that's number one number two all atoms of a given element are identical in uh in their properties so if you have a bucket full of carbon atoms every carbon atom is identical to another carbon atom every oxygen atom is identical to another oxygen atom there is no difference they're completely and totally interchangeable with one another once you have uh down to that level of knowing what kind of atom you have but different atoms of different elements have different properties for instance we know that carbon has a certain mass for each of those atoms and it has a certain boiling point and a melting point all its physical properties what

7:39 it looks like things like that but nitrogen is a totally distinct atom which is different and it has its own properties which are you know its own boiling point a melting point in in its mass and its electrical conductivity and all these other things so all of the carbon atoms are essentially identical all the nitrogen atoms are essentially identical but they are different and distinct from one another now as we go further in the class we're going to talk about what makes them distinct from one another but i'll give you the punch line right now and that is that these two different atoms have different numbers of protons and neutrons and electrons that they are comprised of that is what makes them different right we'll talk about that later but for now just know that you have different atoms and they have different uh characters and that makes them distinct from one another so if you want to use the cartoon the little ball

8:26 analogy here so we have a red atom here right it's not really red but we say that this is an oxygen atom it has a certain mass if you have a bunch of these things together you can measure its boiling point its melting point its conductivity it's flammability it's density all the things that we can measure we can ascribe to this thing which is an oxygen atom then we have a white one which we call a hydrogen atom and of course it's not really white we just paint it white so we can tell the difference here and all of the hydrogen atoms again have all the same properties the density and all the other characteristics and they're all essentially uh uh identical to the other hydrogen atoms and everything in nature is just different combinations of how we can combine all of these different elements on the periodic table the most famous of which of course is water and

9:13 we're going to talk about this a lot later of course we're not done with water but we have oxygen and we have connected to it with these little sticks here we have the two hydrogen atoms so h2o right but hydrogen and oxygen do not have to combine with each other in these ratios in other words two hydrogens for every one oxygen in fact if you stick another oxygen on the end of this thing you'll have h2o2 which is not water that's hydrogen peroxide and of course you could combine these things in other ways and come up with other what we call now called molecules so the fundamental atomic theory of matter is saying that everything around you the difference between h2o hydrogen peroxide and water is basically the amount the number and

10:00 the type of the elements that are all connected together if you could get in there and zoom in on it and look at so number one elements are made up of extremely small atoms number two all atoms of a given element are the same number three different elements have different properties of course we know this and here we are at number four we cannot change atoms into different atoms by chemical reactions people tried to turn lead into gold for many many many years because they wanted to get rich because gold was valuable and they want you know we see things changing all around us so it was perfectly logical you know we see iron rusting for instance right we now know that's combining the iron with the oxygen in the air but people didn't know about the atomic theory back when they were doing alchemy so they were taking a heavy metal called lead and trying to change

10:45 it change it into an another element called gold and that's just not going to work not by chemical reactions okay we now actually can do weird things like this but we have to use nuclear reactions where we rip apart the atom and we change the actual atom's structure by either adding protons or taking them away or whatever so we can do lots of things now but chemical reactions do not change the types of atoms in the chemical reactions chemical reactions we're going to learn later is basically all about the transfer of the electrons which are on the outside of the atom and rearranging the atoms into different combinations and transferring those electrons and rearranging the atoms that's what we do here we cannot change one element into another that is an element or a one of the propositions of the early

11:30 atomic theory number five atoms can neither be created nor destroyed in chemical reactions this is one that took a long time for people to really understand and start to believe so if you believe that everything is made of these fundamental atoms can you destroy one how would you well it turns out that you can't and we call that the law of conservation of mass we're going to talk about it more in a second but in a chemical reaction whatever you start with if you know all of its mass uh and then you do the reaction and you collect everything on the other end and if you measure the mass of everything after the reaction is over then the beginning mass should be exactly equal to the ending mass because if a chemical reaction is just rearranging the atoms it's like different legos if you rearrange the legos then you should

