Although the VSEPR theory is correct in predicting that CH4 is tetrahedral, NH3 is trigonal pyramidal,

Question

and H2O is bent, the HNH angle in NH3 is only 107.3° and the HOH angle in H2O is only 104.5°.
Explain these deviations from the expected tetrahedral angle of 109.5°.

I think it has something to do with the electron repulsion but I'm not exactly sure. I remember my professor talking about a cube.

Answer 1

CH4 has no unpaired electrons. Both NH3 and H2O do (NH3 has 1 set and H2O has two sets). You're right. Those unpaired electrons repel nearby electrons differently and the HNH and HOH bonds are not 109.5 but smaller.