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in most topics you have to get pretty advanced before you start addressing the philosophically interesting things but in chemistry it just starts right from the get-go with what's arguably the most philosophically interesting part of the whole topic and that's the atom and the idea of then atom is philosophers long ago and you could look it up on the different philosophers who first philosophized about it they said hey you know if I started off with I don't know if I started off with an apple if I started off with an apple and I just kept cutting the Apple I only draw a nice-looking Apple just so it doesn't look just like a hardier there you go have a nice looking Apple you just kept cutting it smaller and smaller pieces so eventually you get a piece so small so tiny that you can't cut it anymore and I'm sure some of these philosophers went out there with a knife and tried to do it and they just felt that it all if I could just get my knife a little bit sharper I could cut it again and again so it was completely philosophical construct which frankly in a lot of ways isn't too different the ha to how the atom is today it's really just a a a mental abstraction that allows us to describe a lot of observations we see in the universe but anyway these philosophers said well at some point we think that there's going to be some little point part of an Apple that they won't be able to divide anymore and they call that an atom it doesn't have to just be for an Apple they said this is true for any substance or any element that you encounter in the universe and so the word atom is really Greek for uncuttable uncuttable are indivisible uncuttable now we know that it actually is cuttable and even though it is not a trivial thing it's it's not the the smallest form of matter we know we now know that an atom is made up of other more fundamental particles than let me let me write them so we have the we have the neutron and I'll draw in a second of how they all fit together and in kind of the structure of an atom we have a neutron do have a proton and we have electrons electrons and you might already be familiar with this if you look at the old videos about about atomic project you'll see a drawing that looks something like this let me see if I can draw one so you'll have you know something like that and you'll have these things spinning around that look like this you know them orbits that look like that and maybe something that looks like that and the general notion behind these kind of nuclear drawings and I'm sure they still show up at some government defense labs or something like that is that you have a nucleus at the center of an atom you have a nucleus at the center of an atom and we know that a nucleus has neutrons and protons neutrons and protons and we'll talk a little bit more about which elements have how many neutrons and how many protons and then orbiting orbiting and I'm going to use the word orbit right now although we learn in about two minutes that the word orbit is actually the incorrect or even the mentally incorrect way of visualizing what electron is doing but this the old idea was that you have these electrons that are orbiting around the nucleus very similar way the Earth orbits around the Sun or the moon orbits around the earth and it's been shown that that's actually a very wrong way and it there's a lot of and when we when we cover quantum mechanics we'll learn why this doesn't work what are kind of the contradictions that emerge when you try to model an electron like a planet going around going around the Sun but this was kind of the original idea and frankly I think this is kind of the idea that is is is the most mainstream way of viewing an atom now I said an atom is philosophically interesting why is it philosophically interesting because the what we now view is the accepted way of viewing an atom really start to blur the line between kind of you know our our physical reality and and you know everything in the world is just information there really isn't any such thing as true matter or true particles is the way we define them in our everyday life you know what for me a particle it's like a grain of sand I can pick it up touch it well you know a wave that could be like a sound wave it could be this you know change in energy over time but well especially when we do quantum mechanics that it all gets jumbled up as we start we'd start approaching the scales or the size of an atom but anyway I said this was an incorrect way of doing it what's the correct way so it turns out I see this is a picture not a picture really this is also a depiction so so it's an interesting question when I just said you know how can you have a picture of an atom because it actually turns out that the wave most wavelengths of light that we especially the visible wavelengths of light are much larger than the size of an atom so it's not like something you can even you know everything else we quote/unquote observe in life it's by reflected light but all of a sudden when you're dealing with an atom reflective light is almost you could almost view it as too big or too blunt of an instrument with which to observe an atom now this is a depiction of a helium atom a helium atom a helium atom has two protons and two neutrons or at least this helium atom has two protons and two neutrons and the way they depicted here in the nucleus right there maybe these are the two I'm assuming there's using red for proton and purple for Neutron Neutron seems like purple seems like more of a neutral color and they're sitting at the center of this of this of this atom and then this whole haze around there those are the two electrons that helium has or at least this helium atom has maybe you could you know gain or lose an electron but these are the two electrons you say hey Sal that doesn't make you know how can two electrons be this this.blur that's kind of smeared around this this atom and that's where it gets philosophically interesting so you cannot describe an electrons path around a around a nucleus with the kind of traditional orbit idea that we've encountered when we look at planets or if we just you know imagine things that kind of a larger scale it turns out that an electron you cannot know exactly it's a momentum and location at any given point in time all you can know is a probability distribution on where it is likely to be and the way they depicted this black is a higher probability you're much more likely to find the electron here than you are here but the electron really could be anywhere it could even be here even though it's completely white