| Molecular Shapes and Polarity | | Valence electrons determine molecular shape. Molecules are three-dimensional entities and therefore best depicted in three dimensions. We can translate the two-dimensional electron-dot structure representing a molecule into a more accurate three-dimensional rendering by using the model known as valence shell electron-pair repulsion, also called VSEPR (pronounced ves-per). | | VSEPR theory | | According to this model, any given pair of valence-shell electrons strives to get as far away as possible from all other electron pairs in the shell. This includes both nonbonding pairs and any bonding pairs not taking part in a double or triple bond. (Pairs in a multiple bond stay together because of their mutual attractions for the same two nuclei). The VSEPR model talks about the repulsion between pairs of electrons, not between the two electrons in a pair (The electrons in a pair don?t repel each other because of their opposite spins). It is this striving for maximum separation distance between electron pairs that determines the geometry of any molecule. The two dimensional electron-dot structure for methane, CH4, is |  | | In this structure, the bonding electron pairs (shown as straight lines representing one electron from each atom) are set 90 degrees apart because that is the farthest apart they can be shown in two dimensions. When we extend to three dimensions, however, we can create a more accurate rendering in which the four bonding pairs are 109.5 degrees apart: |  | | Previous | Next |
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