so as we delve into our study of chemistry we do have to look a little bit at the history how did this come about how was this all developed so we're going to delve just briefly into historical atomic theory there are some excellent videos uploaded to your online learning management system that you guys can go through and look they're labeled historical atomic theory i highly recommend you check these videos out they will give you a better description than i can give on how these experiments themselves actually worked they'll be able to show you a better model for it and actually show you animation etc and just overall give you a better again visible description of what's happening but let's go ahead and just talk about some of the things that did happen and some of the important people that factored into it so first we've got democritus 460 to 370 bc and he figured out that everything is composed of atoms he believed that atoms were indestructible and indivisible now we've since figured out that we can break an atom down into its protons neutrons and electrons but at this point even just knowing that everything was composed of atoms was huge that was a huge breakthrough in the world of science next up we're going to talk briefly about robert boyle we will come back to boyle's name later on in the semester he is known for boyle's law it's a gas law and that's what he's most famous for but boyle helped actually develop the definition of an element and was able to kind of um for four supposed elements actually show people they weren't actually a different element than we already had so he really helped bring about this definition of what is an element how does that correlate to what we know about atoms and about the matter around us boyle also really emphasized the importance of experimentation and empirical observation hence the book the skeptical chemist 1661. he before this time based on aristotle and other philosophers from the time a theory would be developed an experiment that was done would be matched to a theory boyle really pushed against this and found that he would much rather do an experiment and then use theory to back up the experiment develop the theory based on the experiment not matching experiment to theory and it really broke through what we know today even today we have a lot of people that do a lot of modeling a lot of computation a lot of what we would consider to be more theoretical work but experiment always trumps or wins the game we have to use experiment to figure out what's truly truly happening we can use the knowledge base we've built up based on other things we've learned to help better identify what's likely what's not but experiment is still what truly wins and so boyle was the first one to really delve into these quantitative experiments and developing an experimental definition of that element we have antoine lavoisier and he is known for the law of conservation of mass he figured out that mass was actually conserved in a chemical reaction so the total mass of the reactants equals the total mass of the products we can change what we have what species we have we can create different compounds different molecules but overall mass will be conserved he did this he also really delved into combustion reactions and a lot of his work was discovered because of his work and um kind of his experimentation and his um work within the work of combustion so we will talk about combustion a lot as a recurring theme throughout this semester and so he did a lot of the initial combustion work that we now take advantage of the knowledge for today next we have joseph prost and he is known for the law of definite proportions he found that substances have the same makeup regardless of how it was made so it doesn't matter what i make it has the same composition or the same makeup the same sub the same pieces go together in the same ratios to make whatever substance i want so hence sodium chloride kind of my go-to example all the time sodium chloride it doesn't matter how you made the sodium chloride it's always going to have the same percentage of sodium the same percentage of chloride within the overall compound and john dalton john dalton was known for the law of multiple proportions he also has dalton's atomic theory so we're going to delve a little more into doc into dalton in just a minute but in john dalton figured out that all atoms in a given element are identical so if i have a single element all of those atoms are identical now we now know that's not 100 true because we can have isotopes but the number of protons within an atom will be identical for that element atoms of different elements vary in mass and size that is correct they vary in mass and size because they have a different number of protons neutrons and electrons and again multiple proportions just that reactants will always combine in a set of whole number ratios the next thing i want to discuss is this infograph um i really like this com um this company or this group compound interest it's a guy that makes some really nice graphics that just kind of explain things better for students to see or for everyone to see we love them as chemists they break down the concepts in a simple format and they make them visually appealing to see so that's where this is coming from actually absolutely giving credit to compound interest they don't they put them out as open resource letting us use them so uh i'm very excited about that i love the way this is broken down it's just an easier way for you to see but here we're going to talk about dalton a little bit more thompson rutherford boer and schrodinger so back to dalton we just talked about how he really came about with this law of multiple proportions and that is correct but he drew upon this idea of atoms um in his theory so he was thinking about you know what is an atom and his theory his theory specifically stated that atoms were indivisible those of a given element are identical and the compounds are combinations of different types of atoms the positives of his theory was that it recognized that atoms of a particular element differ from other elements that is correct the drawback to his theory was that we now know that atoms are actually divisible we can actually break them down smaller because they are composed of subatomic particles they're composed of protons neutrons and electrons and atoms the smallest piece we can have where it still behaves as an element but we can break that down farther next we have thompson who is known for the plum pudding model often times you'll hear this called as the choc chocolate chip cookie model um i don't actually know what plum pudding is apparently if you know what plum pudding is this makes more sense to you i just know this as the plum pudding model but i think of it like a chocolate chip cookie he discovered electrons so he discovered these electrons in these atoms and so he actually developed what we call again this plum pudding model or think about a chocolate chip cookie where you've got this mass this whole atom and it shows that the atom is composed of electrons that are completely scattered throughout a spherical cloud of positive charge so he believed that the positive charge was just kind of an entire cloud and the electrons were scattered throughout so think about a chocolate chip cookie with a um just cookie the entire cookie itself with all the chocolate chips incorporated in is your positive charge your protons your neutrons and they didn't really know about neutrons at that point your protons are causing the positive charge but it's just kind of dispersed throughout and then your electrons are randomly set throughout that entire atom the positive part of his theory is that it did recognize that