(Polar molecules, Non-polar molecules, etc.)
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I know this may be a stupid question, but how can you determine the shape by doing the Lewis Structures. I understand you can see all the bonds and the lone pairs but I am just confused because I'm afraid that if I draw it out that I won't be able to determine the shape from that.
When you draw the lewis structure, you look at the central atom and count how many atoms it's attached to and how many lone pairs it has. This will give you the AXE notation (A = central atom, X = # of attached atoms, E = # lone pairs). Each AXE notation corresponds to a certain shape that I think we just need to memorize.
I think the point is to show you the lone/bonding pairs so that you can find the VSEPR formula (Ex. CCl4 is AX4 which would tell you that its shape is tetrahedral). I also remember Prof. Lavelle saying that if you put atoms symmetrically around the central atom, that's a pretty safe bet for getting an accurately shaped Lewis structure.
Once you can make the most stable lewis structure, the regions of electron density, and and the lone pairs, you should be able to correspond these numbers with a specific shape. You'll need to memorize the names and the characteristics for this one. For example, ammonia (NH3) has 3 regions of electron density and 1 LP, which creates the template AX3E (where A=central atom, X=number of bonded atoms, and E=number of LP) which corresponds with the trigonal pyramidal shape.
To add onto this, the number of atoms bonded to the central atom will almost always be the best indication of the molecular shape. The book even states that when naming the molecular shape, only the positions of the atoms are considered. For example, if there are three atoms bonded to the central atom, you know for sure that the molecule either has a trigonal planar shape or a trigonal pyramidal shape. If there is a lone pair of electrons (in addition to these three atoms) you cannot conclude that it has a tetrahedral shape (because the lone pair is not an atom). The presence of this lone pair would confirm that the shape is a trigonal pyramidal shape because the repulsive effect of the lone pair would cause the three other bonds to move closer to each other.
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