It's easy to predict the next BP disasters. Why?

Explain how the VSEPR model can be used to predict molecular shape?

• I know that electron groups (bonding pairs & lone pairs) constantly try to repel each other, and that LP - LP > LP - BP > BP - BP, but I'm not quite sure how exactly VSEPR is used to predict molecular shape. Any help would be appreciated.

• Answer:

  Hi there! I think you wouldn't be happy to hear this, but there's quite a fair bit of memorizing involved in this! But to help you understand how to memorize, you must first know how to count electron densities. An electron density is basically any kind of bonding (single, double, triple, hydrogen, dative, adductive) around the central atom of the molecule + lone electrons or pairs of lone electrons. Note: 3 lone electrons would be considered 2 electron densities since 2 will make up a pair and the third will be counted as 1 electron density on its own. You can see how many electron densities there are by simply drawing out the molecule, including the lone electrons. So after you have learnt how to recognize electron densities, you compare it to your pre-memorized electron pair geometry. 2 electron densities (ED) would mean it is linear, 3 ED is trigonal planar, 4 is tetrahedral, 5 is trigonal bipyramidal and 6 is octehedral. You may ask how in the world these structures came up, well you just have to visualize the way bonds bond to the central atom. You may want to look up the Internet for visuals. Anyway, after you have determined the initial shape, VSEPR now allows you to determine the MOLECULAR GEOMETRY of the molecule by counting the number of lone pairs or lone electrons. All structures without any lone pairs will have the same molecular and electron-pair geometry. For a trigonal planar structure, 1 lone pair = bent. For a tetrahedral structure, 1 lone pair = trigonal pyramidal 2 lone pairs = bent For trigonal bipyramidal structure, 1 lone pair = seesaw shaped 2 lone pairs = T-shaped 3 lone pairs = linear For octehedral structure, 1 lone pair = square pyramidal 2 lone pairs = square planar FINALLY, to answer your question, the VSEPR model helps us determine molecular shape by allowing us to "take out" lone pairs from the electron-density model to reveal the tangable structure that we will actually see, just keep in mind that the lone pairs acts as an invisible bond, and the molecular shape is simply what the molecule looks like taking all these invisible bonds into account. Gosh. I hope I didn't confuse you further :S