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    How does the shape of water molecules (i.e. its 104.45º angle) relate to its polarity?
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    (Original post by iseekanswers)
    How does the shape of water molecules (i.e. its 104.45º angle) relate to its polarity?
    http://www.elmhurst.edu/~chm/vchembo...s/206water.gif

    The shape of a molecule, and so the bond angle, is based on a fundamental principle
    regions of high electron density repel one another as far apart as possible.

    The regions of high electron density have the same negative charge, so they repel one another. A bond angle of 104.5* keeps the regions of high election density as far apart as possible

    I'm not sure what you mean by polarity.
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    (Original post by iseekanswers)
    How does the shape of water molecules (i.e. its 104.45º angle) relate to its polarity?
    a polar bond can be envisioned as something in between the most ideal ionic and covalent bond. In reality, many covalent bonds are not ideally covalent, the electron density in covalent bonds tend to be attracted towards one side of the atoms involved in bonding due to electronegativity differences.

    Electronegativity results from different electrostatic interactions between different atoms containing different protons with the electrons involved in covalent bonding. This creates an unequal sharing of electron density known as polarisation, leading to what we call polar bonds.

    However, molecular symmetry is important in determining overall bond polarity.

    whereas in linear molecule like CO2, despite electronegativity differences between C and O, the dipole moments exerted due to this cancel each other out - they are vector quantities - directions matter (like force)

    in H2O, the molecule is bent, the dipoles are exerted along the bent symmetry, and they do not cancel out - i.e. there is a net dipole moment - therefore water is a polar molecule.

    in CH4, yes each C-H bond is polar, but due to the tetrahedral symmetry and all are C-H bonds, then the net dipole is cancelled out - CH4 is non polar. See this trend though,

    CH4 - non polar

    CH3Cl - polar

    CH2Cl2 - polar

    CHCl3 - polar

    CCl4 - polar

    Hope you get the ideas. I think with time, and experiencing and practising more examples, you'd get better.
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    (Original post by shengoc)
    a polar bond can be envisioned as something in between the most ideal ionic and covalent bond. In reality, many covalent bonds are not ideally covalent, the electron density in covalent bonds tend to be attracted towards one side of the atoms involved in bonding due to electronegativity differences.

    Electronegativity results from different electrostatic interactions between different atoms containing different protons with the electrons involved in covalent bonding. This creates an unequal sharing of electron density known as polarisation, leading to what we call polar bonds.

    However, molecular symmetry is important in determining overall bond polarity.

    whereas in linear molecule like CO2, despite electronegativity differences between C and O, the dipole moments exerted due to this cancel each other out - they are vector quantities - directions matter (like force)

    in H2O, the molecule is bent, the dipoles are exerted along the bent symmetry, and they do not cancel out - i.e. there is a net dipole moment - therefore water is a polar molecule.

    in CH4, yes each C-H bond is polar, but due to the tetrahedral symmetry and all are C-H bonds, then the net dipole is cancelled out - CH4 is non polar. See this trend though,

    CH4 - non polar

    CH3Cl - polar

    CH2Cl2 - polar

    CHCl3 - polar

    CCl4 - polar

    Hope you get the ideas. I think with time, and experiencing and practising more examples, you'd get better.
    thanks.
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    one thing though: can you explain in more depth as to why tetrahedrally symmetrical molecules are nonpolar?
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    (Original post by iseekanswers)
    one thing though: can you explain in more depth as to why tetrahedrally symmetrical molecules are nonpolar?
    Dipoles in bonds are vector quantities, they have both magnitude and direction, and as such can be resolved along three dimensional axes.

    If you do this with tetrahedral species you will see that, providing the four substituents are identical, the vectors cancel out.

    If one of the substituents is not the same as the others the molecule will be polar to some extent ...
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    (Original post by charco)
    Dipoles in bonds are vector quantities, they have both magnitude and direction, and as such can be resolved along three dimensional axes.

    If you do this with tetrahedral species you will see that, providing the four substituents are identical, the vectors cancel out.

    If one of the substituents is not the same as the others the molecule will be polar to some extent ...
    how does that differ with water?
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    (Original post by iseekanswers)
    how does that differ with water?
    I don't know what you mean by "differ".

    Water has two polar bonds that can be resolved in two dimensions x and y (horizontal and vertical). It only has three atoms and so can lie on a two dimensinal plane.

    With the oxygen atom at the top the vectors cancel out in a horizontal direction, but add up in the vertical. Here's the diagram:

 
 
 
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