Average atomic mass | Atoms, isotopes, and ions | AP Chemistry | Khan Academy

The thing that I've always found amazing about chemistry is it's an entire field of science that we as human beings have developed to actually understand what is happening at an almost unimaginably small scale. In particular, we're going to 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, 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 going to appreciate in a few seconds how small of a scale it is, we're going to have to have some units of measurement. 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 mass unit, and it's historically denoted as amu.

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 ten to the negative twenty-seven 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 like this feels like a bit of a hairy number here, one point six zero. Why did they define it that way? 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 is a little bit more than one; it's about 1.007 atomic mass units. But it's approximately 1, 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 1/2000 of a proton or 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.

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.

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. There are versions that have one or two neutrons, but most—99.98 percent 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.

When we talk about versions of a given element, there's a fancy word for it—they're called isotopes. The different isotopes, they'll all have the same number of protons because they're talking about the same element, but they'll have different numbers of neutrons.

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 because the mass of a proton is 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 an 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 eighty percent 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—twenty percent of what we observe of that element 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, but their 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 called this number atomic weight. And I've always complained about it because it's really talking about mass, not weight. If you don't know the difference, you will learn that at some point in the future. 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 numbers 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, its 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.