Worked example: calculating ion charge | High school chemistry | Khan Academy

So we're asked what is the charge of a calcium ion with 18 electrons. So pause this video and see if you can work that on your own. I will give you a little bit of a tip: a periodic table of elements might be useful to see where calcium sits on that periodic table of elements.

So why don't you pause this video and see if you can figure out the charge of that calcium ion?

All right, so what defines the element is actually how many protons it has, and that's what we have right over here. Its atomic number is 20. That's how many protons it has. So we could say the number of protons, which provide positive charge, is 20.

And then we know the number of electrons is 18. That's negative charge, so I'll just write it here, number of electrons. All right, I'll abbreviate it right over there or I'll shorten it; that is 18. And this has a negative charge.

So if you want to know the net charge, you take the number of protons, the positive charge, and subtract out the number of electrons. And so that leaves you with a positive two charge. Twenty minus eighteen is positive two, and we will denote that with a 2 plus.

So some people might write this as calcium two plus, just like that, to show that it is a calcium ion. It's likely the situation maybe where calcium originally had 20 electrons and 20 protons, so then it would not be an ion; it would just be a neutral atom. But maybe it lost those, it lost two of those electrons, and so then it got a positive two or a two plus charge.

Let's do another example over here. So if I were to ask you what is the charge of an ion that has seven protons, eight neutrons, and ten electrons, pause this video and think about what that would be.

Well, we can confirm that that indeed would be an ion because it has a different number of protons than it does electrons. And if you want to figure out the charge, you just take the number of protons, seven, which are the positive charges, and you subtract out the negative charges. That's why you're subtracting; you subtract out the electrons.

So seven minus ten would be equal to negative three. And so I would say you'd often denote that as saying a 3 minus charge. And if you wanted to write down what that ion is, we can go back to the periodic table of elements.

We can see that if you have seven protons, by definition, you are talking about nitrogen. So that would be a nitrogen ion that you would denote like that: it has a negative 3 or 3 minus charge.