Molecular Geometry vs. Molecular Shape

[Grace Hubrig 4C]

I am a little confused on the difference between molecular geometry and molecular shape. Do we have to find the molecular geometry first in order to figure out the molecular shape?

[Sarah H Brown 1L]

Hm, I believe that "molecular geometry" and "molecular shape" are interchangeable. What you might be referring to is finding "electron domain geometry" first to then find the molecular geometry. Essentially, only the position of atoms in a molecule are used to name the molecular geometry (you do not count lone pairs as surrounding atoms). However, understanding the position of the lone pairs is essential to discovering the molecular shape. For example, H₂O has a central atom, O, and two surrounding atoms, the H's. However, O has two lone pairs. If you include the lone pairs, the shape is a central atom O with 4 regions of electron repulsion around it. This makes the electron domain geometry tetrahedral. However, this is NOT the molecular geometry! Now, disregard the lone pairs and look at the shape of H-O-H. The molecular geometry is now bent (or "angular," as I think the book calls it). Looking at the electron domain geometry can help you decide bond angles in molecular geometry. You know the bond angles for tetrahedral are 109.5 degrees. However, the two lone pairs force the two H's closer because lone pairs have stronger electron repulsion than bonded electrons. You can determine that the H-O-H bond angle is <109.5 degrees.