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?
