Now that you have completed the simple VSEPR diagrams and made your models of Linear, Trigonal, and Tetrahedral molecular geometry… it’s time to break the rules.
The rule we’re breaking is the Octet rule. To this point you have limited the central atom to sharing up to 8 electrons. But, If your Lewis Dot Diagram has too few electrons, you add pairs of electrons to the central atom.
Let me show you what I mean. Let’s say that we have XeF2.If you calculate the valence electrons you should have 22.
But when you make the Lewis Dot Diagram, it only has 20 ve–.
So, we add a pair of electrons to the Xe central atom giving us a total of 22 ve–This creates a new Ideal Geometry known as Triangular Bipyramidal, which means it has placement for 5 parts (attached atoms or free electron pairs) around the central atom.
Like Tetrahedrons, we are working now in 3-dimensions instead of 2.
There is also an Ideal Geometry for 6 parts known as Octahedron. Think of the central atom as a 6 -sided die. So atoms can be bonded in 6 places and in 3-dimensions!