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Resonance and dot structures

Video transcript

now that we know how to draw dot structures let's apply our rules to the nitrate anion and we're going to see that we can draw a few different dot structures for this anion and we're going to call those resonance structures of each other but first we need to calculate the total number of valence electrons and so nitrogen is in group five on the periodic table therefore five valence electrons oxygen is in group 6 therefore six valence electrons for each oxygen I have three of them so 6 times 3 is 18 valence electrons plus the 5 for the nitrogen gives me 23 and I have a negative charge right this is an anion here so we have to add one electron to that so 23 plus 1 gives us a total of 24 valence electrons that we need to represent in our dot structure so we know that nitrogen is going to go in the center because oxygen is more electronegative so nitrogen goes in the center nitrogen is bonded to three oxygen so I can go ahead and put them in there like that and let's see how many valence electrons have we represented so far 2 4 & 6 therefore 24 minus 6 right gives us 18 valence electrons left over we're going to put those leftover valence electrons on our terminal atoms which are our oxygens and oxygen is going to follow the octet rule currently each oxygen has two valence electrons around it the ones in magenta so if each oxygen has two each oxygen needs six more to complete the octet and so I go ahead and put six more valence electrons on each one of my oxygens now each oxygen is surrounded by eight electrons so the oxygens are happy we have we added a total of six valence electrons to three oxygens so 6 times 3 is 18 so we've used up all of the electrons that we need to represent and so this dot structure is ov R it has all of our valence electrons here oxygen has an octet so oxygen is happy but nitrogen does not have an octet if you look at the electrons in magenta there are only six electrons around the nitrogen and so the nitrogen wants to get to an octet and there are a couple different ways that we could give nitrogen an octet for example we could take a lone pair of electrons from this top oxygen here and move them into here to share those electrons between that top oxygen and that nitrogen so let's go ahead and draw the resulting dot structure so we would have our nitrogen now with a double bond to our top oxygen our top oxygen had three lone pairs of electrons but now it has only two because the electrons in green moved in to form a double bond this nitrogen is bonded to an oxygen on the left here on the bottom left and an oxygen on the bottom right here so this is a valid dot structure right we followed our steps and we'll go ahead and put this in brackets and put a negative charge outside of our brackets like that so that's one possible dot structure but we didn't have to take a lone pair of electrons from the top oxygen we could have taken a lone pair of electrons from the oxygen on the bottom left here so if those electrons in blue moved in here we could have drawn another dot structure which would have been equally valid right we could have shown this oxygen on the bottom left now bonded to this nitrogen and it used to have three lone pairs now it has only two and now this top oxygen is still a single bond with three lone pairs around it and this bottom right oxygen is still a single bond with three lone pairs around it so this is a valid dot structure as well let's go ahead and put our brackets with a negative charge and then of course we could have taken a lone pair of electrons from the oxygen on the bottom right so I could have moved these in here to form a double bond and so now we would have our nitrogen double bonded to an oxygen on the bottom right the oxygen the bottom right now has only two lone pairs of electrons the oxygen at the top single bond with three lone pairs and in the same situation for this oxygen on the bottom left and so this is once again another possible dot structure and so these these are considered to be resonance structures of each other and the way to represent that would be this double-headed resonance arrow here and I think when students first see resonance structures the name kind of implies that the in this case the ion is resonating back and forth between these three different possible equally valid dot structures and that's not quite what's going on here each of these dot structures attempt to represent the structure of the ion but they're really not the best way of doing that you need to think about combining these three dot structures in a hybrid so a resonance hybrid of each other and so let's go ahead and draw just a simple representation of a way of thinking about a resonance hybrid right so if I kind of combined all three of my dot structures here into one picture right I had a double bond to one oxygen in each of my three resonance structures here and so the top oxygen had double bond and one of them the bottom left and in the middle one and the bottom right and the third one but so in reality if we take a hybrid of all of those things we can think about the electrons being delocalized or spread out among all three of our oxygens and so instead of giving our top nitrogen oxygen instead of making that a double bond right we can just show some electrons being delocalized in that area so so stronger than a single bond but not as strong as a double bond and we could do the same thing between this nitrogen and this oxygen so the electrons are delocalized a little bit here is not a double bond it's not a single bond and the same idea for this nitrogen oxygen in here and one way we know that that the ion looks more like this hybrid is because of bond length when the ion is measured in terms of the bond length all the nitrogen and oxygen bonds are the same length and of course if we thought about one of these resonance structures as being the true picture of the ion let's say this one for example that wouldn't be the case for this on because this double bond here right we know that would be shorter than one of these single nitrogen oxygen bonds and so it's actually more of a of a hybrid with the electrons delocalized throughout and that's the idea of resonance structures here