Lewis structures are a good representation of the structure of a molecule in 2 dimensions. To visualize the structure of a molecule in 3 dimensions however, we need to introduce a model known as the valence-shell electron-pair repulsion (VSEPR) theory.
Tutorial - Molecular Shapes
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VSEPR (often said “vesper”) Theory is based on the idea that electrons really don’t like each other very much. They all are very negative individuals and therefore repulse each other. In terms of structure, this means that the bonds between atoms which are full of electron density will space themselves out as much as possible to avoid electron-electron interaction. So for example if you have a hypothetical molecule with a central atom and two peripheral atoms:
The peripheral atoms will locate themselves (and their bonds) as far away from each other as possible which results in a very linear molecule. In a similar manner, a central atom with 3 peripheral atoms will see those 3 bonds spaced as much as possible. If you think about this for a minute you can probably predict what this will look like.
As we continue to add peripheral atoms the structures become more complicated but are still based on the bonds being as spaced out as the structure will allow. The table below shows each of the possible structures based on the number of regions around the central atom. Note that lone pairs and double and triple bonds are treated in the same manner as single bonds when it comes to repulsion and the structure that forms:
When we talk about structure and specifically VSEPR we often hear the structures referred to in two different manners: Electron Pair Geometry and Molecular Geometry or Shape.
At first these two ways of describing the structure seem exactly the same and in fact, sometimes they are. But the Electron Pair Geometry (EPG) refers to where the electrons are in space around the central atom. This means that in each of the structures above, the first structure containing all of it bonds and the last structure shown with mostly lone pairs has the exact same EPG. The Molecular Geometry (MG) or Shape changes every time a lone pair replaces a bond. The reason for this is that the lone pairs cannot be "seen" by our current technologies. We know they are there because of our electron counts and the MG that we can see. For example, if you look at the T-shape structure, you notice that there are two lone pairs in the five bonding regions around the central atom. Those lone pairs are holding the molecule in the T-shape. If they were not there, then the structure would move into a trigonal planar structure, right? Trigonal Planar would be the lowest energy (most spaced out) configuration of a molecule with 3 bonds. But since those other two pairs of electrons are there, it maintains the T-shape instead.
Example
The structure of ClF3
The EPG for this molecule is Trigonal Bipyramidal and the MG is T-Shaped.
Intro to VSEPR
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