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Introduction to the atom

Learn how atoms are made up of protons, neutrons, and electrons. Elements are defined by the atomic number, the number of protons in the nucleus. The mass of an atom is determined by the total number of protons and neutrons.  Created by Sal Khan.

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  • old spice man green style avatar for user Sarvesh.Mayil
    Why doesn't light pass through objects if 99.99% is nothing or free space?
    (250 votes)
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  • old spice man green style avatar for user Zach Kendrick
    what is the free space made up of?
    (121 votes)
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  • purple pi purple style avatar for user Lilly
    If a piece of string is mostly free space, then how come we can pull it without it breaking?
    (70 votes)
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    • duskpin ultimate style avatar for user Flying Emu
      The same force which is keeping the string together in the first place.

      The chemical bonding in the string between individual atoms forms the string itself, by pulling on the string, you put strain on the bonds and if you pull hard enough, the bonds would break ultimately causing the string to snap in two.
      (60 votes)
  • leaf green style avatar for user Liwei Yang
    If you were to use a knife to cut a piece of string, would there be a chance of an atom and its electron to split apart?
    (75 votes)
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  • old spice man green style avatar for user ♦Patrick Z.♦
    If you were somehow able to, what would you get if you got rid of all the free space in an atom and compressed everything together (The protons, electrons, and neutrons)?
    (29 votes)
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  • mr pink red style avatar for user Gabrielle Steenberger
    Aren't atoms made up of smaller pieces? It has to be made up of SOMETHING right? Could someone show me a biggest to smallest chart type thing?
    (32 votes)
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  • orange juice squid orange style avatar for user Leo Thomas
    How do we know the average mass number of elements on earth? Is it just an estimate of sorts?
    (14 votes)
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    • leaf green style avatar for user Mr. Schleusener
      Yeah, basically. We assume a more or less random distribution of the elements throughout the earth, so in any given sample, we just need to have enough atoms to ensure that our sample average is equivalent to the population (i.e. all atoms of that type) average. Since atoms are so tiny, it doesn't take very much mass of a sample to have enough atoms for that assumption to be pretty good...
      (8 votes)
  • piceratops ultimate style avatar for user Binh1600
    why do we see and feel things like a wall or a table as a solid object when they are really mostly free space
    (6 votes)
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  • spunky sam blue style avatar for user Rohan Purohit
    I had a doubt. Sal says that all of the protons stay inside the nucleus. But if there are protons staying together inside the same nucleus wont the protons repel each other?Then they should move apart and the nucleus must blast. But the atom is quite stable.
    How can this be explained? Please help!
    (6 votes)
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    • piceratops ultimate style avatar for user Just Keith
      You are quite correct, the protons DO repel each other due to having like electromagnetic charges. However, there is another force at work. It is called the strong force (really, that is its name). Both neutrons and protons carry the strong force (but electrons do not). The strong force is what binds the nucleus together, by overcoming the repulsion between the protons.

      But an atom must have just the right balance of protons to neutrons to make a stable nucleus. If there are too few or too many neutrons, the nucleus won't be stable. The details about which combinations of protons and neutrons are stable and which are not is a very advanced topic and is not usually covered in General Chemistry. But, if the ratios are not exactly correct, then the atom will be radioactive and undergo decay.
      (10 votes)
  • leaf orange style avatar for user Golden pair & co
    Hey did you guys know that:

    Silicon is one of the most useful elements to mankind. Most is used to make alloys including aluminium-silicon and ferro-silicon (iron-silicon). These are used to make dynamo and transformer plates, engine blocks, cylinder heads and machine tools and to deoxidise steel. Silicon is also used to make silicones.


    Chlorine is used to disinfect water and is part of the sanitation process for sewage and industrial waste. During the production of paper and cloth, chlorine is used as a bleaching agent. It is also used in cleaning products, including household bleach which is chlorine dissolved in water.


    Xenon is used in certain specialised light sources. It produces a beautiful blue glow when excited by an electrical discharge. Xenon lamps have applications as high-speed electronic flash bulbs used by photographers, sun bed lamps and bactericidal lamps used in food preparation and processing.