there but with some very very very very very low probability and so this function of where an electron is this is called an orbital orbital not to be confused with orbit orbital orbital remember an orbit with something like this it's like the you know Venus going around the Sun so it's just it's very physically easy for us to imagine while orbital is actually a mathematical probability function that tells us where we're likely to find an electron and we'll deal with a lot more of that when we cover quantum mechanics but that's not going to be in the scope of this kind of introductory a set of chemistry lectures but it's interesting write an electron is its behavior so bizarre at that scale that you can't I mean to call it a particle is almost misleading it is called a particle but it's not a particle in the sense that we're used to in our everyday life it's this thing that is that you can't even say exactly where it is it's it can be anywhere in this haze and we'll learn later that there are different shapes of the Haze's as we add more and more electrons to an atom but that to me that's you know it starts to address kind of philosophical issues of what matter even is or you know do the things we look at how real are they are how real are they at least as we've defined reality anyway I don't want to get too philosophical on you but the whole notion of electrons protons they're all kind of predicated on this notion of charge and we've talked about it before when we learned about Coulomb's law and you could you could review : Coulomb's law as videos in the physics playlist but the idea is that an electron has a an electron has a negative charge a proton sometimes written like that has a positive charge and a neutron has no charge and so that was that's what was tempting about the original model of an electron is that they say okay if this thing has positive charges right let's say this is two neutrons and two protons let's say it's the helium atom then we'll have some positive charges here we have some negative charges out here opposite charges attract and so you know if these things had some had some I guess you could come at some velocity they would or enough velocity they would orbit around this it's just the way uh just the way a planet will orbit around the Sun but now we learn even though this is kind of partially true that you know the further away the further away an electron is from the nucleus the further away an electron is from the nucleus it does have more I guess you could I mean it's true potential energy in that it will want to move towards the nucleus but because of all of these the the mechanics at the quantum level it won't just do something simple like move in a path like that like a comet would do around the Sun it actually has a this kind of wave-like behavior where just has this probability function that describes it but the kind of the further away in orbital it does have more potential we're gonna go a lot more into that in future videos but anyway how do you recognize what an element is I've talked a lot about the philosophy and all of that but how do I know that this is helium is it by the number of neutrons it has is it by the number of protons it has is it by the number of electrons well the answer is it's by the number of protons so if you know the number of protons in an element you know what that element is and the number of protons number of protons this is defined as the atomic number atomic number now so how do you is let's say I said something has four protons four protons how do we know what it is well we could if we haven't memorize it we could look it up on the periodic table of elements which we'll be dealing with a lot in this playlist and you'd say oh four protons that is beryllium right there and the atomic mass the atomic number is the number that you see up there and that's literally the number of protons and that is the single-most different that is what differentiates one atom from another if you have 15 protons you're dealing with phosphorus now of a sudden if you have if you have seven say you have 7u protons you're dealing with nitrogen if you have eight you're dealing with oxygen that is what defines the element now we'll talk in the future about what happens with with charge and all of that but or what happens when you gain or lose electrons but that does not change what element you're dealing with and likewise when you have then when you change the number of neutrons that also does not change the element you're dealing with but that leads to an obvious question of well how many neutrons and and electrons do you have well if a if a atom is atomically neutral is its charge neutral that means it has the same number of electrons and so let's say that I have a carbon let's say this this they say carbon it has its atomic number of six and let's say it's its mass number is 12 now what does this mean so listen listen let me say further that this isn't a this is a neutral particle this is a neutral atom so the atomic number for carbon is 6 that tells us exactly how many protons it have so if I were to draw a little model here and this is in no way an accurate model I would draw up 6 2 3 4 5 6 protons in the center now and and the weight of these protons each proton is one atomic mass unit and we'll talk more about that how that relates to kilograms it's a very small fraction of kilogram roughly one I think it's like one point six times ten to the minus twenty seventh of a kilogram so let's say each of these let me write that each of these are one atomic mass unit and that's approximately equal to I think one point six seven times ten to the minus 27 kilograms this is a very small number it's actually there it's almost impossible to to visualize at least it is for me this tells me the the the mass of the entire carbon atom this particular carbon atom and this can actually change from carbon atom to carbon atom and this is essentially the mass of all of the protons plus all of the neutrons and each proton has an atomic mass of one in atomic mass units and each Neutron has an atomic mass of one atomic mass unit so this is really the number of protons number of protons plus the number of neutrons number of neutrons so in this case we have six protons so we must also have six neutrons six neutrons plus six protons now where are the electrons well I said it's neutral so the the proton has an equal positive charge as the electrons negative charge so this is a neutral atom and it has six protons so it also has six electrons let me draw that so we said it has six neutrons in here one two three four five six so that's its that's the nucleus