Polar molecules Watch

madz2802
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Can only covalently bonded atoms be polar? And so only covalent atoms can have permanent dipole-dipole forces?
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Acruzen
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a polar covalent bond is in between a pure ionic and pure covalent bond, if that helps.
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akfc
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(im not too sure but...) i think you can also have hydrogen bonding O-H as polar, oxygen being delta negative and hydrogen being delta positive.
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joostan
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(Original post by madz2802)
Can only covalently bonded atoms be polar? And so only covalent atoms can have permanent dipole-dipole forces?
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An ion is permenantly charged anyway so has no need for dipoles - a giant ionic structure is held together by the electrostatic attractions of the ion which are similar to, but stronger than the dipoles.
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username913907
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(Original post by madz2802)
Can only covalently bonded atoms be polar? And so only covalent atoms can have permanent dipole-dipole forces?
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Yes and no..... Strictly there is no such thing as an ionic bond, just an extremely polar version of a covalent bond. A covalent bond only becomes truly ionic in the limit of an infinite energy difference between the energy of the two atoms. This is clearly not possible but salts like NaCl etc tend towards this.

In order to be polar (have a dipole) there needs to be an asymmetrical distribution of charge density, which is the case for many covalently bonded molecules. This is possible as the covalent bonds inscribe strong directionality to the bonds and the positions are not interchangeable. This is important as it allows the atoms to not sit in a totally symmetrical environment (as is often the case for 'ionic' systems) where no overall dipole is possible.

It is important to realise that you cannot have a dipole in a symmetrical molecule e.g. CO2.
Each bond may be polar, but there is no overall dipole (look up the actual definition of a dipole). Compare to S=C=O which is not symmetric so does have a dipole.
In theory it would be possible to have a dipolar ionic molecule (if metal coordination complexes count). E.g. [Fe(NH3)(OH2)5]3+ will be polar molecule assuming the coordination positions do not interchange rapidly. However the reason they don't interchange rapidly is because there is significant covalency to the bonding.

(ionic bonding is the same strength regardless of the orientation as it is purely electrostatic, whereas covalent bonding has a strong dependence upon the orientation of the bonds leading to the shapes of molecules you will have learnt about. It's not just down to getting the largest distance apart....)

Bear in mind that the 'ionic model' is just that, a model. It is not what actually happens and there are significant deviations from what it predicts in all but the most 'ionic' substances.
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