    Germanium is key to fibre optic cables and also used in high-speed computer chips and plastics as well as infrared radiation. The metal and its oxides are used in military applications like night-vision devices as well as satellite imagery sensors. It is also important for low-carbon technologies such as solar cells.
    (9 votes)
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Video transcript

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 the atom, as 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, and I just kept cutting the apple -- let me draw a nice looking apple just so it doesn't look just like a heart . There you go. You have a nice looking apple, And 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, oh, if I could just get my knife a little bit sharper, I could cut it again and again. So it's a completely philosophical construct, which frankly, in a lot of ways, isn't too different to how the atom is today. It's really just 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 part of an apple that they won't be able to divide anymore. And they called that an atom. And it doesn't just have to just be for an apple they said this is true for any substance or any element to that you encounter in the universe. And so the word atom is really Greek for uncuttable. Uncuttable or indivisible. Now we know that it actually is cuttable and even though it is not a trivial thing, it's not the smallest form of matter we know. We now know that an atom is made up of other more fundamental particles. And let me write that. So the we have the neutron. And I'll draw in a second how they all fit together and the structure of an atom. We have a neutron. We have a proton. And we have electrons. Electrons. And you might already be familiar with this if you look at old videos about atomic projects, you'll see a drawing that looks something like this. Let me see if I can draw one. So you'll have something like that. And you'll have these things spinning around that look like this. They have 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 that 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, and I'm going to use the word orbit right now, although we'll learn in about two minutes that the word orbit is actually the incorrect or even the mentally incorrect way of visualizing what an electron is doing. But the old idea was that you have these electrons that are orbiting around the nucleus very similar to the 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 when we cover quantum mechanics we'll learn why this doesn't work, what are the contradictions that emerge when you try to model an electron like a planet going around the Sun. But this was kind of the original idea, and frankly I think this is kind of the idea that is the most mainstream way of viewing an atom. Now, I said an atom is philosophically interesting. Why is it philosophically interesting? Because what we now view as the accepted way of viewing an atom really starts to blur the line between our physical reality and everything in the world is just information, and there really isn't any such thing as true matter or true particles as the way we define them in our everyday life. You know, for me a particle, oh, it looks like a grain of sand. I can pick it up, touch it. While a wave, that could be like a soundwave. It could be just this change in energy over time. But we'll learn, especially when we do quantum mechanics, that it all gets jumbled up as we start approaching the scales or the size of an atom. Anyway, I said this was an incorrect way of doing it. What's the correct way? So it turns out-- this is a picture, not a picture really, this is also a depiction. So it's an interesting question, what I just said. How can you have a picture of an atom? Because is actually turns out that most wavelengths of light, especially the visible wavelengths of light, are much larger than the size of an atom. 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, reflected light you could almost view it as too big, or too blunt of an instrument with which to observe an atom. Anyway, this is a depiction of 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 depict it here in the nucleus, right there, maybe these are the two-- I'm assuming they're using red for proton and purple for neutron. Purple seems like more of a neutral color. And they're sitting at the center of this atom. And then this whole haze around there, those are the two electrons that helium has, or that at least this helium atom has. Maybe you could gain or lose an electron. But these are the two electrons. And you say, hey, Sal, how can two electrons be this blur that's kind of smeared around this atom. And that's where it gets philosophically interesting. So you cannot describe an electron's path around a nucleus with the traditional orbit idea that we've encountered when we look at planets or if we just imagine things at kind of a larger scale. It turns out that an electron, you cannot know exactly its momentum and location at any given point in time. All you can know is a probability distribution of where it is likely to be. And the way they depicted this, black is a higher probability, so you're much more likely to find the electron here than you are here. But the electron really could be anywhere. It could even to be here, even though it's completely white there, 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. Remember, an orbit was something like this. It's like Venus going around the Sun. So it's very physically easy for us to imagine. While an orbital is actually a mathematical probability function that tells us where we're likely to find an electron. We'll deal a lot more with that when we cover quantum mechanics, but that's not going to be in the scope of this kind of introductory set of chemistry lectures. But it's interesting, right? An electron's behavior is 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 you can't even say exactly where it is. It can be anywhere in this haze. And we'll learn later that there are different shapes of the hazes is as we add more and more electrons to an atom. But to me, it starts to address philosophical issues of what matter even is, or do the things we look at, how real are they? Or 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. You could review Coulomb's laws videos in the physics playlist. But the idea is that 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's what was tempting about the original model of an electron. If they say, OK, if this thing has positive charges, right? So let's say this is