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Worked example: Lewis diagram of the cyanide ion (CN⁻)

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Video transcript

in this video we're going to try to get more practice constructing Lewis diagrams and we're gonna try to do that for a cyanide anion so this is interesting this is the first time we're constructing a Lewis diagram for an ion so pause this video and see if you can have a go at that alright now let's do this together so we've already seen in many videos the first step is to essentially count the total valence electrons that we're dealing with and the reason why we do that is to make sure that we're allocating all the valence electrons to help us there we can look at a periodic table of elements you might already know that carbon has one two three four valence electrons in that second shell it's in the second period so you have four valence electrons from carbon nitrogen has one two three four five valence electrons in its second shells in that second period and so the valence electrons from a neutral carbon and a little nitrogen free atom would be a total of nine valence electrons but we are not done yet because this is not a neutral molecule we have a negative charge here it is an anion it has a negative one charge and so because of that negative one we can think about it having an extra valence electron so let's add a valence electron here why did we do it because of this negative charge so we're dealing with a total of 10 valence electrons now the next step is to try to draw single bonds try single bonds and identify a central atom now we only have two atoms here so really neither feels a central so let me just put a carbon and a nitrogen next to each other here and then let me draw one single bond so by drawing that one single bond I have now accounted for two valence electrons so now I am left with eight valence electrons and so that's the next step allocate remaining valence electrons allocate valence electrons so let me start with the more electronegative let's try to get nitrogen to eight already has two so let's give it three more lone pairs so we have two four six-eight so I have just used up six of these remaining valence electrons so minus six so I have two left to allocate so let me give carbon two valence electrons like that and there I have used up all of my all of my valence electrons now let's see how happy everyone is nitrogen has eight valence electrons hanging around two four six eight but carbon only has four two and four so this is where we think about whether we want to have some extra bonds extra bonds or higher-order bonds so how can we give carbon more valence electrons well what we could do is we could take some of these lone pairs around nitrogen and then use them turn this single covalent bond into a higher-order bond so let's see if we were to take these two and turn it into another covalent bond what is going to happen let me erase all of these and then I'll just draw I'll just draw another covalent bond so nitrogen still has eight electrons hanging around carbon now has six so maybe we can do that again so let me erase these two characters let me erase these two characters and make another covalent bond out of them so let me make a covalent bond out of them and so now what's going on carbon has two four six eight valence electrons hanging around nitrogen has two four six eight valence electrons hanging around so this is looking pretty good but are we done yet the simple answer is no we still haven't represented this negative charge in our Lewis diagram the way that we would do that is say hey this entire molecule you put brackets around it has a negative charge and now we're done we've allocated all of our valence electrons we have our octet rule on all of our atoms that are not hydrogen there's no hydrogen here and we're showing that this indeed is an anion and now we are done