some mystery substance here,

and we know that it's a pure element,

and 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 can take a sample of a substance

and think about the various atomic masses

of the different isotopes in that substance.

And 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.

So what this is telling us is,

this looks like maybe, I don't know,

let's call this 82% of our sample

has an atomic mass of 88 universal atomic mass units.

About, this looks like about 7% of our sample

has an atomic mass of 87 universal atomic mass units.

It looks like 10% has an atomic mass

of 86 universal atomic mass units,

and it looks like about 1% of our sample

has an atomic mass of 84 universal atomic mass units.

And 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 10%, 0.1, times 86,

plus, let's see, it should add up to 100%.

This is 89,

and then this gets us to 99,

so then another 1%,

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 our, even if we were to do 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.