Chemical Bonding: C₂H₂ Lewis Structure

Drawing the Lewis Structure for C₂H₂ - Ethyne or Acetylene

Viewing Notes:

• With C₂H₂ you are going to run out of valence electrons and will have to share more than one pair of electrons between the Carbon atoms.
• Remember that Hydrogen (H) atoms always go on the outside of a Lewis Structure.
• Note that Hydrogen only needs two valence electrons to have a full outer shell.

Transcript: OK, let's try the Lewis structure for C2H2: ethyne. We have it right here. You can see the Hydrogens on the outside, two Carbons in the middle. What's all this? We'll see in a minute. So, first thing we need to do, count those valence electrons for C2H2. We have a periodic table. Carbon is in group 14, or 4A, so it has 4 valence electrons. Carbon has 4. Over here, Hydrogen, in group 1A or 1, that has 1 valence electron, but since I have two Carbons, let's multiply that by 2; and I have two Hydrogens, multiply that by 2, so that equals 8, that equals 2; I add them up, I have 10 valence electrons to work with in this Lewis structure here.

So with 10 valence electrons, let's see where we go next. Put the least electronegative atom at the center. But Hydrogen always goes on the outside. So here it's pretty easy. We've got Carbon, Carbon, and Hydrogen has to go on the outside. All right. Assign two electrons to each bond. Let's put two here, let's put two here, two here, so we've used 2, 4, 6. So we've only got 4 valence electrons left to work with. So we've assigned two electrons to each bond. Now we need to complete the octets on the outside atoms. In this case, Hydrogen right here is already full. It's the exception. It only needs 2 valence electrons to be full, to have a full outer shell or octet. Everything else needs 8, but Hydrogen needs 2.

So we've got 4 valence electrons. Let's put the remaining electron pairs on the central atom, or in this case, atoms. So I need to get 8 around each one. So there's 2, 2. So i've used all my valence electrons, Hydrogens are fine. Two, 4, 6, 8, that Carbon's fine. But this Carbon right here only has 2, 4 valence electrons. So its octet is not complete. It's not a stable molecule that way. So we need to figure out something, somehow to get 8 valence electrons around this Carbon, this Carbon, and keep the Hydrogens full.

If the central atom doesn't have an octet, what we'll need to do is pull some electrons in and share them, to make each atom have more electrons. So let's take these two right here, and we'll put them right here. So now Hydrogens are fine. This Carbon has 2, 4, 6, 8, because it's sharing, right, and this Carbon has 2, 4, 6. Not quite there yet. Let's move another pair in. Let's take those, put them right here. I'm going to make these a little darker.

And now Hydrogens are fine. This Carbon has 2, 4, 6, 8; it has an octet. And this Carbon has 2, 4, 6, 8; it has an octet. And if we count them all up, we have 2, 4, 6, 8, 10 valence electrons, which is what we had to start with, and we've written the Lewis structure for C2H2. We can clean it up a little bit to make it easier to see here. And you notice all those valence electrons being shared in the center.

We could write this as, using these lines. Each line represents a pair of electrons. That's one way to do it. And if you come back to our structure, now you can see these are the single bonds right here. And this triple bond, 1, 2, 3, in the center, holds the two Carbons together—bonds those together. That's the Lewis structure for C2H2, ethyne.