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Thread: why the bond angle of water is 105 instead of 109?

  1. #1 why the bond angle of water is 105 instead of 109? 
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    Answer please.


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    This sounds like another HW question, but i'm going to answer it anyway. It is because the electrical density of the lone pairs does not act like a perfect tetrahedron. It is distributed evenly over the remaining space and pushes the hydrogen atoms closer together.


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  4. #3  
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    Answer please.
    \and for future, think about you ask the question. some people (myself included) wouldnt like answering such a request.
    Stumble on through life.
    Feel free to correct any false information, which unknown to me, may be included in my posts. (also - let this be a disclaimer)
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    Moderator Moderator AlexP's Avatar
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    actually, it's not 105<sup>o</sup>, it's 104.45<sup>o</sup>.
    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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  6. #5  
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    The two lone pairs of electrons on the oxygen atom have greater repulsive forces than the bonding pairs. Therefore the angle is "squeezed" and decreases from 109 to 105.
    In theory, there is no difference between theory and practice. In practice, however, there is.
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    Quote Originally Posted by scientist-to-be
    The two lone pairs of electrons on the oxygen atom have greater repulsive forces than the bonding pairs. Therefore the angle is "squeezed" and decreases from 109 to 105.
    Why when there are in the bond pair also 2 electrons and the lone pair 2 electrons. What is the problem? Also the sp3 hybrid orbitals are on same energy level so, they are all similar by size.
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    Quote Originally Posted by scientist91
    Why when there are in the bond pair also 2 electrons and the lone pair 2 electrons. What is the problem? Also the sp3 hybrid orbitals are on same energy level so, they are all similar by size.
    According to the VSEPR ( Valence Shell Electron Pair Repulsion) theory, the repulsion generated by the lone pairs is greater than that by the bonding pairs, since lone pairs do not have another atom at the end.
    In theory, there is no difference between theory and practice. In practice, however, there is.
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    Quote Originally Posted by scientist-to-be
    Quote Originally Posted by scientist91
    Why when there are in the bond pair also 2 electrons and the lone pair 2 electrons. What is the problem? Also the sp3 hybrid orbitals are on same energy level so, they are all similar by size.
    According to the VSEPR ( Valence Shell Electron Pair Repulsion) theory, the repulsion generated by the lone pairs is greater than that by the bonding pairs, since lone pairs do not have another atom at the end.
    Ok, thank you.
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  10. #9  
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    Quote Originally Posted by scientist-to-be
    The two lone pairs of electrons on the oxygen atom have greater repulsive forces than the bonding pairs. Therefore the angle is "squeezed" and decreases from 109 to 105.
    This is probably what the teacher wants you to say, but it's also wrong. Water is not an sp3 hybrid, and the bond angle isn’t 104.5 because of lone pair repulsion.

    In a water molecule each proton forms a sigma bond to a p orbital on the oxygen, giving an ideal bond angle of 90 degrees. The van der Waals radius of the protons and electrostatic repulsion between the positively charged protons causes them to move apart to around 104.5 degrees. If you consider H2S, which should presumably have a very similar electronic structure to H2O, the protons are almost exactly 90 degrees from each other. Since S is a lot larger than O, the protons are able to be 90 degrees apart without coming too close together. It is easy to prove all this with photoelectron spectroscopy.

    It’s unfortunate that so many low-level textbooks still teach the myth of water being an sp3 hybrid.
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  11. #10  
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    Quote Originally Posted by scientist91
    Also the sp3 hybrid orbitals are on same energy level so, they are all similar by size.
    On a side note, I'm pretty sure this is wrong. The bonding sp3 orbitals would still be lower in energy than the non-bonding sp3 orbitals. Otherwise, why would a bond form at all?
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    Quote Originally Posted by Scifor Refugee
    This is probably what the teacher wants you to say, but it's also wrong.
    I always knew they taught us wrong stuff at school!! Anyway, thanks for the info.., I wasn't aware of that before ( I didn't even know that protons can form pi bonds!)
    In theory, there is no difference between theory and practice. In practice, however, there is.
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  13. #12  
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    They don't form pi bonds, they each form a single sigma bond to an oxygen p orbital.
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