# According to the VSEPR theory, which shape is possible for a molecule with the molecular formula of AB_3 (where the number of total electron groups is unstated)?

May 29, 2017

I know of $3$ molecular geometries: trigonal planar (${\text{BF}}_{3}$), trigonal pyramidal (${\text{NH}}_{3}$), and T-shaped (${\text{ClF}}_{3}$).

#### Explanation:

By shape, I'll assume you mean the molecule's molecular geometry; that is, the arrangement of solely the atoms around the central atom, not the nonbonding electron pairs. We would expect the second element ($\text{B}$) to have one electron shy of a complete valence, because there are three of this element, and one of $\text{A}$. So, element $\text{B}$ must either be a halogen or hydrogen, and element $\text{A}$ must be a nonmetal (it's a molecule) in groups $13$ or higher (groups $12$ and lower are all metals, except for hydrogen, which is a possibility for $\text{B}$).

1.

Let's start with a group $13$ element, specifically $\text{B}$ (boron). The compound boron trifluoride is ${\text{BF}}_{3}$, so it is an ${\text{AB}}_{3}$ molecule. We can predict it's molecular geometry by drawing its Lewis structure:

We can see that since there are no nonbonding electrons left on the central boron atom, and there are three bonding pairs, the molecular geometry of ${\text{BF}}_{3}$ is trigonal planar.

2.

You've probably heard of the compound ammonia, ${\text{NH}}_{3}$. Ammonia is also an ${\text{AB}}_{3}$ molecule, and its Lewis structure is

Since ${\text{NH}}_{3}$ has one nonbonding pair of electrons about the central $\text{N}$ atom, and three bonding pairs, ${\text{NH}}_{3}$ has the molecular geometry trigonal pyramidal.

3.

It is possible to form interhalogen compounds; that is, a molecule consisting of only halogen atoms. An example of this is ${\text{ClF}}_{3}$, which is also an ${\text{AB}}_{3}$ molecule. Its Lewis structure is

We can see that the central $\text{Cl}$ atom has two pairs of nonbonding electrons, and three bonding pairs, which makes the molecular geometry of ${\text{ClF}}_{3}$ T-shaped.