Drawing the Lewis Structure for PCl4-
Transcript: Hi, this is Dr. B. Let's do the PCl4- Lewis structure. Phosphorus has 5 valence electrons. Chlorine has 7, but we have four Chlorines; and we have this negative sign up here. That means it's an ion, and that means we have an extra valence electron so we need to add the extra valence electron here. Five plus 28 is 33, plus 1 is 34 valence electrons. We'll put the Phosphorus at the center, and then we can put the Chlorines around the outside here.
Let's start by connecting each of the Chlorines to the Phosphorus with a chemical bond. Each of these is two valence electrons. So we've used eight valence electrons so far. Now we'll start going around the outside and put valence electrons on the Chlorines to give them eight, or an octet. So we have 2, 4, 6, 8, 10, 12, and 32. That means we have two more, and all the Chlorines have octets. So let's stick those last two right here on the Phosphorus in the center.
So at this point, all of the Chlorines have eight valence electrons, and we're counting that there. These lines count as two. So each one has 2, 4, 6, 8. So they have octets. The Phosphorus has more than 8. It has 10 valence electrons, but that's OK because Phosphorus is in period 3, or row 3, of the periodic table, and it can have more than eight valence electrons. So we've used all of our valence electrons, our structure is good, so this is the Lewis structure for PCl4-.
You could check the formal charges if you want. You'd see that they would be zero, and that makes this the best Lewis structure for PCl4-. This is Dr. B., and thanks for watching.