Drawing the Lewis Structure for SCN-

Viewing Notes:

  • SCN- is one of the more challenging Lewis structures to work with.
  • You'll need to calculate the formal charges to decide which is the best Lewis structure for SCN-.
  • Note that both Lewis structures show in the video have formal charges of -1. The difference is the negative is on the Sulfur (S) atom in one structure and on the Nitrogen (N) atom in the other. Because N is more electronegative it makes more sense for it to have the negative charge. As a result the SCN Lewis structure with the N having a negative one formal charge is the best structure.
  • The Lewis structure for SCN- has 16 valence electrons.

Transcript: This is Dr. B. We're going to look at the SCN- Lewis structure. It's a bit more of a challenging structure, but it does incorporate some of the big ideas about Lewis structures and why we draw them for a molecule. So each of the Lewis structures for SCN- that are drawn here, and they're resonance structures--each of these uses all of the 16 valence electrons that we started with. The formal charges for each one add up to negative 1, and that makes sense because we have a negative 1 up here. Let's take a look at the formal charges for each one of these and see what's going on--which one's going to be the best Lewis structure for SCN-.

So let's do this one first, here. So for the Sulfur on the periodic table, 6 valence electrons. Then we look at our Lewis structure. Nonbonding, there are 6 nonbonding. Then we have 2 involved in a chemical bond. We'll divide that by 2. Six minus 6 minus 1; Sulfur has a negative one charge. The Carbon and the Nitrogen, they both have formal charges of zero. This makes sense, because if you add the formal charges up, you'll see that you have a negative one charge for this molecule, and this negative up here. That makes a lot of sense.

Now let's look at the next Lewis structure here for SCN-. Let's look at this one here and calculate the formal charges. So for this Lewis structure right here for SCN-, we can see that the Sulfur and the Carbon both have formal charges of zero. The Nitrogen, on the periodic table it's in group 5, so it has 5 valence electrons. These right here and here are nonbonding, so 2 plus 2 is 4 nonbonding. And then we have 2 plus 2 again; 4 bonding electrons which we divide by 2: it has a -1 charge. So at this point, we see that both structures are pretty much identical. The only difference is, over here the Sulfur has a -1 charge. Over here, it's the Nitrogen that has the -1.

So we need to make a decision about which is going to be the most likely molecule for the SCN- ion to exist. There's really not a big difference between the two structures that we can see here other than that -1 charge on the Nitrogen. That actually is going to make the difference. Nitrogen, it makes more sense for it to have a -1 charge than it does for Sulfur because Nitrogen's more electronegative. Because of that, this right here is going to be the more likely Lewis structure.

So in a sample, it's more likely that you'll find this compound right here, this resonance structure of the SCN-. but it's not going to be a big difference. They're so similar, and we're only looking at an electronegativity difference here. There'll be a lot of this in the sample, but the majority will be this SCN- ion right here.

This is a good structure to look at and think about because it incorporates a lot of those big ideas like looking at formal charges, resonance structures, and electronegativity. So go back and look over this again. Try to solve it for yourself and make sure you understand these concepts. This is Dr. B., and thanks for watching.