VSEPR Model

Valence

Shell

Electron

Pair

Repulsion

VSEPR is a model used in Chemistry in order to predict a geometric shape of molecules based on the number of valence electrons. 

We can classify molecules into 7 main geometric shapes:

• Linear                                      180°                    Non-Polar
• Bent                                         104.5°                Polar
• Trigonal Planar                   120°                    Non-Polar
• Trigonal Pyramidal           107.3°                Polar
• Tetrahedral                          109.5°                Non-Polar
• Trigonal Bipyramidal       90°/ 120°         Non-Polar
• Octahedral                           90°                      Non-Polar

Before figuring out any of those shapes, we need to draw Lewis Dot Diagram first. Let’s look at some examples:

 

Based on its Lewis Dot structure, we can classify HCN as Linear. 

As you can see above, Sulfur has 5 bonds. So, we would classify this as Trigonal Bipyramidal. 

Now, in this case, Sulfur has only 2 bonds. We would classify this as Bent. Why? HCN also has only 2 bonds but classified as Linear. That is because there are two extra pairs of electrons in the middle atom so those two pairs of electrons will push each of the two bonds down, creating an angle.