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Why do we ignore the lone pair for naming the molecular geometry? One way to look at it is the fact that electrons are infinitely smaller and lighter than nuclei and when looking on modern microscopes, we don’t see them. The lone pair on the nitrogen is important and if it wasn’t there, we would have a hypothetic molecule with a flat/planar geometry: For methane (CH 4), it is tetrahedral and for ammonia (NH 3), it is trigonal pyramidal. However, their molecular geometries are different. This corresponds to a tetrahedral electron geometry: The last two molecules in the examples above (CH 4 and NH 3) are both tetrahedral. This is why we need to identify the Electron and Molecular Geometries.įor the Electron Geometry, we treat the atoms and electrons equally. Notice that the last two molecules have the same steric number (4) but a different number of atoms and lone pairs. Whether it is a single, double or a triple bond, it is atoms + lone pairs for any bond type. However, if you use this formula, you don’t need to worry about the types of bonds. You might have a different formula for the steric number that involves the number of bonds. The sum of the number of atoms and lone pairs is called Steric Number (SN): In total, there are four units around the oxygen in water: This assumes that you already know the Lewis structures, so if you don’t check, this article. For example, in the Lewis structure of water, we can see that it has two atoms and two lone pairs of electrons. However, in actual molecules, they can be atoms or lone pairs of electrons. In the model demonstration above, we said that the blue spheres represent atoms. There can be more than four atoms, however, it is never the case for the carbon, and that is why we won’t get to those since this post is tailed toward organic chemistry. However, the tetrahedral geometry is a better alignment since the angle between the groups is 109.5 o:Įach of these geometries that we have discussed has a name: If you have never heard about the tetrahedral geometry and thought it was 90 o, that is fine, we all did when first introduced to this topic. What do you think it is when there are four atoms connected to the center? When there are three atoms around the central unit, the optimal angle is 120 o: In this case, putting them across at 180 o allows for achieving the optimal geometry: An orientation that puts them as far away as possible with the same angle between all. Keeping in mind that the blue atoms repel each other, propose an optimal geometry for them. The red sphere represents the central atom and the blue ones are connected to it: Let’s use this model examples to illustrate how this works. It is the keyword and all you need to grasp this concept is keeping in mind the idea that atoms want to stay as far apart from each other as possible because of the repulsion between the electrons on them. First, what does VSEPR stand for? It is the Valence Shell Electron Pair Repulsion theory. In today’s post, we will discuss the VSEPR theory which helps us understand the geometry of molecules.
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