right there and then if we were to draw the electrons well I could draw it as from here but if we want to kind of visualize a little better we could say okay there's gonna be six electrons orbiting here one two three four five six and they're gonna be moving around in this unpredictable way that we would have to describe with a probability function and so and the interesting thing about it is most of the mass of a of a of an atom is sitting right in here I mean you might notice that when people care about the mass of a when they care about kind of the atomic mass number of a of an atom they ignore the electrons and that's because the mass of a proton one proton mass wise is equal to 1836 electrons so for thinking about the mass of an atom for all you know for all basic purposes you can ignore the mass of an electron that it's really the the mass of the the mass of the nucleus that counts as the of the as the kind of mass of the atom now you might see this periodic table here and you say okay they gave us the atomic number up there right the atomic number of oxygen is eight it means it has eight protons the atomic number of silicon is 14 it has 14 protons now what is this right here let's say listen carbon urban they have this 12.0 107 that is the atomic weight of carbon let me write this right now atomic atomic weight of carbon and the atomic weight of carbon is 12 point OH 12 point Oh 107 now what does that mean does that mean that you know carbon has six protons six protons and then the remainder the remaining six point oh 107 neutrons it has kind of this fraction of a neutron no it means if you were to average all of the carbon all of the different carbon versions of carbon you find on the planet and you were to average the number of neutrons based on on the different on the on the kind of the quantity of the different types of carbon this is the average you would get so it turns out that carbon the two major forms the main one you'll find is carbon-12 so that's like this so that has six protons and six neutrons and then another isotope of carbon now an isotope is the same element with a different number of neutrons another isotope of carbon is carbon 14 which is much more scarce in the on the planet we don't know how much you know in the universe but on the planet now if you were to average these not just a straight-up average then you would get you know carbon 13 and then the atomic weight would be 13 but you weight this one much higher because this exists in much larger quantities on earth I mean this is pretty much all of the carbon that you see but there's a little bit of this so if you weight them appropriately the average becomes this so most of the carbon you'll find so if you just took found carbon in the in some place on average its weight an atomic mass units is going to be twelve point OH twelve point Oh 107 but that idea of an isotope is an interesting one remember when you change the neutrons you're not changing the actual fundamental element you're just getting a different isotope a different version of the element so these two versions of carbon are both isotopes now I want to leave this video with I think is kind of the neatest idea behind atoms and it's kind of the most philosophically interesting things about them is that the relative size so you know we have this electrons which matter represent very little of the mass of an atom I mean it's you know 1 mm of the mass of an atom or and the electrons and even then those are it's hard to even describe them as particles because they had you can't even tell me exactly where and how fast one of these particles is moving they just have a probability function so most of the atom is sitting that inside the nucleus and this is the interesting thing if you look at an atom on average if you say you know this is my atom and let's say I had two atoms that are bonded to each other and I would say what you know how much of this is actual stuff and when I say stuff that's a very abstract concept because we're talking about the nucleus right because the nucleus is where all the mass is all this stuff it turns out that it's actually an infinitesimally small fraction of the volume of the atom where you know the volume of the atom it's hard to define because the electron can pretty much be anywhere but if you view the volume is where you're most likely to find the electron or you know with 90% probability you're likely to find the electron then the nucleus is on a lot of cases and of the way I think is but one ten-thousandth of the volume so if you think about when you look at something if you look at your hand or if you look at the wall or if you look at your computer 99.999% of it is free space it's nothing it's vacuum it's if you were to just you know if you had ultra small I guess we could call them particles or something most of them would pass straight through whatever you look at so it already starts to kind of question our holds on reality what is there when if you know and this is this is kind of this is fact this isn't theory right here that if you take anything down to the building blocks down to the atomic level most of the space of that kind of quote-unquote object is free vacuum space you could go straight through it if you could get down to that scale I mean know this is this image of a helium atom they say right here this is one femtometer right one femtometer one femto meter did they this is the scale of the nucleus of a helium atom right one femtometer this is one angstrom right and they say that equals a hundred thousand femtometers and just to get a sense of scale one angstrom is 1 times 10 to the negative 10 meters right so the atom is roughly on the scale of an angstrom in the case of helium the nucleus is even a smaller fraction it's 100,000 so when you if you had you know let's say you had liquid helium which you'd have to get very cold to get if you're looking at that most of it is free space if you're looking at an iron bar the great great-great-great-great great majority of it is free space and we're not even talking about what's you know maybe there's some free space inside the nucleus that we could talk about in the future but to me that just blows my mind that most things we look at are not really solid they're really just empty space but they look solid because of the way light reflects on them or the forces that repel us but there really isn't kind of something to to touch there that most of this right here is all free space I think I've said the word free space now and I think I'll leave leave further mind-blowing to the next video
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