Lewis Dot Structure: Molecular Geometry (Review)
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Lewis Dot Structure: Molecular Geometry (Review)


In screencast we want to draw the lewis dot structures of water and ammonia, and use those to understand something about the molecular structure of those molecules So let’s start with water, and determining the lewis structure the first step is to count the number of valence electrons and so in hydrogen, we have only a single valenece electron in each hydrogen atom, mulitply that by two for the two hydrogen atoms, for oxygen we have six valence electrons. And that gives us a total of 8 valence electrons Alright, we use the lewis dot structure to figure out how to distribute those electrons, so we start with a central atom which can never be hydrogen in these diagrams since hydrogen can only form one bond Alright, we surround that oxygen atom with hydrogen atoms. To that central oxygen atom we draw one bond to each of the species that’s involved in this structure Again, once we draw one bond to hydrogen that completes it’s electronic shell since it just has an s-orbital and can only accomidate two electrons We’ve distributed four of the electrons in these bonds out of the total of eight, and so we need to distribute four more and those are distributed in terms of two lone pairs on the oxygen atom. So this is a simple lewis rendering of the structure, but of course these different species occupy three dimensional space, and in general, will be repelling eachother, the lone pairs and the various bonds since they all contain electrons, and electrons will electrostatically repel eachother, and so we know that this will adopt a strained tetrhedron type structure, where the lone pairs will occupy two branches of the tetrahedron and then the the other two will be from hydrogen atoms that I’ve tried to show projected into and out of the screen here And so if you ignore the lone pairs, we see then that a water molecule adopts a bent type structure rather than a linear structure due to the replusion between the different substituents of the oxygen atom whether they be bonds to hydrogen atoms or the lone pairs. We can do the same analysis for ammonia nitrogen has five valence electrons, we then get three times one for the hydrogen atoms which give us a total of eight valence electrons, just like in water in this case though, the electronic structure picture is a little bit different, and we around the nitrogen with hydrogen atoms, we again start by drawing the bonds to each of the components. This saturates all of the hydrogen atoms they each have two electrons now that are shared with nitrogen, we’ve distributed six of the eight electrons and the last two electrons have to go in the nitrogen in terms of a lone pair. Again, this kind of planar structure does not minimize the repulsion between the electrons that are participating in this bond, and so instead We adopt a tetrahedral type configuration Where we have hydrogen atoms that form trigonal pyrimidal type structure wit hthe nitrogen, and then the lone pair projecting up from the nitrogen. So we can see how analysis of these simple molecules in terms of their lewis dot structure helps us explain why they take the shapes that they actually take instead of what you might imagine just from the very simple molecular formula where you migh expect water would be linear and that ammonia would be triangluar. In order to reduce repulsions with their lone pairs, they actually adopt tetrahedral type geometries and that accounts the shapes that they have

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