 what we've just looked at. Methane, ammonia and water are all based on the tetrahedral geometry, but depending on how many bonds and how many lone pairs are on the central atom, the true shape of the molecule can be either tetrahedral, trigonal pyramidal or bent. Our VACPR theory has a shorthand that allows you to summarize the bonds and lone pairs around the central atom and therefore to see more easily what the molecular shape will be. This is the AXE notation. A represents the central atom, the X's are surrounding atoms and the E's are lone pairs. So try this. Which of these three VACPR shorthand notations on the left matches with each molecule on the right? See if you can pause the video and work it out. So methane with four bonds around the central atom and no lone pairs is denoted AX4E0. Ammonia with three bonds and one lone pair is denoted AX3E1 and water with two bonds and two lone pairs is AX2E2. So let's summarize. When you're drawing a VACPR structure, first of all draw out its Lewis structure. It's really important. Secondly, work out its shape. Count the number of electron groups, bonds and lone pairs on the central atom. Work out the geometry that it's based on and then the actual shape. And then finally if you can work out the bond angles. Start from the usual bond angle for that geometry and then account for lone pairs and double bonds which exert a greater repulsion than single bonds. Sometimes you may not be able to give an exact bond angle but you might be able to say I know that this bond angle is going to be say less than 109.5 or less than 120 degrees. As with any model, VACPR has its limitations. So it works for a certain number of molecules but there are also plenty of molecules for which VACPR does not correctly predict their shape. But this doesn't mean that the VACPR model is worthless. It works very well for a large number of molecules and that's why we learn it. Okay so here's your task.