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Explain why the boiling temperature of PH3 is greater than that of CH4. Watch

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    How do you know that PH3 has dipole-dipole and the other one doesn't just from a question like this and no other information given?
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    (Original post by yaya1)
    How do you know that PH3 has dipole-dipole and the other one doesn't just from a question like this and no other information given?
    You should be able to draw out the Lewis structure of each molecule and from that derive the shape of the molecule. Once you do that you see that PH3 is trigonal pyramidal, while CH4 is tetrahedral.

    From the shapes you get the polarity.
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    So does trigonal pyramidal have more dipole than tetrahedral is there trend?
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    (Original post by yaya1)
    So does trigonal pyramidal have more dipole than tetrahedral is there trend?
    Trigonal pyramidal structures are not totally symmetrical. They are symmetrical about a vertical centra of rotation, but they have a non-symmetrical horizontal plane.

    Put simply, there are three groups on one side of the central atom and none on the other. This makes them polar if the attached groups are of different electronegativity to the central atom.



    In this case the phosphorus and the hydrogen have different electronegativity and the hydorgen atoms are relatively positive.

    This means that at the bottom of the molecule there is a region of relative positive charge while at the top there is a region of relative negative charge. This charge separation is called a molecular dipole.

    In the case of CH4 all of the individula dipoles cancel out (although there is very little electronegativelty difference between C and H anyway), and the molecule does not have an overall dipole.



    With experience you can 'see' this by the symmetry of the molecule. Tetrahedral structures are symmetrical in many ways.
 
 
 
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