Question

Can nonpolar molecules have polar bonds?

Answer 1

Sometimes, there can be.

If there is, it must all cancel out. If you consider `"CO"_2`, we know that's nonpolar linear:

`:stackrel(..)"O"="C"=stackrel(..)"O":`

but we should note that it has two dipole moment vectors in the `z` direction:

`stackrel(larr)( :stackrel(..)"O"=)"C"stackrel(rarr)(=stackrel(..)"O": )`

These are of equal magnitude but opposite direction, so they cancel out perfectly. Thus, `"CO"_2` is considered nonpolar.

However, it doesn't mean `"CO"_2` can't be polarized, which is another story. It has two infrared bands near `"2360 cm"^(-1)`.

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These correspond to `"CO"_2`'s asymmetrical stretching and bending vibrational modes. It has two nondegenerate modes that are IR-active as shown below:

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If it were not possible for `"CO"_2` to develop a net nonzero dipole moment (thereby changing its dipole moment, one prerequisite for an IR band, the other being the asymmetry of the vibrational mode), we wouldn't see these infrared bands.

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We could have treated these identical-magnitude dipole moment vectors as `vecl = << 0,0, 1 >>` and `vecr = << 0,0,-1 >>`. In that case, we can show that they cancel upon summing them together:

`vecl + vecr`

`= << 0,0,1 >> + << 0,0,-1 >>`

`= << 0,0,0 >> = vec0`

which is a dot, representing the nonpolar result of two dipole moment vectors of identical magnitude and perfectly opposite directions summing together.