Lecture #15
  Text: acid structure not in text; molecular geometries (VSEPR) is section 13.13
  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.
Lecture Outline Molecular Geometries

VSEPR Model

Steric number (and coordination number)

Electronic geometries

(Ideal) Molecular geometries
A former "target practice" question inquired as to which of the two OH groups on the bottom of the vitamin C structure shown here is actually the one providing the acidic hydrogen. The suggestion was that resonance stabilization of the resulting negative ion would lead to the correct choice.  
The question of why you don't get eight oxygens in an oxygen molecule as is achieved for sulfur can be answered by noting that such an oxygen cluster is even more stable if it breaks into four diatomic molecules. For sulfur, such is not the case. Sulfur's ring is actually puckered, and such a subtle geometrical detail serves as a transition into the next major topic: molecular geometries.
Obtaining molecular geometries once a Lewis structure is known. The Lewis structure enables you to determine the steric number of each atom. The steric number is the number of bonded atoms and lone pairs around an atom.
Valence electron arrangement corresponding to a molecule of type AB2, an example being BeCl2. Steric number =2. (Coordination number =2.)
Valence electron arrangement corresponding to a molecule of type AB3. Steric number =3.
Valence electron arrangement corresponding to a molecule of type AB4. Steric number = 4.
Valence electron arrangement corresponding to a molecule of type AB5. Steric number = 5.
Valence electron arrangement corresponding to a molecule of type AB6. Steric number = 6.
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.