Lecture #16
 
  CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Text: Section 13.13 Molecular Geometries

VSEPR Model

Steric number (and coordination number)

Electronic geometries

(Ideal) Molecular geometries
Valence electron arrangement corresponding to a molecule of type AB2E (where E symbolizes a non-bonding or lone pair of electrons). Steric number = 3.
A three dimensional view of AB2E
Valence electron arrangement corresponding to a molecule of type AB3E (where E symbolizes a non-bonding or lone pair of electrons). Steric number = 4.
A three dimensional view of AB3E
A structure of the type AB2E2. Steric number = 4.
A structure of the type AB4E. Steric number = 5.
Previous molecule type showing three dimensional structure as influenced by the lone pair.
A structure of the type AB3E2. Steric number = 5.
Previous molecule type showing three dimensional structure as influenced by the lone pairs.
A structure of the type AB2E3. Steric number = 5.
Previous molecule type showing three dimensional structure as influenced by the lone pairs.
A structure of the type AB5E. Steric number = 6.
Previous molecule type showing three dimensional structure as influenced by the lone pair.
A structure of the type AB4E2. Steric number = 6.
Previous molecule type showing three dimensional structure as influenced by the lone pairs.
The relative effectiveness of different electron pair repulsion combinations. The example illustrated shows how the HOH bond angle is distorted down to 104.5o from its ideal tetrahedral angle of 109.5o
An illustration of how the differences between electron pair repulsions leading to the preferred see-saw geometry of a molecule of type AB4E.
Further distortions from the ideal VSEPR geometry of a molecule of type AB4E.
Here is the rationale behind choosing the T-shaped molecular geometry for two lone pairs and three bonding pairs about a central atom. Two of the choices have six unfavorable repulsions at 90o. The other has only four such repulsions. See it?
Illustrating the imbalance when SF4's lone pairs distort bond angles and bond lengths from the ideal VSEPR geomtry.
In a molecule of type AB4E2, the lone pair repulsions make the preferred molecular geometry that corresponding to square planar. This gives minimum lone pair-lone pair repulsions
The molecular geometry of structure AB5E and the distortions from the ideal square pyramidal molecular geometry caused by the lone pair of electrons.
Additional considerations that lead to distortions from ideal VSEPR molecular geometries. (These are not in the textbook.)
Effect of multiple bonds.
Illustration of double bond effect in POCl3
The effect of a lone single electron.
Illustrating the lone electron effect by comparing ClO2- to ClO2