Worked example: Identifying an element from its mass spectrum | AP Chemistry | Khan Academy

So let's say that we have some mystery substance here, and we know that it's a pure element. We need to figure out what it is. Well, scientists have a method, and we go into the details or more details in other videos called mass. Sometimes it's known as mass spectrometry or mass spectroscopy.

It's a technique where you could take a sample of a substance and think about the various atomic masses of the different isotopes in that substance. That's what we have right over here. They tell us the mass spectrum for an average sample of a pure element is shown below.

So let's say it's this pure element. This is telling us that this looks like maybe—I don't know—let's call this 82 percent of our sample has an atomic mass of 88 universal atomic mass units. About this looks like about 7 percent of our sample has an atomic mass of 87 universal atomic mass units. It looks like 10 percent has an atomic mass of 86 universal atomic mass units, and it looks like about 1 percent of our sample has an atomic mass of 84 universal atomic mass units.

So from this information, we can try to estimate what the average atomic mass of this mystery element is. We could calculate it as 0.82 times 88 plus—let's call this 7—so 0.07 times 87 plus 0.1 times 86 plus unless it should add up to 100 percent. This is 89, and then this gets us to 99. So then, another 1 percent, 0.01 times 84.

And so if we were to do this calculation, this is our estimate of the average atomic mass of this element. We could type this into a calculator and get some number and then look that up on a periodic table of elements, or we could just try to estimate it.

We can see that it's going to be close to 88 because that's where the highest percentage is. When we're taking the weighted average, we have the highest weight right over there. But these other isotopes, these other versions of the element that have a different number of neutrons, which changes its atomic mass, they're going to bring the average down. So our average atomic mass is going to be a little bit less than 88.

So let's look up a periodic table of elements. What element here has an atomic mass a little bit less than 88? Well, Yttrium is 88.91, but we know it can't be that because none of the isotopes have an atomic mass above 88, so we can rule out Yttrium.

Strontium is looking pretty good—it's exactly what we predicted, a little bit less than 88. And Rubidium is a lot less than 88. So even if we did the calculation, we could feel confident we're not going to be as low as Rubidium.

So I'm feeling very confident—just eyeballing it, just estimating—this is going to be a little bit, have an average atomic mass a little bit less than 88, which tells me that this is Strontium.