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)?

1 Answer
May 29, 2017

Answer:

I know of #3# molecular geometries: trigonal planar (#"BF"_3#), trigonal pyramidal (#"NH"_3#), and T-shaped (#"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 (#"B"#) to have one electron shy of a complete valence, because there are three of this element, and one of #"A"#. So, element #"B"# must either be a halogen or hydrogen, and element #"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 #"B"#).

1.

Let's start with a group #13# element, specifically #"B"# (boron). The compound boron trifluoride is #"BF"_3#, so it is an #"AB"_3# molecule. We can predict it's molecular geometry by drawing its Lewis structure:

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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 #"BF"_3# is trigonal planar.

2.

You've probably heard of the compound ammonia, #"NH"_3#. Ammonia is also an #"AB"_3# molecule, and its Lewis structure is

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Since #"NH"_3# has one nonbonding pair of electrons about the central #"N"# atom, and three bonding pairs, #"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 #"ClF"_3#, which is also an #"AB"_3# molecule. Its Lewis structure is

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We can see that the central #"Cl"# atom has two pairs of nonbonding electrons, and three bonding pairs, which makes the molecular geometry of #"ClF"_3# T-shaped.