Worked example: Identifying isotopes and ions | Chemistry | Khan Academy

An isotope contains 16 protons, 18 electrons, and 16 neutrons. What is the identity of the isotope? I encourage you to pause the video and see if you can figure it out. I'll give you a hint: you might want to use this periodic table here.

All right, so I'm assuming you've had a go at it. An element is defined by the number of protons it has. So, if someone tells you the number of protons, you should be able to look at a periodic table and figure out what element they are talking about.

So, because it is 16 protons, well, we can go right over here to the atomic number. What has 16 protons? Well, anything that has 16 protons, by definition, is going to be sulfur, right over here. So, I could write a big S.

Now, the next thing we might want to think about is the mass number of this particular isotope. Remember, an isotope: all sulfur atoms are going to have 16 protons, but they might have different numbers of neutrons. So, the sulfurs that have different number of neutrons, those would be different isotopes.

So, in this case, we have 16 protons, and we have 16 neutrons. So, if you add the protons plus the neutrons together, you're going to get your mass number. So, 16 plus 16 is 32.

Now, let's figure out if there's going to be any charge here. Well, the protons have a positive charge, the electrons have a negative charge. If you have an equal amount of protons and electrons, then you would have no charge. But in this case, we have a surplus of electrons. We have two more electrons than protons, and so we have a surplus of the negatively charged particles.

We have two more, so we're going to have a negative two charge, and we would write that as 2 minus. So, this is actually an ion; it has a charge.

So, this is the isotope of sulfur that has a mass number of 32. The protons plus the neutrons are 32, and it has two more electrons than protons, which gives it this negative charge.

Let's do another example where we go the other way. We are told we are given some information about what isotope and really what ion we're dealing with because this has a negative charge. Then we need to figure out the protons, electrons, and neutrons.

Well, the first thing that I would say is, well, look, they tell us that this is fluorine. As soon as you know what element we're dealing with, you know what its atomic number is when you look at the periodic table, and you can figure out the number of protons.

Remember, atomic number is the number of protons, and that's what defines the element. That's what makes this one fluorine. So, let's go up to our periodic table, and we see fluorine right over here has an atomic number of nine. That means any fluorine has nine protons.

So, let's scroll back down. Just because it is fluorine, we know we have nine protons. Now, what else can we figure out? Well, we know we have a negative charge right here. We could use this as a negative one charge, and so we have one more electron than we have protons.

Since we have nine protons, we're going to have 10 electrons. Finally, how many neutrons? Well, remember the neutrons plus the protons add up to give us this mass number. So, if you have nine protons, well, how many neutrons do you have to add to that to get to 18?

Well, you're going to have to have 9 neutrons. Nine plus nine is 18.