Video Transcript
Using the AXE method, which of the following AXE notations corresponds to a molecule with a trigonal pyramidal shape? (A) AX one E three, (B) AX four, (C) AX two E two, (D) AX three, or (E) AX three E.
The question is asking us to use the AXE method, which is a method that can be used with Lewis structure diagram to categorize polyatomic molecules and ions into groups of preferred three-dimensional geometry. The AXE group will allow us to predict the preferred three-dimensional geometry of the molecule or ion. So we must identify the notation that represents the AXE group that would take on a trigonal pyramidal shape. In an AXE notation, the A represents the central atom in the structure. X represents bonded electron domains on the central atom, which can be a single or multiple bond between the central atom and an outer atom. And E represents the nonbonded electron domains on the central atom.
Nonbonded electron domains can also be called lone pairs. A lone pair is a pair of valence electrons that are not shared in a covalent bond. And in the case of X notation, we would count only the lone pairs present on the central atom. Finally, 𝑚 and 𝑛 are both integers, with 𝑚 representing the number of bonded electron domains on the central atom and 𝑛 representing the number of nonbonded electron domains.
Since we must identify the AXE notation that corresponds to the trigonal pyramidal shape, let’s have a look at a molecule that takes on this shape. A molecule we see often in chemistry is ammonia, or NH3. In this Lewis structure diagram of ammonia, we can see there nitrogen is the central atom of the structure. Nitrogen has one nonbonded electron domain, or in other words one lone pair, and three bonded electrons domains due to the three single bonds with three hydrogen atoms. Using this information, we can predict the three-dimensional geometry of ammonia which arises due to the repulsion between the bonding and nonbonding domains surrounding the central atom.
The domains will situate around the nitrogen to maximize the distance and minimize the repulsion between domains. In the case of ammonia, the shape of a triangle-based pyramid forms, which we can imagine the lone pair at the peak of the pyramid and the outer hydrogen atoms forming the base. This molecule takes on a trigonal pyramidal shape. So if we were to write the AXE notation for ammonia, we would start with A, representing the central nitrogen atom, then X with a subscript of three, representing the three bonded electron domains, and finally E, representing the one nonbonded electron domain. Note that the subscript of one is implied, so we don’t need to write it.
We can see that the answer choice that matches the AXE notation for a structure with a trigonal pyramidal shape is (E) AX three E.
