Introduction to the atom | Chemistry of life | Biology | Khan Academy
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Introduction to the atom | Chemistry of life | Biology | Khan Academy


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.

About James Carlton

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100 thoughts on “Introduction to the atom | Chemistry of life | Biology | Khan Academy

  1. Just began reading The Making of the Atomic Bomb by Richard Rhodes and this video really helped me wrap my head around what an atom is. I'm hooked.

  2. I've noticed that some periodic tables have different weight numbers. Does anyone know why this is or does that mean they discovered changes in some of them at some point.

  3. Thanx 🙂
    @Trying to understand the so invisible atom by looking for reoccurring patterns: How I see it is not that most of the world is free space around a core, but most of the world is moving electrons creating a fuzzy kind of bonding; someone once explained it with cotton candy. The compact core, and less compact external layers of really fast electrons. These less compact layers are the ones that allow the bonding between atoms based on the nucleus force pull; something like our atmosphere or the cytoskeleton in cells, they are not free space as they can appear to be, but rather like an electronic haze of very fast movement creating a bond; can we really easily break the structures of a fast moving electron without binding it to another atom?! I think of the neutron as nucleolus of the atom's 'dna' or signature, the proton creates it's 'atmosphere' (and real realm) by attracting with the 'echoing' electrons. For maybe this is another kind of atomic signature, like the prokaryotic dna, it signs its actual charge, not just the neutron signature of potential weight. #The 'electronic haze/atom field' has no cell wall, just a tight knit of electrons?

  4. Thank you, sir! Ive decided to go back to school and after a six year absence and having never taken a chemistry class I wanted a nice little crash course and this helps tremendously. Thanks again!

  5. Could it be that atoms are so incredibly small that the movement of the electrons are unimaginably fast around the nucleus, faster than the speed of light, and that's the reason we can't explain a lot of shit at the quantum level? because what would faster than the speed of light look like? perhaps that's why the strange phenomena of quantum mechanics come in.

  6. I can't show you my gratitude for posting this awesome video……my chemistry teacher is no match for you… I love your lectures but I wish I could ask some questions……..THANKS

  7. Grrreat lecture. I have been captivated by the mystery of chemistry since age 8, and this man shares my sentiments. This lecture in particular brings the wonder and mystique of chemistry and atoms to the viewer beautifully. He's got it, and he shares it! Not one of my teachers of Ivy League professors can match him. Thanks.

  8. because apples have air pockets in them so when sliced they can be sliced in to the smallest particle maybe not as small as an atom but enough for show and tell

  9. Thank You so much
    I have watched many videos and not understood anything they were saying. You are the first person I actually understand

  10. I am a 5th grader and I am able to study this even if its like something l would if i was in 11th grade so thank you so much

  11. Thank you soooo much!! I just found your channel, and I am not sleeping tonight.. This is the first video I sat through, and I feel so smart now! I have never taken so much notes.. The color-coding is soo helpful. Thank you very much..May god bless your soul..!:)

  12. I did not know that this was a Scientific fact that there is actually nothing between smaller particles. I wrote an elaborate mail to Derek of Minute Physics regarding this asking questions on a much higher level which he has not responded to. I added many layers to it and asked many questions, perhaps many which Scientists do not have an answer to/for.

  13. So, if we could temporarily scatter the subatomic particle cloud of our atoms or negate the charge by feeding in, or taking out of it, a number of protons to balance out the repelling force and then we did the same with the object we are facing, would we then be able to walk through the object?

  14. These first 3 videos to intro to chem are horrible. He is not a good teacher, Khan should've did these videos.

  15. Khan Academy is great, but they should really make sure they understand the questions being asked of them before flagging them as off-topic and stifling constructive scientific debate. What are they there for if not to encourage learners to question things and find out the answers? In the moderators' quest for aesthetically-perfect discussion boards, they are acting in a counter-productive way which goes against their ethos of helping to educate.

  16. I realized today that free space, or empty space, or absence is a form of potential energy….the potential of an empty canvas or a plot of land….So the atom is full of empty space meaning its full of potential energy, the more space between the nucleus and electrons means more potential energy, perhaps the potential to become something larger like an element ….right? … Also drawing a connection between potential, and probability….perhaps a larger orbit means its more likely to bond, so higher potential…?

  17. i have a doubt in my mind for so long that, I think the atom is still uncuttable coz u cant rip the nucleus apart from atom to get protons and neutrons. Yeah u can say it is not the tiniest thing

  18. From now on I will enroll myself in Khan Academy. So great, many things I have learn from Calculus to Physics. May God bless you. Thank you!

  19. So….if you can't be certain where an electron really is (in the case of an atom with a single shell), can it be said that there is "more certainty" as to an electrons position (in the first shell), in regards to an atom with a full second shell ? Seems to me that the full second shell would "trap" the 2 electrons into a definable area. Am I thinking this wrong ?

  20. I´m a chem teacher and when I have to talk about the atom, I also first tell them about what the old philosophers were thinking when created the word "atom".

  21. In a moment…u told the weight….
    12
    Carbon……1.67×10.27…but
    6
    If u get it in the predictibly way…..by getting on 12
    Carbon
    6
    You clearly get 1.63 kg….atomic mass is changed in 0.05…..u added it in the P+
    In an average…..in average…u get 6.0102+0.0105….so we get 6.0107…🤔🤔🤔🤔 u are quit true….but….not much… .plz explain it in the e- P+ and ¡-¡….so plz cooperate…..i wanna find it out……because 90%probability is to find electron….and 99.98 percent of vacum……then 1femtometer is it… am i right?..then what is there in the vacum….if it is true…..we can pass through walls😂 6.63×10^24

  22. Sir , electron moves around the nucleus , so in nucleus proton and neutron are present . proton have + charge and electron have – charge , opposite charges attract each other so why they do not collapse?

  23. Mr. Khan, You don't know it, but my 7 year old granddaughter will be watching you as her math and science Instructor! I appreciate you and your website. This is so informative, the visuals and explanations are just superb! Thank You! I am a Baby Boomer and if I had had you in High School as a Chemistry Teacher; I would not have dropped the class!!!

  24. So, you're saying that there is an empty space in the atom between the nucleus and electrons and it is easy to move through that space? electrons don't rotate around the nucleus? In a still state, just popup here and there around the nucleus? what is that haze? That's the vigorous rotation of the electrons around the nucleus. May be the orbits aren't so geometrically perfect, but there is a constant movement and rotation of the electrons around the nucleus. You can't go through the space of an atom between nucleus and electrons, because it's like an epicenter of a tornado. You can see through, but you can't get through. Your explanation is very deceiving in that part.

  25. Hello there I think your am awesome teacher I have a question. I really would like to have an overall understanding of science chemistry physics including quantum physics but have no idea on where to start and what path to take. Can you give me the curriculum I should follow in your opinion

  26. Thank you so much, can we think all the proton in the different atom is the same, so do the electron and neutron, are they same in the different atom?

  27. Thanks Man I Learned this In 8 grade In Now in hs I'm learning biology in 9 grade Thanks Again god bless you

  28. The orbital symbol makes sense but it is after all just a symbol of an atom, since weve never seen one.
    So its a silly depiction as it cant "indicate" how many electrons the atom has.
    I like the old orbit symbols personally

  29. my science teacher doesn’t really teaches us, justs let’s us read the book. you are my second science teacher, thank you!!!

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