Why do we describe the #"ELECTRONIC"# geometry of the water molecule as tetrahedral, but its molecular geometry as bent?

1 Answer
Mar 20, 2017

Answer:

#"Well, there are 4 electron pairs around the central oxygen atom,"# #"so.........."#

Explanation:

There are 4 valence electron pairs in water: 2 bonding, the #O-H# bonds; and 2 non-bonding, the #O# lone pairs. VESPER dictates that the most stable geometry of these bonding and lone pairs is tetrahedral.

And thus to a first approximation, the #/_H-O-H# should be #109.5^@#, which of course is the ideal tetrahedral angle - certainly this is the #/_H-C-H# bond angle we observe in methane.

However (and there is always a #"however"#), because TWO of the electron pairs around oxygen are LONE pairs, these tend to lie closer to the oxygen atom. And these lone pairs tend to compress the #/_H-O-H# bond angle down from the tetrahedral angle to approx. #104.5^@# by electrostatic repulsion of like charges. We may make the same argument for the ammonia molecule, #NH_3#, #"trigonal pyramidal"# BUT tetrahedral to a first approximation. Capisce?