electrons are components of an atom he did discover electrons the negative point of his or drawback to his theory was that they didn't have a nucleus and so at the time his theory actu absolutely explained what we knew but going forward his this lack of a nucleus did not explain experimental observations that later were found then comes rutherford there's apps i know there's a video about rutherford's gold foil experiment i highly recommend you check it out but rutherford took a thin sheet of gold foil and he fired positively charged alpha particles at this gold foil what he found was that most of these alpha particles pass through with very little deflection meaning they just passed right through the sheet nothing obstructed their path it was straight through and through but every so often these alpha particles would be deflected at large angles this was only possible if the atom was mostly empty space so those alpha particles were mostly passing through that empty space but every so often they would hit a central concentrated area that had a lot of positive charge which is what rejected those alpha particles alpha particles have a positive um alpha particles were rejected by that positive charge and kicked off at a large angle so rutherford was able to develop this concept of a center positively charged nucleus advantages to his theory is that we did realize the positive charge was actually localized in your nucleus we no longer had this um a full plum pudding model but it didn't explain why the electrons remain in orbit around the nucleus niels bohr comes along 1913 he modifies rutherford's model a little bit by stating that electrons move around your nucleus in orbits of fixed sizes and energies the electron energy was quantized meaning we could measure the amount of energy and electrons could not occupy values of energy between fixed energy levels so niels bohr really brought about this idea of different energy levels that these electrons reside in so the positive state's theory was that it did propose um stable electron orbits and they explained emission spectra of some elements we are going to talk about emission spectrum more in chapter eight when we talk about the quantum mechanical model but if you've ever been driving down the street and you see like a restaurant or any store at all that has like an open sign and it's lit up really bright looks kind of fluorescent to you that's an emission it's emitting a certain color of light based on the gas that's inside of it the elemental gas that's inside the disadvantage to bohr's model was that he he thought the moving electrons should emit energy and collapse into the nucleus this model did not work well for heavier atoms along comes schroedinger and schrodinger stated that electrons do not move in set paths around the nucleus as previously believed by bohr but rather they move by wave pattern and what we're going to discover when we talk we're discuss shorting germore in chapter eight we're going to discover there is that electrons can behave as both particles and waves and there's very there's differences why we have to treat a part electron as a particle but also a wave so schrodinger decide to said that it is impossible to know the exact location of an electron instead we have what we call clouds of probability which we call orbitals meaning we're most likely to find an electron in this area and so the advantage to schrodinger's method is that it does show that electrons do not move around them nucleus and orbits but rather in clouds where their position is uncertain we can never know exactly where an electron is going and how fast it's going we can know one or the other short insurance method is still the most widely accepted as it is the most accurate model of the atom that we have so let's go ahead and talk about the law of definite proportions or fixed mass proportions basically what the law of definite proportions showed us was that it doesn't matter what size sample we have we will have the same ratio of one element to the other so let's go ahead and look at sodium chlorine or sodium chloride a 100 gram sample of sodium chloride contains 39.3 grams of sodium and 60.7 grams of chlorine so the mass of the chlorine to the mass of the chloride we see this is a ratio of 1.54 so we've got 1.54 more chloride ions than sodium if i have a 200 gram sample of sodium chloride this has 78.6 grams of the sodium ions and 121.4 grams of the chlorine or technically chloride ion we'll talk about the difference of that in this chapter actually but if i take the difference here 121.4 divided by 78.6 i again get this ratio of 1.54 a 58.44 gram sample of sodium chloride it's a weird number um you're going to figure out why at the end of this chapter that number is actually specific and why you picked that number it's known as the molar mass or formal formula mass of a sodium chloride formula unit but in this case i would have 22.99 grams of sodium and 35.45 grams of chloride if i take that ratio again i see a ratio of 1.54 so the law of definite proportions shows us that it doesn't matter what size sample i have sodium chloride always has the same ratio it has the same percentage of sodium ions to chloride ions within the sample no matter if i have a 100 gram sample a 200 gram sample a 1 000 gram sample it does not matter it will be the same ratio next we have the law of multiple proportions so the law of multiple proportions shows us that there is a whole number relationship between these molecules or compounds that are formed but we can actually form them with different ratios so carbon can be combined with oxygen to form two different substances carbon dioxide and carbon monoxide why do we care well because one we breathe out really easily the other one not so well and the other one can kill us unfortunately so this is carbon dioxide this is called the space filling model it shows you the space of the atoms comparative to each other and this would be carbon monoxide so the red atom there is an oxygen atom the black is carbon and we see that the carbon is actually a little bit bigger in size than oxygen that is correct we'll talk about that in chapter nine about size of the atoms we find is that again carbon combined with oxygen to form two different things carbon monoxide contains 1.33 grams of oxygen for every one gram of carbon so its ratio is 1.33 if i take oxygen divided by carbon co2 on the other hand contains 2.67 grams of oxygen for every 1 gram of carbon if i take these ratios 2.67 divided by 1.33 i see i've got a whole number here of 2. so there is a whole number relationship between the oxygen masses this must be significant dalton developed the atomic theory to help explain this law of multiple proportions i hope this helps when reviewing atomic theory this is kind of the background of atomic theory we use law of multiple proportions and law of fixed proportions kind of intuitively in the back of our minds and we often mention the law of conservation of mass throughout this entire course so please watch the videos for the historical atomic theory and the different animations different simulations none of them are very long videos but i think they'll help you a lot while picking it up keep these two laws in the back of your mind meaning law of multiple portions and law fixed proportions and remember the law of conservation of mass is going to factor into the rest of your general chemistry courses in gen chem 1 and gen chem 2 we're going to see this concept of law of conservation of mass coming back into play throughout the entire course it will actually benefit us a lot when we're just not sure how to approach a problem we think about law of conservation of math and it will help us get us back on the right track