12:16 still have the same number of legos before and after and it turns out that that is true if you're very careful in measuring the mass before and after reaction you'll find that the mass is always the same before and after now things get tricky when you for instance burn a wood in a campfire but if you're careful you see what's going on when you burn the wood is you have the ash left behind but some of that carbon is combining with oxygen and it makes co2 carbon dioxide and also carbon monoxide co and also water h2o and those things float up into the air and they disappear you can see it coming up you know well you can't see the co2 you see the the soot coming up you see that but the co2 and the co and even the water vapor are basically invisible but if you could collect everything floating up from the

13:03 campfire and also collect the ash and we're very very careful about it then you would find that the mass before and the mass after the reaction is over is the same that is another proposition set forth in the atomic theory of matter as as as put forth by dalton all right uh number six uh reactions only rearrange the atoms we talked about that and number seven compounds form when uh atoms unite which we call bonding in a modern language in fixed whole number ratios so this is a biggie this is a real biggie so at first you have to hypothesize that everything is really not different everything is really made up of a small number of little things called atoms but that these atoms can bond together and connect together in

13:48 only hole ratios what that means is if i want to bond you know an oxygen to a hydrogen together like this it has to be one to one or one to two or one to three or one to four or two to four or two to six or one to eight it has to be a whole number of ratios because you have to have an entire oxygen atom or an entire hydrogen atom in order to bond in other words you cannot you cannot cut an atom in half and have the chemical reaction everything is bonding in whole number ratios because the atoms are whole things that cannot be basically split in half now of course we know now that we can split atoms in half but that's not chemistry that's nuclear physics that's not a chemical reaction so we're ignoring that for right now so real quick from the top these are all things that you should kind of know from

14:35 earlier things but we're codifying it into the what we now call the atomic theory of matter number one elements are made of extremely small atoms number two all atoms of a given element are identical number three different atoms of different elements are different of course their properties are all different and distinct from one another number four you cannot change from one atom into another atom by chemical reaction that doesn't happen number five atoms are neither created nor destroyed in chemical reactions number six reactions only rearrange the numbers of the atoms you have that are reacting together like legos and number the last number here seven compounds form when atoms unite in whole number ratios you can't split an oxygen in half it has to be one and then a fixed whole number of

15:21 something else bonding with it that's the atomic theory of matter all right now right on the heels of that as i said lots of experiments were done where they would take certain uh chemicals and we could see or they could see back then that they reacted together and they could collect the products so they didn't really know in the beginning that it was small little individual indivisible atoms that came later but they did a lot of experiments with how much uh mass reacted with how much of something else and how much was produced on the on the uh for the products they did a lot of uh of calculations and collecting of data and so i'm not going to write this one down either because this is so uh i think something that everybody mostly knows but if not it's not that hard to understand we already

16:07 mentioned it it's called the law of conservation of mass and that says that the total mass before the reaction is equal to the total mass after the reaction so if i wanted to say conservation i just said i wouldn't write it down but here i go the conservation of mass we now this no you can write this as a big sentence if you want to i could just say the mass before reaction rxn is reaction is equal to the mass after reaction but you have to be careful because if you're burning something you have to collect all the gases if uh you know you just have to be very careful and

16:52 meticulous but the mass before is always equal to the mass after in a chemical reaction all right so that's dalton's atomic theory of matter uh it's all true with the exception of when it talks about that you can't turn one atom into another that's true you can't do that with chemical means but now that we know that atoms are made up of smaller things we of course we can break atoms up we can add protons we can create some of these elements in the laboratory by shooting protons in an accelerator so we actually can fiddle around with i don't want to say changing one element into another but certainly altering what an atom is by either injecting protons and electrons and doing other things and high energy accelerators but that is not chemistry chemistry is all about electron transfers and rearranging what the atoms are so now what we have to do

