Now that we know how to find the general structure of a molecule using Lewis Dot diagrams, we can actually find the three-dimensional shape of a molecule AND BUILD IT! We can find the shapes of molecules using the VSEPR (Valence Shell Electron Pair Repulsion) Theory.
The idea is that atoms and electrons found around a central atom are repelled by each other because of the negative charges of the electrons surrounding each (remember opposites attract +/- and similar charges repel -/- or +/+). In other words, everything bonded or attached to the central atom wants to be as far away from everyone else as possible.
So let’s look at NH3 for example.
Step 1: Calculate the valence electrons
1 x N = 1 x 5 = 5
3 x H = 3 x 1 = 3 5+3 = 8 ve–
Step 2: Draw the Lewis Dot Structure
Step 3: Calculate the ABE type
Each letter of ABE stands for a part of the molecule
A= A central atom. If it has a central atom, write an A
B= Attached atoms. Count the number of atoms attached to the central atom and write that as a subscript of B example: B3
E= Free Electron Pairs. If their are extra electron pairs on the central atoms we count them as pairs and write that as a subscript of E (if there are no pairs, do not write E. If there’s only 1 pair, just write E) example: E2
NH3 has a central atom, three attached atoms, and 1 electron pair. So the ABE structure would be AB3E
Step 4: Find the Ideal Geometry.
I like to think of the ideal geometry as places on the central atom with thing happening. If there is only 2 places where things are happening (attached atoms or free electrons) then it’s Linear. If there’s three places, it’s Trigonal. If there’s four, it’s Tetrahedral.
Below is a great link to a video explaining the different shapes and where they come from.
Since NH3 is an AB3E structure there are 4 places where things are happening (B 3 + 1 E) so NH3 is Tetrahedral.
Step 5: Molecular Shape.
Along with the ABE chart is the list of Ideal Geometry and Molecular shape. The shape is based on the idea that the other atoms want to be as far apart as possible AND that free electron pairs need a lot of space to roam.
NH3 has 1 free pair of electrons that float to the top of the molecule as if they were in a balloon. The three Hydrogens then act as a tripod for the entire molecule holding it up. Therefore we say the shape of NH3 is pyramidal.
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!