In the cyanate ion, OCN-, carbon is the central atom. (b) Which resonance structure makes the greatest contribution to the resonance hybrid? Which makes the least contribution? Explain.

Video transcript

welcome back everyone. We need to consider the ion carbon is essential atom and its resonant structures identify the structure that contributes the most and the structure that contributes the least to the resonance hybrid justify your answer. So we're going to begin by drawing out our structures of thigh assassinate by first calculating total valence electrons. And so beginning with our atom of sulfur, we recognize that it being in group six A corresponds to six valence electrons where then we have carbon, which we recall is located in group four A. On the periodic table corresponding to four valence electrons. And then nitrogen being in group five A. On the periodic table will correspond to five valence electrons where we have this minus one charge which tells us that we gain one electron. And so we would add one electron here for our total of valence electrons equaling 2 16 electrons total. And so now for our structure we want to recall bonding preference. So we would have carbon in the center, it's surrounded by nitrogen and sulfur. Where carbon we want to recall should have four bonds. So we would place four bonds where we have a triple bond between carbon and nitrogen. And so now we have our carbon happy and stable. But now we're calling that nitrogen has a bonding preference of having three bonds and one lone pair on itself. We would add one lone pair here and then going to our sulfur atom. We stated that it has six valence electrons and right now it only has one of those six valence electrons in a co violent bond directly attached to itself. And so we would need to add loan pairs where we have three sets or yes, three sets of lone pairs on our sulfur atom which contributes to its formal charge of minus one. Recall that we calculate formal charge by taking the group number of that atom subtracted from the bonds on that element subtracted. Or sorry, added to non bonding electrons. And so for the formal charge of sulfur, we would prove that it's minus one by taking its group number being in group six. A subtracted from the bonds on it being just one bond added to the non bonding electrons being a total of 246 non bonding electrons. So plus six, we would have six minus seven, which is why we have a formal charge of minus one for our sulfur atom here, as we stated. And so now that we have this structure written out, we're going to move this below. And sorry, wrong thing. We're going to move this structure below so that we have some more room so that we can compare it to our resonance structures which are going to form from our loan electrons, causing our bonds to shift. So if we were to shift one of our lone pair of electrons from our sulfur to a bond between carbon and sulfur, this would now create a total of five bonds on carbon, which carbon would not allow. And so this bond here on carbon would have to move as a lone pair to nitrogen. And this would create our first resident structure where we would have a double bond between carbon and sulfur, where carbon is then double bonded to nitrogen and nitrogen now has two sets of lone pairs on itself, contributing to now a formal charge of minus one on nitrogen. Based on our calculation for formal charge. And so this leaves a total of two lone pairs on sulfur, where it now has a neutral formal charge because it has its total of six electrons directly attached to itself, where to our shared in a covalin bond between carbon. And we can actually continue and form a second resident structure where we would also move our second lone pair from sulfur as a bond between carbon and sulfur, where carbon again would not be happy with that additional bond. So it would transfer one of its bonds to nitrogen as another lone pair and let's go ahead and draw that going in this direction. And so now to create our next resident structure, we have sulfur triple bonded to carbon now where it now just has one lone pair on itself and our nitrogen atom is now going to have a total of three lone pairs on itself, increasing its formal charge to now minus two, where sulfur will now have a formal charge of plus one and we know that because we can count directly attached to it +12345 valence electrons where it would prefer to have six as we stated. And because it's missing one, it has that formal charge of plus one. Now that we have our resonance structures drawn out, we want to recall that the larger the atomic radius of an atom will correspond to a greater ability to carry a negative charge. We can also think of election negativity where we would see that the greater the election negativity of an atom will correspond to a greater ability to carry a negative charge. So we'll just put an equal sign because the same statement applies there. And so we would confirm to completely enter this prompt that the structure that contributes the most is going to be structure A. Will label this a structure A will label this as structure B and this third resident structure as structure see So structure they will say contributes the most. And we know that because not only does it have a total minimum formal charge of minus one from our sulfur atom here, but the minus one charges on our atom of sulfur. Because we would recognize that sulfur on the periodic table is the most election negative in comparison to carbon and nitrogen. Because we recall that electro negativity on the periodic table increases as we go towards the top right of our periodic table. And so we would say that also based on atomic radius sulfur is also a larger atom compared to carbon and nitrogen. So it does have more of an ability to handle that negative charge. So this is also based on increasing atomic radius trend on our periodic table. So to further justify our answer, we would say that structure A contributes the most because sulfur or rather will say because it bears minimum formal charge and sulfur is larger compared to carbon and nitrogen. So it handles it's negative formal charge the best and as far as our structure that would contribute the least to the resonance hybrid. We would confirm that structure C contributes the least. And we can see that it does because it has a net formal charge of -2, which we can see is attached to this nitrogen atom here where apologies for not drawing that bonding, but we should have a single bond between carbon and nitrogen there. And as we stated, it will contribute the least to residents because it bears the most formal charges on our atoms. And nitrogen is a smaller adam Compared to carbon and sulfur based on its position being in group five A. So it's not favorable to carry a negative charge for that nitrogen atom. And so these two statements as well as our structures A. N. C. Would be our final answer to complete this example as the resident structures that would contribute most and least. And the justifications for why for this prompt, I hope that everything I reviewed was clear if you have any questions, please leave them down below and I'll see everyone in the next practice video.