17:39 is go and talk about two more laws i'm going to show you what they are we're going to read them and then i'm going to explain they're much easier to understand with an example the first one is the following and again you might think this is obvious and that's just because you were born understanding this from second grade you were learning what atoms were there is something called the law of constant composition all right you might also see this written as the law of definite proportions because it has kind of two different names but it's the law of constant composition it says that all samples of a compound have the same composition which are the ratios of the elements that are present there so what i mean by that is if i have if i take h2o by mass if i meticulously measure how much

18:27 hydrogen is in this water molecule and how much oxygen is in this water fuel by water molecule by mass by the way how would i do that i would take a bucket of water and i would have to break up the water molecule right and we know we can actually do that by passing an electric current through the water molecule and you'll see bubbles forming on one wire and other bubbles forming on the other wire and those bubbles are the hydrogen and the oxygen gas so if you're really careful and you do this experiment without blowing yourself up so don't do it all right don't do it because it's flammable hydrogen is very flammable but if you're careful and you collect the oxygen and the hydrogen what you're gonna and if you suppose that that's coming from the water which is in the liquid form there then what we find when we do that experiment and collect everything and

19:12 measure its mass is the mass percent of hydrogen in water is approximately 11.11 percent and the mass percent of oxygen in water is approximately equal to 89.99 percent right so what this means the law of constant composition means that no matter if i take a bucket full of water from this part of the bucket or from this part of the bucket or from this part of the bucket all water molecules are the same and if i can peer inside and see what they're made of and now we know they're made of hydrogen and oxygen in these ratios then what we find out is

19:58 that if we break it apart and do the experiment we might find that the the mass of the water turns out to be only 11 percent hydrogen in terms of its mass and almost 90 percent oxygen in terms of its mass and no matter how many different samples we take of different different buckets of water we're always going to get the same exact ratio of hydrogen and oxygen by mass that's the law of constant composition and basically just says all samples of the compound have the same composition which is the ratio of the atoms and that is because they're all made of the same atoms and the atoms are all identical and so if we all know they're all made of h2o and all the hydrogens are identical in the universe and all of the oxygens are identical in the universe then we know when we measure the percent mass of any compound no matter where the sample comes from we're going to get the

20:43 same answer all the time constant composition all right now notice that we now know that water is h2o so there's actually two atoms of a hydrogen and one atom for oxygen so there's actually more hydrogen in the atom in terms of the actual atoms there's two atoms of the h's the hydrogen and one of the oxygen so the two to one ratio of hydrogen to oxygen but the mass percentage of hydrogen is really small and the oxygen mass percentage is really really high how can that be how can we have way more hydrogen double the amount of hydrogen atoms but yet have much less percent of the hydrogen by mass well you probably realized it that if you were to go to the periodic table and look for the mass of the hydrogen it is much less it's a

21:29 much less massive atom than oxygen in other words oxygen has pro more protons and neutrons in there that make it more massive so even though the molecule itself only has one oxygen atom and two of these these hydrogens don't have much mass at all so when you add it all up it works out that the hydrogen actually only accounts for about 11 whereas the oxygen counts for much more so these atoms are combining in whole number ratios which we now call molecules but all of these atoms have different masses and so constant composition no matter how many samples you take you're going to get the same ratios by mass or by the number of atoms no matter where you look remember back in you know 200 years ago they couldn't zoom in and look at the atoms i mean we can't even really do that now right we have some images of

22:14 atoms but uh i wouldn't say they're really images they're they're they're more of graphs of showing showing the results of a experiment more than anything it's not like a visible light picture they can't really see atoms so easily i mean you like i said there's some experiments but they're not like color photographs of atoms what we really can do is measure the mass when we break apart water so the constant composition is more related to the mass is going to always give you the same ratios now that's pretty easy to remember the law of conservation of mass says the mass before a reaction is equal to the mass after the law of constant composition just says you're going to get the same percent masses uh by element uh no matter how many different times you do the sample or how or where you get the sample from as long as the material is the same these are fairly easy to understand this one right here

