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  • - [Instructor] The thing that I've always found

  • amazing about chemistry,

  • it's an entire field of science

  • that we as human beings have developed

  • to actually understand what is happening

  • in an almost unimaginably small scale.

  • In particular we're gonna be thinking about the atomic,

  • and even the subatomic scale.

  • And by looking at that scale we can then begin to understand

  • the universe in which we live in,

  • the scale in which we live in,

  • and even be able to make predictions about what will happen,

  • and make things that are useful for human beings.

  • So if we're going to operate at this small of a scale,

  • and we're gonna appreciate in a few seconds

  • how small of a scale it is,

  • we're going to have to have some units of measurement.

  • And this video is going to focus on mass.

  • How do we measure mass at such a small scale?

  • Well to do that the chemistry community

  • has historically used something called an atomic mass unit.

  • I'll write it here, atomic, atomic mass unit,

  • and it's historically denoted as AMU.

  • And more recently,

  • the more modern version of this

  • is the unified atomic mass unit,

  • that is denoted by just a U instead of an AMU.

  • So how does a unified atomic mass unit

  • connect to our units of mass

  • that we might use on a larger scale

  • like, say, grams or kilograms.

  • Well, the unified atomic mass unit is defined as

  • 1.660540 times 10

  • to the negative 27 kilograms.

  • So when you see something like this,

  • you might have a few reactions.

  • Your first reaction, which would be an appropriate reaction,

  • is that wow, 10 to the negative 27 power is very small.

  • To appreciate it you could write it out,

  • it would be zero point and then 26 zeros

  • and then you would have one six six zero five four zero.

  • So very, very, very small, really unimaginably small.

  • We can only try to abstract it with things like mathematics.

  • The other thing you might appreciate

  • is this feels like a bit of a hairy number here,

  • 1.660540, why did they define it that way?

  • And the answer to your question is,

  • this definition makes it a lot cleaner

  • when we think about the mass of whether it's an atom

  • or the constituents of an atom like a proton or a neutron.

  • Roughly speaking the mass of a proton

  • is approximately one unified atomic mass unit.

  • The mass of a neutron

  • is approximately one unified atomic mass unit.

  • It actually turns out that a proton's

  • a little bit more than one,

  • it's about 1.007 atomic mass units,

  • but it's approximately one.

  • And the neutron is actually a little bit more

  • than even a proton,

  • it's 1.008 approximately unified atomic mass units.

  • Now an electron's mass is actually far smaller

  • than either of these,

  • it's actually almost one two thousandth

  • of a proton or a neutron,

  • and so you can imagine an atom

  • which is made up of protons

  • and usually neutrons and electrons as well,

  • the mass is mainly going to be

  • the protons and neutrons in the nucleus.

  • And so if you know the number of protons

  • and neutrons in the nucleus,

  • you're going to have a pretty good sense

  • of its atomic mass.

  • And you can see that indicated

  • on a periodic table of elements which we have here.

  • And we will study the periodic table of elements

  • in a lot more detail in other videos.

  • But you can see a couple of interesting elements.

  • One, you have the abbreviation of a given element,

  • H represents hydrogen.

  • The number on top on this periodic table,

  • that's the atomic number,

  • and that tells you how many protons it has.

  • And an element is defined by the number of protons.

  • So any atom that has exactly one proton in its nucleus

  • is going to be hydrogen by definition.

  • Any atom that has exactly 20 protons

  • in its nucleus is going to be calcium by definition.

  • Any atom that has exactly 36 protons in its nucleus

  • is going to be krypton by definition.

  • So what would you expect the mass of a hydrogen atom to be?

  • Pause this video and think about it.

  • Well we know that all hydrogen atoms

  • by definition have one proton,

  • but it actually turns out

  • there's different versions of hydrogen

  • that can have different numbers of neutrons.

  • Most of the hydrogen in the universe

  • actually has zero neutrons, zero neutrons.

  • There are versions that have one or two neutrons,

  • but most, 99.98% roughly, of hydrogen in the universe

  • has one proton, zero neutrons,

  • and if it's a neutral hydrogen

  • it's going to have one electron.

  • And when we talk about versions of a given element

  • there's a fancy word for it, they're called isotopes.

  • And the different isotopes,

  • they'll all have the same number of protons

  • 'cause they're talking about the same element,

  • but they'll have different numbers of neutrons.

  • And so if this is the most common form of hydrogen.

  • What do you think its mass is going to be?

  • Well its mass is going to be essentially the mass

  • of a proton plus an electron,

  • and roughly speaking it's going to be the mass of a proton

  • 'cause the mass of a proton's

  • going to be so much larger than the mass of an electron.

  • And so you would expect

  • that its mass is approximately one unified atomic mass unit.

  • Now if you were to precisely look at

  • the mass of a proton and a electron,

  • if you add them together, you actually get something

  • that's a little bit closer to 1.008.

  • And you actually see that

  • right over here on the periodic table of elements.

  • Now this number, although it is pretty close

  • to the mass of the version of hydrogen

  • that I just described,

  • it's actually a weighted average

  • of the various versions of hydrogen.

  • It's just close to this version

  • because this version represents most of the hydrogen

  • that we actually see around us.

  • If for example you had two versions of an element,

  • some hypothetical element,

  • and let's say that 80% of the element

  • that we see is version one

  • and version one has a mass

  • of let's call it five atomic mass units,

  • and then version two, it's the remainder, 20%,

  • of what we observe of that element,

  • it has an atomic mass of six atomic mass units.

  • You would get a weighted average here

  • of 5.2 unified atomic mass units.

  • And that's actually how these numbers are calculated.

  • They are not just the mass of one type of that element,

  • they're a weighted average mass

  • of the various isotopes, of the various types.

  • And so this number on a periodic table of elements

  • is known as the average atomic mass,

  • average, average atomic atomic mass.

  • Now in older chemistry books,

  • and this is actually the case

  • when I first learned chemistry,

  • they call this number atomic weight.

  • And I've always complained about it

  • because it's really talking about mass and not weight.

  • If you don't know the difference

  • you'll learn that at some point in the future,

  • and it's really talking about average atomic mass.

  • Now I'll give you one little detail

  • that might be useful to you.

  • Sometimes you'll hear something called relative atomic mass.

  • It actually turns out this periodic table of elements,

  • because it does not write a little U

  • after each of these numbers,

  • it's essentially these number are unitless,

  • so it's really talking about relative atomic mass.

  • So it's saying, hey on average, for example,

  • the mass of a carbon atom

  • is going to be roughly 12 times that of,

  • on average, the mass of a hydrogen atom.

  • If they put the units here,

  • then that would actually truly be average atomic mass.

  • But for our purposes, as we go into chemistry,

  • you can look at these numbers,

  • and say okay, if oxygen has a relative atomic mass of 16,

  • it's average atomic mass

  • is going to be 16 unified atomic mass units.

  • And as we will see in the future,

  • this understanding of average atomic mass

  • will prove to be very, very useful.

- [Instructor] The thing that I've always found

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平均原子質量|原子の構造と性質|AP化学|カーンアカデミー (Average atomic mass | Atomic structure and properties | AP Chemistry | Khan Academy)

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    林宜悉 に公開 2021 年 01 月 14 日
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