Worked example: Finding the formula of an ionic compound
About this video
This worked example finds the formula of calcium bromide. Calcium is in Group Two, so it loses two electrons to form Ca2+, while bromine is a halogen that gains one electron to form Br-. Since the charges must cancel, two bromide ions are needed for every calcium ion, giving CaBr2.
You'll be able to
- use an element's group to predict the charge on its ion.
- explain why the positive ion is written first and what the -ide ending means.
- combine ions so their charges cancel to write an ionic formula like CaBr2.
Key words
- Ionic compound
- A compound made of positive and negative ions whose charges cancel out.
- Cation
- A positively charged ion, formed when an atom loses electrons.
- Anion
- A negatively charged ion, formed when an atom gains electrons.
- Alkaline earth metals
- The Group Two elements, which tend to lose two electrons.
- Halogen
- A Group Seven element such as bromine, which tends to gain one electron.
Transcript
- [Instructor] Let's now see if we can come up with the chemical formula for the ionic compound calcium bromide. And like always, if you are inspired, pause the video and see if you can come up with it on your own. All right, so the convention is that we write the positive ion first and so that's a pretty good clue that calcium is going to be the positive ion.
Now let's look at the periodic table to confirm that it's likely that calcium would ionize as a cation. Well, calcium is right over here in Group Two, and Group Two elements, also known as alkaline earth metals, they tend to ionize by losing two electrons and that's because they have two electrons in their outermost shell and they would like to lose them.
And so when calcium ionizes, it is going to be, it is going to ionize as Ca2+. Now, let's look at the bromide part. The -ide tells us that this is going to be a negative ion or it's going to be an anion. And if you look at where bromine sits in our periodic table, right over here, we see it is a halide.
We see that it likes to gain an electron and so it makes sense that it's going to be our anion. And so bromine would like to gain an electron to have eight electrons in its outermost shell. So, our bromide anion is going to look like this. It's going be to 1-.
It's gonna wanna gain an electron, that's what the elements in this group like to do. Now, what is the formula going to be, and remember, the key here is for an ionic compound, especially one that has, well, we don't see any net charge here, for an ionic compound, these things are going to cancel each other out.
The charge of the calcium cation is going to cancel out with the bromide anion. So how is that going to happen? Well, have you 2+ here, you only have 1- here, so you're gonna have to have two bromides for every of the calcium ions. So this is going to be, for every one of the calciums, you're going to have two bromides.
So it's going to be like this, Br2, and there you have it, that is the chemical formula for calcium bromide. And how did we know that we have two bromides for every calcium? Well, because when calcium ionizes, it's going to be 2+, it's a Group Two element right over here.
And bromine only gets a -1 or a 1- charge, so you're gonna need two of the bromides for every one of the calciums.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.