VSEPR Theory

What is it?
Valence
Shell
Electron
Pair
Repulsion


This theory allows us to understand and thus be able to predict the structures (or shapes) of molecules in three dimensions. We know that like charges repel, so electron pairs are located as far apart from each other as possible around the central atom. The geometry of a molecule will be the ideal structure that will best prevent the repulsion that are acted upon the electrons.


If there were two bonded electron pairs around an atom, they would situate themselves in exactly 180º. This shape would be called LINEAR.

Domain - region of space that occupies the electron pair

Here is a chart that can help you predict the geometries and angles for molecules based on the NUMBER of BONDED ELECTRON PAIRS

However, when there are lone pairs, the geometry of a compound would be different as shown above because they exert a larger repulsive force. 
So when compounds like NH3 would be expected to have a trigonal planar shape, the hydrogen atoms would actually have the shape of a tetrahedral because there are four electron pairs.

Notice that the repulsive force exerted by the lone pair causes the hydrogen atoms to move closer to one another. This creates a smaller bond angle between the atoms. This is because lone pairs takes up more space than bonded pairs. We can say that the more lone pairs there are, the smaller the bond angle will be.

Here is a chart that includes lone pair structuring that may help:

Also keep in mind that double and triple bonds may also act the same way as lone pairs, reducing the bond angle as well. This is because the domain of the multiple bond takes up more space, thus pushing the other atoms away.

Enjoy!