hello this is another lecture topic video over the atom now the idea the atom isn't a new one by any stretch of the imagination so if we go all the way back to 5th Century BC in ancient Greece there was a philosopher called a Democritus right he had already started to think about uh the thing that we know as the atom all right but the problem was many of his contemporaries just didn't quite buy into it so it freeze it later so we're talking uh the late 1700s and now we're in England uh where we have John Dalton coming into the scene all right John Dalton was a school teacher so he's very very educated um and in fact he had a classical education he knew about these ancient philosophers and their teachings about the atom all right and so what he did was he revived and essentially formalized the concept of the atom and because of this he's often known as the father of modern day chemistry so through his research what he did is he came up with four postulates and these four postulates essentially led to the atomic theory okay and so what are those four postulates well the first one was each element is made up of tiny particles called atoms now what he said was that these atoms were indivisible in other words they can't be broken down into simpler things now we now know that that's not true we know that there are things called neutrons and there's protons and they're electrons right and those three things come together to give us essentially what we think of as the atom now of course he didn't know that then this is just apostolate so you know we can revive things through through a scientific experimentation right but that was one of his first postulates and essentially the rest of these slides are all about how did we come up or discover uh what were the experiments that led us to discover these other uh particles that kind of go into making the atom as we know it his second postulate was that the atoms of a given element are identical okay so if we have you know Neon gas every is the same all right um and conversely the atoms of different elements are different in some way some fundamental way okay um so then there's the third postula that chemical compounds are formed when atoms of different elements combine with each other so what we know is that we can take carbon and we can take oxygen and through experimentation we can form carbon dioxide okay but if we change up the ratios of the carbon to oxygen we can also make something called carbon monoxide right a given compound always has the same relative numbers and types of atoms but we can combine them in different ratios to get different compounds okay so then the fourth postula all right is that chemical reactions involve reorganization of the atoms all right chemical reactions we have to reorganize how the atoms are connected together all right so we change the way they're bound together the atoms themselves are not changed in the chemical reaction it's just how they are interacting with each other so again this is late 1700s all right and where we now have this atomic theory and so essentially what we wanted to do from this point all right is to go through these experiments that allowed us to characterize the atom and the first one happened in the late 1800s so this is where J.J Thompson comes on to the scene and he's kind of pictured here off to the right okay and so what he did okay at the time there were many scientists doing research with radiation and I'll oftentimes what these experiments relied on were the use of these cathode ray tubes and that's kind of what's being shown right here okay and so what do we have we have this this sealed tube as a partially evacuated okay and then we have two electrodes two metal electrodes we have negative electrode okay that's called our cathode and then we have this positive electrode and that's called our anode well what we can do is we can connect these two electrodes with a high voltage source okay so we can provide some source of electric potential and then what we see when we do this is that we get the stream of particles emanating from the negative electrode and preceding towards the positive electrode it is the stream of of of of of particles that is called a cathode ray so that's where it gets its name from okay so then what we can what they started to do or what JJ Thompson started to do okay is he could now apply an external electric field okay so what he did here and that's what's being shown right here this applied electric field now we have two more electrodes okay here's our positive pole here's our negative all right and what he found was that when we put this um kind of uh external electric field applied this beam this cathode ray if you will actually steered or veered away from the negative pull and is actually attracted foreign and so that gave him this idea that these cathode rays were actually con actually consisted of negatively charged particles and these negatively charged particles were then given the name electrons electrons so we have now the idea of electrons being formulated and these are negatively charged particles all right that essentially can come from a given uh metal okay and so the other thing he did was he started to change all right the applied electric fill so he did kind of like um a gradient of Applied electric fields and by doing that what it allowed him to do was to Derm determine the charge to mass ratio on these electrons okay and what he found is that 1.76 times 10 to the 8th coulombs per gram that is our charge to mass ratio and again he figured that out by varying the supplied electric field okay now the amazing thing about this is that it was reproducible and what do I mean by that different metals different Metals okay at this at this cathode okay again this is our cathode this is our anode different Metals gave the exact same results it's reproducible so what did that tell him it tells him that the electrons they are fundamental to all atoms okay fundamental to all atoms all right it also told him that if an atom has this fundamental negatively charged particle that there should be some sort of positive particle because people knew at the time that at and we as we know now that atoms are electrically neutral okay and so what that allowed him to do was to come up with this idea of his Plum post model where an atom essentially is this very diffuse kind of cloud if you will of positive charge and embedded within that cloud of positive charge a very diffuse Cloud a positive charge all right we're negative electrons okay so again Plum Pudding model just ice