23:02 is the one that is much much more difficult for what more stu most students to understand so we're going to read it several times and then we're going to talk about it with a real example so here it is the law of multiple proportions we're going to read it a few times when two different compounds of the same two different elements are compared the mass of one element that combines with a fixed underlying fixed mass of the other element the second element are in the ratio of small whole numbers now i know what this means but every time i read it it noodles my brain it the words just jumble and it doesn't make a lot of sense so let's read it one more time and then i'm going to show you with an example and you're going to be like oh i got that that makes sense after he writes down an actual example so when two compounds of the same two

23:48 different elements are compared the mass of one element that combines with a fixed mass of the second element are in the ratio of small whole numbers basically what this boils down to is it's essentially telling you that these compounds are always going to combine in ratios like we know that we can get h2o right and we know that we can get h2o2 that's hydrogen peroxide right we know that we can get uh we know that we can get co carbon monoxide we know that we can get co2 carbon dioxide there's many examples i can list here you can change the subscripts you can increase the number of atoms but you can never have co 1.5 you can never have c

24:33 3.72 oh you can and you know that intuitively because you know that there are these things called atoms and they're like legos and they have to combine but back in the day they didn't really know that so this is basically a law that essentially predicts that these elements are combining in whole fixed ratios that's why it's called the law of multiple proportions proportion is related to the word ratio and basically when you boil it down it basically says that when you combine different elements together they're going to combine in whole number ratios and when we do experiments to measure measure the amount of mass in each of the different uh molecules that we create we see that they're related to one another so let's give let's give a good example i think let's give a couple of examples of this okay

25:18 let's take the um the compound carbon monoxide right carbon monoxide is going to be right here and then we're going to say co2 that's carbon dioxide right here and i'm going to put a little squeaky right in the middle so i don't hopefully don't bleed over into one column from another so let's say we do some experiments and we figure out that this is three grams of carbon and we add it to four grams of oxygen that's what we we start out with three grams of carbon and we figure out by reacting it that it only soaks up four grams of oxygen not five not six not seven it only reacts with 4 grams and of course now we know it's because the masses of these things are fixed and the way in which they can react they have to be whole numbers so it can't

26:04 just react with 4.5 grams or 6.72 grams it's only going to join together in certain ratios because the atoms have fixed masses and we can only combine these things these atoms and whole number ratios but they didn't know that at the time so they wrote this law down back when all this was being discovered all right now for the carbon dioxide let's say that we start out with the same three grams of carbon right and because the molecule has an extra oxygen actually what happens is eight grams of oxygen is what reacts to it now it reacts with it now i think this should make sense to you right because we see the only difference between these molecules is there's an extra oxygen atom and since you know that each oxygen atom is the same if you double the number of oxygen atoms you should double the amount of mass that

26:49 reacts with the fixed amount of carbon notice that i started with three grams of carbon in each case to make it easy to compare because these are the same then if you have four grams reacting of oxygen here you should have double the amount eight grams reacting here so here it says when two compounds of the same two elements carbon and oxygen are compared the mass of one element that combines with a fixed mass of the second element notice it's a fixed mass here uh are in a ratio of uh small whole numbers all right so what we figure out after we allow this reaction to occur is that what you get i'll put a little arrow for like an equal sign you get seven grams of co right because you start with law of conservation of mass three plus four is seven you have to

27:36 have seven grams of the product and when you uh let this thing wrap what do you think is going to happen here you should get 11 grams of some something called co2 because you add these together and you get 11. so so far everything makes sense this is kind of what you would expect what we want to do is i want to rename this law in your head instead of calling it the law of multiple proportions i want you to call it in your head ratio of ratios because that's really what you're doing if i had to name it i know it's kind of a silly name also it's not a great name but if it were me and i were trying to be descriptive i would actually call it the ratio of ratios law because this is what we're gonna do what we're going to do is we know how these things what ratio these react in and we know what ratio this other

