vsepr Theory and Model
All elements have between one and eight electrons in their outer shell. This is true regardless of the atomic number of the element as can be seen when using the electron shell filling pattern. These outer shell electrons are divided evenly into four equally spaced regions around the nucleus. These four regions are called orbitals and each can hold a maximum of two electrons. Since all electrons are negatively charged and since like charges repel, the orbitals are pushed as far away from each other as possible. 
If we are familiar with the two dimensional version of electron dot diagrams, we end up with simple flat pictures such as these to the right, for single atoms. Here you can see an electron dot diagram for each element from Lithium through to Neon. These flat pictures are fine for individual atoms, and they even allow us to make reasonable looking molecules like this:
If we replace each PAIR of electrons with a line, we get the standard simple molecular diagram for 1-Propanol:
The Three Dimensional Model 
Atoms and molecules are not flat, so we need to take 3-dimensional space into account when considering vsepr. The best way to learn this is to get a ball of modeling clay or the like and stick four matches in it, representing the four orbitals. If we rearrange these four sticks until they are all as far apart from each other as possible, we end up with a tetrahedral shape like that in the picture to the right. The dark "leg" at the bottom of the image is an orbital that is going backwards and down from the sphere.Obviously the bonds between the atoms will be affected by this positioning on the orbitals in 3-dimensional space. The conventional method of showing the orbital or bond projecting backwards and down is with a dotted line. Doing this, we can have another look at the molecule of 1-Propanol: 
This is a good model to build out of modeling clay and matches; it's easier to see these in 3D. Double Bonds 
Since double bonds have a greater quantity of electrons in them, they exert more of a push on the remaining orbitals. In terms of modeling and drawing molecules correctly, this means that the angle between the two remaining orbitals will be smaller than the angle between each of the orbitals and the double bond itself.Triple Bonds 
Any atom that is involved in a triple bond, such as Nitrogen in N2, has only one orbital remaining. As with all like charges, this orbital will move as far away from the others as possible. In the case of the triple bond, the remaining orbital will be at the opposite end of the atom, again resulting in a flat molecule. This is as predicted by the VSEPR model; the orbitals are as far away from each other as possible.Return from VSEPR to The Chemistry or return to the Green Planet home page for more Solar Power Facts. |
