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Worked example: Finding the formula of an ionic compound

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Video transcript

let's now see if we could 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 could come up with it on your own alright so the convention is is that we write the positive ion first and so there'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 2 and group 2 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 they would like to lose them and so when calcium ionizes it is going to be it is going to ionize as CA 2 plus 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 bro means it's 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 is going to 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 to be one minus it's going to want to gain an electron that's what these 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 we're going to have no these things are going to cancel each other out the charge of the calcium anion the calcium cation is going to cancel out with the bromine there was a bromide anion so how is that going to happen well you have two plus here you only have one minus here so you're going to have to have two bromides for every of the calcium ions so this is going to be you're going for every one of the calcium's you're going to have two bromides so it's going to be like this B are two 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 two plus it's a group two element right over here and bromine only gets a negative 1 or 1 minus charge so you're going to need two of the bromides for every one of the calcium's