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A molecule's polarity has two main contributing factors: symmetry and bond polarity. In order for a molcule to have poles, it must be asymmetrical and contain polar bonds. In the case of HI, the molecule is asymmetrical, however the difference in the electronegativities between hydrogen (with an electronegativity of 2.1) and iodine (with an electronegativity of 2.5) is technically too small for the H---I bond to be polar. However, there is a characteristic associated with polar molecules called its "dipole moment." The value of a polar molecule's dipole moment can be calculated by multiplying the bond length of the molecule (from nucleus to nucleus) by the charge of the molecules. Since iodine has such a large atomic radius, and therefore a larger bond radius than the hydrogen halides HF, HCl, and HBr, it can be thought of as having a very slight dipole moment. Therefore, HI is a polar molecule.

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16y ago

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