Worked example: calculating ion charge

0:00- [Instructor] 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. And 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.

0:20So 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. It's a atomic number is 20, that's how many protons it has.

0:40So we could say 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 electrons, I'll abbreviate it right over there or I'll shorten it. That is 18. And this has negative charge.

1:01So if you wanna 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 positive two charge, 20 minus 18 is positive two, and we will denote that with a two plus. So some people might write this as calcium 2+ just like that.

1:22To show that it is a calcium ion, it's likely a situation maybe where the 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.

1:43Let'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 10 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.

2:05And if you wanna 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 10, that would be equal to negative three.

2:20And so I would say you'd often denote that as saying a three minus charge. And if you wanted to write down what ion that is, once it's gonna go back to the periodic table of elements, we can see that if you have seven protons, by definition you are talking about nitrogen.

2:37So that would be a nitrogen ion that you would denote like that. It has a negative three or a three minus charge.