Drawing the Lewis Structure for CN- (Cyanide ion)

Viewing Notes:

  • CN- is a negative ion (called an anion). It is highly poisonous.
  • There aren't enough valence electrons available for each atom to obtain an octet without sharing more than one pair. Therefore CN- has a triple bond between the carbon and oxygen atom.
  • For the CN- Lewis structure there are a total of 10 valence electrons available. C has 4, N has 5, and the negative sign indicates an additional valence electron.

Transcript: Hi, this is Dr. B. Let's do the Lewis structure for the cyanide ion. We know it's an ion because it has a charge up here. In this case, it's a negative charge so it's an anion. Let's count the valence electrons. On the periodic table, Carbon's in group 4, sometimes called 14; and Nitrogen is in group 5 right next to it, so it has 5 valence electrons; and this negative means we have an extra valence electron to work with. Let's add that in there. We have a total of 10 valence electrons to work with.

So let's draw our Carbon and our Nitrogen right here, and let's take and put the electrons around--let's do a bond first, so there's 2 electrons. Now we've bonded the Carbon and Nitrogen together. We've used 2, let's put them around here, 4, 6, 8, 10, and we can check for octets. Nitrogen's fine, it has 8 valence electrons. Carbon only has 4. Let's take and move these electrons right here. Let's put them in the center. And now Carbon has 6, that's better, and Nitrogen, it still has 8 (2, 4, 6, 8), so it's sharing, it keeps those 8. Let's put two more in and see what happens. Nitrogen: 2, 4, 6, 8. Carbon also has 8.

So that's the valence structure for CN-, the cyanide ion. (Clean this up a little bit.) You could also write it in the structural formula and that would look just like this right here, where your three pairs of electrons are represented by three straight lines.

This is Dr. B., and thanks for watching.