Chemical Bonding: OCN^- Lewis Structure

Drawing the Lewis Structure for OCN^- (Cyanate ion)

Viewing Notes:

• OCN^- is a negative ion (called an anion).
• There aren't enough valence electrons available for each atom to obtain an octet without sharing more than one pair. Therefore, OCN^- has a triple bond between the Carbon and Nitrogen atom.
• While there are other ways the draw the Lewis structure for OCN^-, the one with the triple bond between the Carbon and the Nitrogen will have formal charges that make the most sense. In this case that means that the negative charge will be on the Oxygen because Oxygen is more electonegative than Nitrogen.
• For the OCN^- Lewis structure there are a total of 16 valence electrons available.

Transcript: This is the OCN- Lewis structure: the cyanate Ion. for the cyanate ion, we have a total of 16 valence electrons. That includes this negative up here. Carbon is the least electronegative; we'll put that at the center. Then an Oxygen here, and a Nitrogen over here. We'll put 2 electrons between atoms to form a chemical bond. Then we'll go around the outside, so we have 2, 4, 6, 8, 10, 12, 14, 16. We've used all our valence electrons at this point. Oxygen and Nitrogen have 8 valence electrons, so they're good. But the Carbon only has 4. We're going to need to share valence electrons from the outer atoms with the Carbon so it can have an octet.

The question is, do we share from the Oxygen or from the Nitrogen or both? Nitrogen is less electronegative than Oxygen. That means it's more likely to give up valence electrons, to share them. So let's move them from the Nitrogen. So let's take 2 from here and form a double bond. Now the Carbon has 6, so we need to move 2 more. Let's put those in there. So now Nitrogen, it has 8, but the Carbon also has 8. We're still only using 16 valence electrons.

When we calculate the formal charges, we'll find that the Oxygen has a formal charge of -1, Carbon is 0, and Nitrogen is 0. That makes a lot of sense: we have a -1 up here. The negative charge in our OCN- Lewis structure is on the most electronegative atom. So this looks like a pretty good Lewis structure. We could form double bonds between the Oxygen and then between the Carbon and Nitrogen. You'd see that that would work from an octet standpoint, but when we check our formal charges, we'd have a -1 on the Nitrogen. That doesn't make sense, because Nitrogen is less electronegative.

So this right here is the best Lewis structure for OCN-. I'm Dr. B., and thanks for watching.