Why Is the Angle Between Hydrogen Atoms in Water Molecules 105 Degrees?

[johnhb]

In "Six Easy Pieces", Chapter 1, Richard Feynman discusses how the angle between hydrogen atoms in a water molecule is 105 deg. Why is this? Would it be the same for a heavy water molecule?

John

 

[DrDu]

Probably Feynman discussed it because he found it interesting or illuminating!

 

[johnhb]

Thank you, Dr.Dru, but I was wondering why the angle is 105 deg?

John

 

[DrDu]

Yes, but what is the problem? I thought Feynman gave an explanation. Is there anything unclear with his explanation?
One of the easiest explanations is the VSEPR (Valence Shell Electron Pair Repulstion) theory.
According to it the free and bound electron pairs repell each other. In H2O there are two free electron pairs and two bound ones, hence they try to arange in a tetrahedral configuration.
The bound electron pairs extend over two atoms, hence the repulsion between them is a little bit smaller whence the angle between the bound pairs is a little bit smaller than 109.5 deg, the ideal tetrahedral angle.