 So let's briefly go back and do a little Lewis structure revision. We're going to draw the Lewis structure for CHCl3, which is called trichloromethane or used to be known as chloroform. So first we've got to total up the valence electrons, four for carbon, one for hydrogen, three times seven for the three chlorines. That gives us a total of 26. Then we decide on the central atom. Carbon forms four bonds and hydrogen and chlorine usually form only one bond each, so carbon should be in the middle. I draw bonds to join the atoms together. That's four bonds, so that uses up eight of my electrons and I still have 18 left. Hydrogen now has a full outer shell. Being in the first period, it only needs two electrons to get a full outer shell. So I can't put any lone pairs on it. Each of the chlorines, however, only has access to two electrons at the moment from the bond, so they need some lone pairs to get a full outer shell. With 18 electrons to distribute, I have enough for six electrons on each of the chlorines, which gives each one a full octet. Now, this is fine for a 2D representation, but what does the molecule really look like? I mean, what shape is it? Is it a flat cross shape, like we've drawn it here, or do the atoms poke out of the page somehow? The Lewis model gives us no information about this. So if we want to go further, we need another model that will describe its shape.