28:22 compound reacts in i want to figure out i want to write down the ratio here and the ratio here of how they react and i want to compare these ratios together and we're going to find out something interesting so what you want to do is write down this ratio but you always want to to do the top number and divide it by the bottom number because remember when you write down ratios you need to write them down as a fraction that's a ratio so since 4 is bigger i'm going to put 4 grams of oxygen on the top in a ratio with 3 grams of carbon right here and when i take four and divide by three notice i put the units four grams of oxygen is locked in a ratio with three grams of carbon okay so four divided by three comes out to one point three three three now the threes go on forever i'm

29:08 truncating it at the third position here then what are the units i have here i have grams of oxygen per grams of carbon so what this means is that 1.333 grams of oxygen react for every 1 gram of carbon because that's what the units tell me yes it really was four grams reacting with three grams but when you do the division it's like going a hundred miles or a hundred kilometers in 50 seconds yes you actually went 100 kilogram 100 kilometers in in uh 50 seconds but your speed is better when you to really compare everything the same you actually do the division 100 divided by 50 is really two kilometers per uh second now so 100 kilometers this is the same thing

29:55 as 100 kilometers so if you do 100 kilometers in 50 seconds then when you divide this this comes out to two and the unit is kilometers per every one second two kilometers per every one second so we really reacted four grams of oxygen with three grams of carbon that's the ratio but that's the same thing as 1.333 grams of oxygen for every one gram of carbon that's the ratio of the reaction that's going on here okay now let's do the same thing big number divide by small number eight grams of oxygen in ratio with three grams of carbon what is eight divided by three you get 2.667 and the units are still here grams of oxygen per grams of carbon so this one reacts in the ratio of 2.67 grams of oxygen per

30:41 every one gram of carbon so what the law of multiple proportions says is that if we find the ratio that's happening in this reaction and the ratio that's happening in this reaction which we just figured out and we compare these ratios 2.667 compared to 1.333 you're going to find something interesting again we take big number divided by small number so 2.667 and what was the units here it was grams of oxygen per grams of carbon and we're going to put that in ratio or divide with the other one we're comparing two different molecules 1.333 grams of oxygen per grams of carbon now we have the exact same units on the top and the bottom so just like any unit

31:27 conversion they're going to cancel and what do you get when you take 2.667 divided by 1.333 you get exactly two the number two and the number two can be written as a fraction two over one which can be written as a ratio of two in ratio with one what is saying that's why i said i want to rename this thing to the following we call it a ratio of ratios we take the ratio of how this molecule reacts the ratio of how this reacts and then we get those answers and we take a ratio of those and the answer always comes out to a ratio of whole numbers right of small whole numbers so now that we've done this let's read the thing again and see if it makes any sense when two compounds of the same two elements are compared two compounds same

32:13 two elements carbon and oxygen are compared of course the the the subscripts are different but it's the same two elements and we are comparing so we're good up to here the mass of one element that combines with a fixed mass of the second element that is a a wordy way of saying take the ratio of of of the things that are reacting together uh of the second element are in the ratio of small whole numbers so we take these ratios and then we take these two numbers and put them in ratio and we always get a whole number there now that's great but why do we care you got to put yourself in the position of someone who doesn't really know about atoms you take some carbon and you let it react with oxygen you might think that it's always going to react the same way but it doesn't because some of the

33:00 times it reacts to make this and some of the time it reacts to rank this but you don't know that all you know is that you get some mixture of results one of which is looks different than the other and when you actually measure the the the masses and the composition of what you get is the products you figure out that sometimes it reacts in a ratio of three grams of carbon to four grams of oxygen to make some product and then sometimes it reacts with three grams of carbon with eight grams of oxygen to make some kind of product and you know that these must be different substances somehow but you're not totally sure about the atomic nature of what's going on so what you find out is okay if you were really going to study this for the first time and try to figure out the atomic theory what you would do is you would look and see how they compared how do they react together

