Chemical Bonding: POCl₃ Lewis Structure

Drawing the Lewis Structure for POCl₃

Viewing Notes:

• In the POCl₃ Lewis structure Phosphorus (P) is the least electronegative so it goes in the center.
• In the Lewis structure for POCl₃ there are a total of 32 valence electrons.
• If you check the formal charges for POCl₃ you'll find that you need a double bond between the Phosphorous and Oxygen atom in order to have the formal charges equal zero.

Transcript: Hi, this is Dr. B. Let's do the POCl3 Lewis structure. On the periodic table, Phosphorus in group 5 or 15, 5 valence electrons; 6 for the Oxygen; and then 7 for Chlorine, we do have three Chlorines. If we add them up: 5 plus 6, 11; plus 21: 32 valence electrons. We'll take and put the Phosphorus at the center. It is the least electronegative. Put an Oxygen up here, and then Chlorines around the Phosphorus.

So we have 32 valence electrons. Start by forming chemical bonds; a pair there, a pair there; so we've formed the bonds. We've used eight. Let's go around the outside: 10, 12, 28, 30, and 32. Each of the atoms has eight valence electrons around it. We've used 32 that we started out with. So this looks pretty good. Everything has eight valence electrons, the octets are full; we've used 32 total valence electrons, which is what we started out with.

So this is a pretty good structure. We should, however, check our formal charges. Phosphorus is in period 3 of the periodic table. It can have more than eight valence electrons. So let's see what that looks like. Looking at the formal charges for Phosphorus, on the periodic table, Phosphorus has 5 valence electrons. Up here, we have every pair involved in bonds, that's going to be zero. And we have 2, 4, 6, 8 bonding valence electrons, which we'll divide by two. Which gives us a formal charge of plus one for the Phosphorus. When we calculate the Chlorine, we find that that is zero, and the Oxygen has a -1 formal charge.

So this might not be the best structure for POCl3. We see this plus one on the Phosphorus, and then we have a negative one on the Oxygen. We know we can move these two valence electrons to form a double bond right here with the Oxygen. So by moving the two valence electrons here into the center, we've formed a double bond, and you can see that the formal charges for each of the atoms is now zero.

So by forming that double bond, and having our formal charges all at zero, this Lewis structure for POCl3 is actually a better structure; a more stable structure than the one we previously had. So this is Dr. B., and thank you for watching.