Wording of Questions Regarding Molecular Shape

(Polar molecules, Non-polar molecules, etc.)

Moderators: Chem_Mod, Chem_Admin

Alicia Beebe
Posts: 37
Joined: Fri Apr 06, 2018 11:01 am

Wording of Questions Regarding Molecular Shape

Postby Alicia Beebe » Wed Jun 06, 2018 12:06 am

In discussion last week, we were working through problems of determining molecular shape. I think I understand the concepts and process well, but got confused on the wording. It seems to me that there are two different answers for molecular shape based on if you include the lone pairs in the naming, or don't include them. For example, If an atom had two bonds and one lone pair, it would be trigonal planar since there are three regions of electron density, but we would refer to it as angular when not including the lone pair in the naming, correct?
So, my question is how to determine which answer to give, for example on a quiz or test. I understand you kind of have to figure out "with the lone pair" to determine "without the lone pair," but in what instance (meaning with what wording of the question) would each of the answers be given?

NatalieSDis1A
Posts: 39
Joined: Fri Apr 06, 2018 11:05 am

Re: Wording of Questions Regarding Molecular Shape

Postby NatalieSDis1A » Wed Jun 06, 2018 12:26 am

You are correct in saying that an atom with two bonds and one lone pair would be called angular. No matter the wording of the question, we would never call a molecule with 2 bonds and one lone pair trigonal planar. Trigonal planar has 3 bonds while angular has 2 bonds and one lone pair. However, one reason to consider the 3 regions of electron density and associate them with trigonal planar is to derive the bond angle of an angular molecule. Since an atom with 3 bonds has angles of 120 degrees we can derive that 2 bonds and a lone pair (which has more repulsive power than a bond) would form an angle < 120 degrees.

jessicasam
Posts: 36
Joined: Wed Nov 15, 2017 3:02 am
Been upvoted: 1 time

Re: Wording of Questions Regarding Molecular Shape

Postby jessicasam » Wed Jun 06, 2018 10:40 am

You may be confused between the difference of electron geometry and molecular geometry. When naming using the electron geometry, all the regions of electron density are included. So an atom that has two bonds and one lone pair would be trigonal planar since there are three regions of electron density. When naming using molecular geometry, this atom would be referred to as angular because it is only describing the shape of the atom using the bonds. Both names are correct, it just depends on which type of geometry you are being asked about.


Return to “Determining Molecular Shape (VSEPR)”

Who is online

Users browsing this forum: No registered users and 1 guest