33:45 how much of carbon reacts with how much of the oxygen in terms of mass and grams of what you mix together to react that's what you would do you would try to see how they react and you would say that the ratio of this reaction is 1.33 grams of oxygen to one gram of carbon but sometimes you mix it together and it mixes and it reacts in a ratio of 2.66 grams of oxygen to carbon and you scratch your head and you say why is it different why is it different what could possibly because you're assuming that it's always making the same product the whole time but then you realize every time you do this reaction an experiment the ratio you get here is exactly double the ratio here and you see that because when you divide them you always get a number of two and then you do the same exact reaction or the same exact procedure with something else maybe nitrogen combining with oxygen

34:32 so after you get this interesting result you try to do the same thing with maybe two other different elements that make multiple different compounds and you find out that this result occurs and happens repeatably and reliably all the time that whenever you figure out how two things combine in terms of their mass and then combine find out how they combine uh in a different way to make a different substance and you take these ratios you always find that this ratio is related to this one by a ratio of whole numbers in other words it's not random ratios you get they're related to each other if you were going to use that to formulate an atomic theory which they did what would it cause you to think it would probably cause you to think that the reason that the different substances that are created are basically made of building blocks that can only combine

35:19 in certain ways because what this thing is really telling you when you get a ratio of 2 here 2 to 1 where this is the number over here and this is the number here the units are grams of oxygen compared to grams of carbon grams of oxygen compared to grams of carbon what it's telling you is that there's twice as much oxygen per gram of carbon in this molecule i'm gonna say that again because the ratio is two to one it means there is exactly twice as much oxygen as compared to carbon in this molecule compared to this one and the way that that happens is if the oxygen can only come in discrete chunks which we now call atoms oxygen cannot be added to the molecule just any which way you want you can't just add a fraction of an oxygen uh a substance to it it only comes in

36:04 chunks which we now call atoms and that leads to atomic theory that's why this even though it's a little wordy and even though it's a little weird it's always taught in chemistry classes because it basically leads you to conclude that atoms are combining and making these different compounds even though maybe the compounds behave differently and have different masses they're made of the same underlying things in fixed ratios so let's take that before we close out this lesson and do the exact same thing that we just did over here with instead of co2 and co let's take a look at nitrogen combining with oxygen no2 and nitrogen combining with oxygen into o so these are two completely different compounds but they both contain nitrogen

36:50 and they both contain oxygen and they're going to have obviously different masses so we can use the law of multiple proportions let's say that for the no2 what you end up seeing when you do the experiment is that 2.28 grams of oxygen combines with one gram of the nitrogen okay this is the ratio this is the amounts that are reacted together right when you supply one gram of nitrogen this is the only amount of oxygen that reacts with it not 2.5 not 2.7 this is the amount that reacts with one gram of nitrogen and you do the same thing over here you find out that 0.570 grams of oxygen combines with one gram of nitrogen so when you make this substance over here and you again supply one gram of nitrogen it only takes up and reacts with this much oxygen not 0.6

37:36 not 0.8 not 0.9 only this amount okay so if you were going to study how these were related to each other what would you do you'd probably want to figure out the ratio of these two things that are reacting together bigger number divided by smaller numbers so you would say you would say 2.28 grams of oxygen in a ratio with one gram of nitrogen right and then over here you do the same thing and even though i usually say bigger number divided by smaller number we have to write the ratio in the same top and bottom numerator so over here we're going to write 0.570 grams of oxygen in a ratio with one gram of nitrogen now it's this one's easy because there's only a one on the bottom so when you divide it you get 2.28 and the units are here as grams of

38:22 oxygen compared with grams of nitrogen and then over here you're dividing by one so you get 0.570 grams of oxygen per gram of nitrogen so what this is saying is when you react and make this compound it's 2.28 grams of oxygen for every 1 gram of nitrogen that's the ratio and this is 0.570 grams of oxygen for every 1 gram of nitrogen so these are the ratios that we have here what the law of multiple proportions says is that once we have these ratios these ratios themselves when you divide one by the other should be a whole number of fixed ratio of whole numbers all right so what we do then is we come down here and we say all right what is

