Introduction to ions | High school chemistry | Khan Academy

So my apologies. I just had a deviated septum surgery yesterday. But what we're going to talk about in this video is the notion of an ion.

So before we talk about ions, we're just going to talk about the idea of an element. For example, if I have carbon, carbon is an element. Let's say I have one atom of carbon. What do we know about that one atom of carbon? Well, by definition, an atom of carbon has six protons. So it's going to have six protons, and if it is neutral carbon, it is going to have the same number of electrons. It is going to have six electrons, and that's what makes it neutral. You have the six positive charges and the six negative charges.

Now you could have a carbon ion, although they aren't that typical. The way you get an ion is if you don't have an equal amount of protons and electrons. So for example, if you had 6 protons and 5 electrons, what would that be? Well, we still have six protons, and remember, protons are what define what element we're dealing with. So we're still dealing with carbon.

But now we have one more positive charge than we have negative charge. So this will be carbon with, you could write it as a one plus charge like that, or you could even write it like this. This you would just view as a carbon atom. This you would now call an ion because it has that net charge; it has a different number of protons and electrons.

What if you were to go the other way around? What if you were to have 5 protons and 6 electrons? What would this be? Well, remember, protons define what element you're dealing with. So now if you look at what element has five protons, we're dealing with boron. So this is going to be boron. Neutral boron would have five protons and five electrons, but this one has one extra electron, so it has one extra negative charge.

So you could write it like this one minus, or you could just say it has a negative charge. So this is a boron ion right over here. As soon as you have an imbalance between protons and electrons, you no longer would call it an atom; you would call it an actual ion.

Now let's do an example question dealing with this. So our question tells us, "An atom of platinum has a mass number of 195." So let's just look up platinum on our periodic table. Platinum is sitting right over here, if you could see it.

An atom of platinum has a mass number of 195, and 195 looks pretty close to that atomic mass we have listed there, and it contains 74 electrons. How many protons and neutrons does it contain, and what is its charge?

Alright, so let's think about this a little bit. So we're dealing with platinum. So by definition, platinum has 78 protons, so we know that it has 78 protons. They’re telling us it has 74 electrons.

So just from that, we see we have four more protons than electrons. So you're going to have a positive four charge. Four more of the positive things that you have of the negative things. So you could write this as platinum with a plus four charge.

This is a platinum ion, a positive platinum ion. The general term when we're talking about a positive ion, we're talking about a cation. That is a positive ion up there. When we talked about boron being negative, a negative ion, that is an anion. This is just to get ourselves used to some of the terminology, but we're not done answering the question.

They say an atom of platinum has a mass number of 195 and contains 74 electrons. How many protons and neutrons does it contain, and what is its charge? We figured out its charge. We know the protons; by definition, platinum has 78 protons. But what about its neutrons?

Well, protons plus neutrons is going to be equal to our mass number. It's going to be equal to 195. So we have 78 protons plus, I'll say, n for neutrons is equal to 195. Subtract 78 from both sides, and what do you get? The number of neutrons is equal to 117.

Let's see if I subtract 78, I'll get 117 neutrons. So 117 neutrons, and we're done. But the big idea is—and we say atom when we have the same number of electrons and protons—that's when we are neutral. But as soon as we become non-neutral, we have either more electrons or more protons, and this is true of an atom or molecule.

We will then call it an ion, and if you want to be a little bit more specific, a positive ion is called a cation, and a negative ion is called an anion.