two neutrons and two protons. Let's say it's a helium atom. Then we'll have some positive charges here. We have some negative charges out here. Opposite charges attract. And so if these things had some velocity, enough velocity, they would orbit around this, just the way a planet will orbit around the Sun. But now we learn, even though this is partially true, that the further away an electron is from the nucleus, it does have more, it's true, potential energy. In that it will want to move towards the nucleus, but because of all 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 this kind of wave-like behavior, where it just has this probability function that describes it. But the further away an orbital, it does have more potential. We're going to 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, this is defined as the atomic number. Now, so let's say I said something has four protons. How do we know what it is? Well if we haven't memorized 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 number is the number that you see up there. And that' s literally the number of protons. And that is what differentiates one atom from another. If you have fifteen protons, you're dealing with phosphorus. And all of a sudden, if you have seven 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 charge and all of that. Or what happens when you gain or lose electrons. But that does not change what element you're dealing with. And likewise, 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 electrons do you have? Well, if an atom is charge-neutral, that means it has the same number of electrons. So let's say that I have carbon. Its atomic number is six. And let's say its mass number is twelve. Now what does this mean? And let me say further that this is a neutral particle. This is a neutral atom. So the atomic number for carbon is six. That tells us exactly how many protons it has. So if I were to draw a little model here, and this is in no way an accurate model. I'll draw six-- two, three, four, five, six protons in the center. And the weight of these protons, each proton is one atomic mass unit, and we'll talk more about how that relates to kilograms. It's a very small fraction of a kilogram. Roughly I think it's 1.6 times 10 to the minus 27th of a kilogram. So let's say each of these are one atomic mass unit, and that's approximately equal to, I think, 1.67 times 10 to the minus 27 kilograms. This is a very small number. It's actually almost impossible to visualize. At least it is for me. This tells me the mass of the entire carbon atom, of 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 plus the 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 proton has an equal positive charge as the electron's 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 the nucleus right there. And then if we were to draw the electons-- well, I could draw it as a smear, but if we want to kind of visualize it a little better, we could say, OK, there's going to be six electrons orbiting. One, two, three, four, five, six. And they're going to be moving around in this unpredictable way that we would have to describe with a probability function. And so the interesting thing about it is, most of the mass of an atom is sitting right in here. I mean, you might notice that when people care about the mass, when they care about the atomic mass number of an atom, they ignore the electrons. And that's because the mass of a proton, one proton mass-wise, is equal to 1,836 electons. So for thinking about the mass of an atom, for all basic purposes, you can ignore the mass of an electron. It's really the mass of the nucleus that counts as the mass of the atom. Now, you might see this periodic table here, and you say, OK, they gave us the atomic number up there. 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 see, in carbon. In carbon they have this 12.0107. That is the atomic weight of carbon. Let me write this. Atomic weight of carbon. The atomic weight of carbon is 12.0107. Now, what does that mean? Does that mean that carbon has six protons and then the remainder, the remaining 6.0107 neutrons, it has kind of this fraction of a neutron? No. It means if you were to average all the different versions of carbon you find on the planet and you were to average the number of neutrons based on 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 on the planet. We don't know how much in the universe, but on the planet. Now, if you were to average these, not just a straight-up average, then you would get 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-- if you just found carbon someplace, on average its weight in atomic mass units is going to be 12.0107. 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 what I think is kind of the neatest idea behind atoms. And it's the most philosophically interesting things about them. It's that the relative size-- so, we have these electrons, which represent very little of the mass of an atom. It's 1/2000 of the mass of an atom are the electrons. And even those, it's hard to even describe them as particles, because 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 inside the nucleus. And this is the interesting thing. If you look at an atom on average, if you say this is my atom. Let's say I had two atoms that are bonded to each other. And I were to say, 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 the stuff. It turns out that it's actually an infinitesimally small fraction of the volume of the atom where-- the volume of the atom is hard to define, because the electron can pretty much be anywhere, but if you view the volume as where you're most likely to find the electron, or with 90% probability you're likely to find the electron, then the nucleus is, in a lot of cases and the way I think about it, it's about 1/10,000 of the volume. So if you think about it, 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. 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 hold on reality. What is there when, if-- and 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. This image of a helium atom, they say right here this is one femtometer. Right? One femtometer. This is the scale of the nucleus of a helium atom, right? One femtometer. This is one angstrom, right? And they say that equals 100,000 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 1/100,000. So if you had-- 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, 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 something 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 further mind-blowing to the next video.