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Video transcript

so we have the formula for an ionic compound here and the goal of this video is what do we call this thing it clearly involves some cobalt and some sulfur but how would we name it well the convention is is the first element to be listed is going to be our cation and if we look at cobalt over here we see that it is a d-block element and d-block elements are tricky because you don't know exactly how it will be our cation it's going to be our positive ion but we don't know what the charge on each of those cobalt is actually going to be so now let's look at the anion let's look at the sulfur or as an anti on the sulfide so let me underline that and on the periodic table we see sulfur is out here that it would that its group it would want to gain two electrons in order to have a complete outer shell it's just like oxygen it wants to gain two electrons so the sulfide anion will look like this so it will have sulfur when it is will have a two minus charge just like oxygen just like everything else in this group it would want to gain one two electrons so that its outer shell looks like that of a normal noble gas looks like that of argon so we can use this as a clue to figure out what must be the charge on the cobalt because we have three of the sulfides we have each of the sulfides has a mate two minus charge and we have three of them so that's going to give us a six minus charge all in and then the cobalt the cobalt we have two of them and so these two cobalt have to offset these this six minus charge they have to have a six plus charge well that means that each of them need to have a three plus charge if each of these have a three plus charge and you have two of them that you're going to have six plus on the positive side and you're going to have six minus from the sulfides and the reason why this is useful for us is now we can name this we would call this we would call this ionic compound cobalt cobalt three cobalt and you would write with Roman numerals here cobalt three sulfide cobalt three sulfide now I know what you might be thinking hey when we look at other ionic compound I didn't have to write the charge of the cation there and the reason why the convention is to do it here and then actually let me I don't have to but I don't have to write in an uppercase there so let me rewrite it is cobalt cobalt three sulfide the reason why I wrote that three in this case is because the cobalt can take on multiple charges it's known as polyvalent poly valent if something is in Group one you know it tends to have a positive one charge Group two tends to have positive two charge if it's a halide tend to have negative one charge but these metals here in this D block here they could have they could be ionized in multiple ways and so that's why we have to figure out what the charge of the cobalt is and we write it here in the name we would call this cobalt three sulfide you can have other other ionizations of cobalt and other ionic compounds but this one is cobalt three sulfide and you could go either way you could say okay if the cobalt has a charge of three plus you can figure out its formula right over here or as we just did in this video you could go from the formula to the actual name