Bond angle of Hydrogen Sulphide

[Priyadarshini]

Hydrogen sulphide has the same structure as water. The bonding in water is 104.5 degrees. As both have two bond pairs and two lone pairs of electron, shouldn't the bond angle in hydrogen sulphide be 104.5 degrees? Why is it 92 degrees? Is there anyway one can calculate the bond angles?
Thanks.

 

[jambaugh]

The bonding angle will depend significantly on the fact that the S atoms are bonding via electrons in one higher orbital state and so the hydrogen nuclei will be farther from the sulfur. Just thinking about it intuitively this would allow the hydrogen atoms to be at a smaller angle from each other in the sulfide case if their distance from each other didn't significantly change. Picture an isosceles triangle, if you lengthen the equal size you decrease the inner angle.

Remember that the two hydrogen atoms (in water) don't just interact with the central atom but also with each other. Consider also why they have a bonding angle other than 180deg. There's complex interplay in the wave aspect of the electrons as one defines the common orbitals in the bonding of the three atoms. Solving that system is not something you can tackle with pencil and paper. The spacing of the various nuclei will affect how the electrons wavelength gets wrapped around the them so as to match up phases and constitute a stationary state. Then imagine tweaking each spacing as you look for a minimum energy case.