 Valence shell electron pair repulsion, or VESPA, is a model used to predict the shape of molecules based on an understanding of subatomic and molecular structure. Alright, that's cool. But what does that actually mean? So we know about Lewis structures and how they look like these, and that they're a great way to understand how each atom in a molecule is bonded to each other. However, they can only give us a limited understanding for what the molecule actually looks like in a 3D space. Think of it like what sort of information you'd know from this drawing, compared to this drawing. Well, from the simple one, you know structural information, what's connected to what, such as that the windows are in the walls and the roofs on top. However, you don't know how far back the house goes or how big the balcony is to the side of the house. So this model gives information about the structural and spatial components of the house, or molecule. So Lewis structures have been used to draw the relationships within a molecule since 1916, and they still are because they are a fantastic model, and they do convey a lot of information, and we'll still be using them throughout your chemistry career. They are drawn based on bonding pairs of electrons, so what's connected to what? And they also address lone pairs of electrons by drawing them in, but not always. So in 1940, Sidwig and Powell proposed that the shapes of molecules are dependent on the number of valence electrons, and then this was furthered in 1957 by an Australian scientist Sidney Niam and Canadian Ronald Gillespie. They looked at how these electrons repel each other, and how this would apply to the shape of a molecule. Lone pairs and bonding pairs of electrons repel each other, but they are still connected to each other. So in order to minimize this repulsion, they move as far away from each other as possible. And this is Vespa theory. The Vespa theory states that each atom in a molecule will achieve a geometry that minimizes the repulsion between electrons in the valence shell of that atom. So while the Lewis structure for ammonia, NH3, may look like this, the molecule doesn't lie flat, because the electron pairs could be further away from each other, like this. This is a tetrahedral shape because it looks like a tetrahedron. So this solid triangle indicates that the hydrogen's poking out of the page, so it's coming towards us, while the dashed line indicates that it's going into the page, so away from us. That's all for this video, but further on we'll have a look at how to get from Lewis structures to Vespa diagrams.