39:08 what is 2.28 and let's go and put the units grams of oxygen per grams of nitrogen in a ratio with what we have over here 0.570 grams of oxygen per grams of nitrogen so we have this unit canceling completely with this unit so you have a dimensionless result if you take 2.28 and divide by 0.570 you get exactly four and you know that the number four is the same as four over one which is the same as four in ratio with one so what did the law of multiple proportions say says when two compounds of the same two elements are compared the mass of one element that combines with a fixed mass of the second element are in ratio of small whole numbers what it is saying is take the ratios that combined the

39:55 ratio of each of the different molecules that form and then put those two numbers in ratio it's a ratio of ratios here we found the ratio was 1.33 here we found the ratio was 2.667 put those in ratio we get 2 to one and that meant that there was twice as much oxygen as compared to carbon in the second molecule when we do the same exact thing here we take the ratio of these we take the ratio of these and we take these numbers and put them in ratio and we get exactly four to one which means this one has four times the amount of whatever's on top oxygen as compared to nitrogen this means four this molecule has four times the amount of oxygen by mass as it does nitrogen when you compare it to the other molecule now does that make sense

40:41 i mean it made sense here because what we basically said is co2 has double the amount of oxygen as co that makes sense but this one's a little harder to look at what we're claiming is that this molecule has four times the amount of oxygen because oxygen's on the top as compared to this one well it's tricky because you see this one is in with only one nitrogen and this one has two of them so that that makes it a little tricky to compare them what you really need to do is divide this even though we can't break an atom up we can mathematically divide this by two so we can have nitrogen each having a number one here so if you divide this by two uh what you'll get is n1 and then oxygen one half if you divide each number by two oxygen will be one half now i know that you can't cut an oxygen

41:27 atom in half i know that but mathematically you can still divide them and say okay this if this can become a one then this bit can become a one half and since the nitrogens in both cases then would have a one you would have a two here compared to a one half here and then this would be four times bigger than one half one half times four is two so when you put them on equal footing and make this when you divide by two if you make it n1 oxygen to the one half then this molecule is indeed four times more oxygen by mass when it's compared to nitrogen as compared across the different molecules so we covered a lot in this lesson we covered the historical evolution of atomic theory we first just kind of outlined what the

42:13 atomic theory is and it should seem fairly familiar to you because we're all grow up now knowing that everything's made of atoms but once upon a time not too long ago nobody knew atoms existed they just thought there was an infinite variety of amazing different things out there and we didn't really know why they existed and even things that look different from each other we now know um it just depends on how the atoms combine to make their properties different for instance sodium sodium is an element on the periodic table that's a metal and it reacts violently in water okay it actually uh combi it generates a lot of heat and it reacts violently with water chlorine is at room temperature a gas that will kill you but if you put them together you make something called sodium chloride which doesn't it's not a metal at all

42:58 and it's not anything that's going to kill you like chlorine will kill you it's something we eat every day so the properties of the substance change dramatically after we combine them together and that idea that everything is made of a small amount of building blocks really set off the modern age of chemistry the rest of the lessons here we're all just trying to kind of like give you a little practice with how we applied this thing and the three most important are the conservation of mass the mass before the reaction will always be equal to the mass of everything that you collect after the reaction constant composition means if you measure the ratios by mass of any of any sample of a compound you're always going to get the same ratios because all of the co all the molecules in the compound are all basically identical little molecules or or compounds that are

43:44 joined together we now know this because they're joined uh based on individual atoms together and then the constant or the uh law of multiple proportions is basically another way of saying that every all of these uh molecules that we find in nature are just various combinations of these things called atoms or elements same thing right and so that was a lot of a great deal of proof for the theory of atomic atomic theory of matter so i'd like you to watch this probably two times maybe even three times to make sure you understand what is being said here then follow me on to the next lesson we're going to solve a few mathematical problems to get practice with these ideas