In some molecules, when all the bond dipole moments
are summed together, the overall dipole moment is zero. In such a case, the molecule is
said to be nonpolar.
Is carbon dioxide polar or nonpolar?
In other molecules, when all the dipole moments
are summed up, there is a net dipole moment.
Do you know how to explain that water is a polar molecule?
In summary,
to determine whether a molecule is polar or not, we need to sum up all the
bond dipole moments of the molecule by
adding up all its bond dipole moments (ie - vectors).
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When the molecule does not possess a net dipole moment, the molecule is
nonpolar.
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When the molecule possesses a net dipole moment, the molecule is
polar.
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